diff options
| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 04:39:33 -0700 |
|---|---|---|
| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 04:39:33 -0700 |
| commit | a9322e9a3e5d2f5543809794555a3fad17e0f3d1 (patch) | |
| tree | 6da90fa795d26817308b415d60fda51eb4ef310e /12299-0.txt | |
Diffstat (limited to '12299-0.txt')
| -rw-r--r-- | 12299-0.txt | 7095 |
1 files changed, 7095 insertions, 0 deletions
diff --git a/12299-0.txt b/12299-0.txt new file mode 100644 index 0000000..7cd69a8 --- /dev/null +++ b/12299-0.txt @@ -0,0 +1,7095 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 12299 *** + +THE MECHANICAL PROPERTIES OF WOOD + + + + +[Illustration: Frontispiece. _Photo by the author_. + +Photomicrograph of a small block of western hemlock. At the top +is the cross section showing to the right the late wood of one +season's growth, to the left the early wood of the next season. +The other two sections are longitudinal and show the fibrous +character of the wood. To the left is the radial section with +three rays crossing it. To the right is the tangential section +upon which the rays appear as vertical rows of beads. X 35.] + + + + +THE MECHANICAL PROPERTIES OF WOOD + +_Including a Discussion of the Factors Affecting the Mechanical +Properties, and Methods of Timber Testing_ + + +BY SAMUEL J. RECORD, M.A., M.F. ASSISTANT PROFESSOR OF FOREST +PRODUCTS, YALE UNIVERSITY + + +FIRST EDITION FIRST THOUSAND + +1914 + + +BY THE SAME AUTHOR + +Identification of the Economic Woods of the United States. +8vo, vi + 117 pages, 15 figures. Cloth, $1.25 net. + + +TO THE STAFF OF THE FOREST PRODUCTS LABORATORY, AT MADISON, +WISCONSIN IN APPRECIATION OF THE MANY OPPORTUNITIES AFFORDED AND +COURTESIES EXTENDED THE AUTHOR + + + + +PREFACE + + + +This book was written primarily for students of forestry to whom +a knowledge of the technical properties of wood is essential. +The mechanics involved is reduced to the simplest terms and +without reference to higher mathematics, with which the students +rarely are familiar. The intention throughout has been to avoid +all unnecessarily technical language and descriptions, thereby +making the subject-matter readily available to every one +interested in wood. + +Part I is devoted to a discussion of the mechanical properties +of wood--the relation of wood material to stresses and strains. +Much of the subject-matter is merely elementary mechanics of +materials in general, though written with reference to wood in +particular. Numerous tables are included, showing the various +strength values of many of the more important American woods. + +Part II deals with the factors affecting the mechanical +properties of wood. This is a subject of interest to all who are +concerned in the rational use of wood, and to the forester it +also, by retrospection, suggests ways and means of regulating +his forest product through control of the conditions of +production. Attempt has been made, in the light of all data at +hand, to answer many moot questions, such as the effect on the +quality of wood of rate of growth, season of cutting, heartwood +and sapwood, locality of growth, weight, water content, +steaming, and defects. + +Part III describes methods of timber testing. They are for the +most part those followed by the U.S. Forest Service. In schools +equipped with the necessary machinery the instructions will +serve to direct the tests; in others a study of the text with +reference to the illustrations should give an adequate +conception of the methods employed in this most important line +of research. + +The appendix contains a copy of the working plan followed by the +U.S. Forest Service in the extensive investigations covering the +mechanical properties of the woods grown in the United States. +It contains many valuable suggestions for the independent +investigator. In addition four tables of strength values for +structural timbers, both green and air-seasoned, are included. +The relation of the stresses developed in different structural +forms to those developed in the small clear specimens is given. + +In the bibliography attempt was made to list all of the +important publications and articles on the mechanical properties +of wood, and timber testing. While admittedly incomplete, it +should prove of assistance to the student who desires a fuller +knowledge of the subject than is presented here. + +The writer is indebted to the U.S. Forest Service for nearly all +of his tables and photographs as well as many of the data upon +which the book is based, since only the Government is able to +conduct the extensive investigations essential to a thorough +understanding of the subject. More than eighty thousand tests +have been made at the Madison laboratory alone, and the work is +far from completion. + +The writer also acknowledges his indebtedness to Mr. Emanuel +Fritz, M.E., M.F., for many helpful suggestions in the +preparation of Part I; and especially to Mr. Harry Donald +Tiemann, M.E., M.F., engineer in charge of Timber Physics at the +Government Forest Products Laboratory, Madison, Wisconsin, for +careful revision of the entire manuscript. + +SAMUEL J. RECORD. +YALE FOREST SCHOOL, _July 1, 1914_. + + + + +CONTENTS + + + + PREFACE + + PART I THE MECHANICAL PROPERTIES OF WOOD + + Introduction + Fundamental considerations and definitions + Tensile strength + Compressive or crushing strength + Shearing strength + Transverse or bending strength: Beams + Toughness: Torsion + Hardness + Cleavability + + PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF + WOOD + + Introduction + Rate of growth + Heartwood and sapwood + Weight, density, and specific gravity + Color + Cross grain + Knots + Frost splits + Shakes, galls, pitch pockets + Insect injuries + Marine wood-borer injuries + Fungous injuries + Parasitic plant injuries + Locality of growth + Season of cutting + Water content + Temperature + Preservatives + + PART III TIMBER TESTING + + Working plan + Forms of material tested + Size of test specimens + Moisture determination + Machine for static tests + Speed of testing machine + Bending large beams + Bending small beams + Endwise compression + Compression across the grain + Shear along the grain + Impact test + Hardness test: Abrasion and indentation + Cleavage test + Tension test parallel to the grain + Tension test at right angles to the grain + Torsion test + Special tests + Spike pulling test + Packing boxes + Vehicle and implement woods + Cross-arms + Other tests + + APPENDIX + + Sample working plan of United States Forest + Service + Strength values for structural timbers + + BIBLIOGRAPHY + + Part I: Some general works on mechanics, materials of + construction, and testing of materials + Part II: Publications and articles on the mechanical + properties of wood, and timber testing + Part III: Publications of the United States Government on + the mechanical properties of wood, and timber + testing + +ILLUSTRATIONS + + Frontispiece Photomicrograph of a small block of western + hemlock + 1. Stress-strain diagrams of two longleaf pine beams + 2. Compression across the grain + 3. Side view of failures in compression across the + grain + 4. End view of failures in compression across the + grain + 5. Testing a buggy-spoke in endwise compression + 6. Unequal distribution of stress in a long column due + to lateral bending + 7. Endwise compression of a short column + 8. Failures of a short column of green spruce + 9. Failures of short columns of dry chestnut + 10. Example of shear along the grain + 11. Failures of test specimens in shear along the + grain + 12. Horizontal shear in a beam + 13. Oblique shear in a short column + 14. Failure of a short column by oblique shear + 15. Diagram of a simple beam + 16. Three common forms of beams--(1) simple, + (2) cantilever, (3) continuous + 17. Characteristic failures of simple beams + 18. Failure of a large beam by horizontal shear + 19. Torsion of a shaft + 20. Effect of torsion on different grades of hickory + 21. Cleavage of highly elastic wood + 22. Cross-sections of white ash, red gum, and eastern + hemlock + 23. Cross-section of longleaf pine + 24. Relation of the moisture content to the various + strength values of spruce + 25. Cross-section of the wood of western larch showing + fissures in the thick-walled cells of the late + wood + 26. Progress of drying throughout the length of a + chestnut beam + 27. Excessive season checking + 28. Control of season checking by the use of S-irons + 29. Static bending test on a large beam + 30. Two methods of loading a beam + 31. Static bending test on a small beam + 32. Sample log sheet, giving full details of a + transverse bending test on a small pine beam + 33. Endwise compression test + 34. Sample log sheet of an endwise compression test on + a short pine column + 35. Compression across the grain + 36. Vertical section of shearing tool + 37. Front view of shearing tool + 38. Two forms of shear test specimens + 39. Making a shearing test + 40. Impact testing machine + 41. Drum record of impact bending test + 42. Abrasion machine for testing the wearing qualities + of woods + 43. Design of tool for testing the hardness of woods + by indentation + 44. Design of tool for cleavage test + 45. Design of cleavage test specimen + 46. Designs of tension test specimens used in United + States + 47. Design of tension test specimen used in New South + Wales + 48. Design of tool and specimen for testing tension at + right angles to the grain + 49. Making a torsion test on hickory + 50. Method of cutting and marking test specimens + 51. Diagram of specific gravity apparatus + + TABLES + + I. Comparative strength of iron, steel, and wood + II. Ratio of strength of wood in tension and in + compression + III. Right-angled tensile strength of small clear + pieces of 25 woods in green condition + IV. Results of compression tests across the grain on + 51 woods in green condition, and comparison with + white oak + V. Relation of fibre stress at elastic limit in + bending to the crushing strength of blocks cut + therefrom in pounds per square inch + VI. Results of endwise compression tests on small + clear pieces of 40 woods in green condition + VII. Shearing strength along the grain of small clear + pieces of 41 woods in green condition + VIII. Shearing strength across the grain of various + American woods + IX. Results of static bending tests on small clear + beams of 49 woods in green condition + X. Results of impact bending tests on small clear + beams of 34 woods in green condition + XI. Manner of first failure of large beams + XII. Hardness of 32 woods in green condition, as + indicated by the load required to imbed a + 0.444-inch steel ball to one-half its diameter + XIII. Cleavage strength of small clear pieces of 32 + woods in green condition + XIV. Specific gravity, and shrinkage of 51 American + woods + XV. Effect of drying on the mechanical properties of + wood, shown in ratio of increase due to reducing + moisture content from the green condition to + kiln-dry + XVI. Effect of steaming on the strength of green + loblolly pine + XVII. Speed-strength moduli, and relative increase in + strength at rates of fibre strain increasing in + geometric ratio + XVIII. Results of bending tests on green structural + timbers + XIX. Results of compression and shear tests on green + structural timbers + XX. Results of bending tests on air-seasoned + structural timbers + XXI. Results of compression and shear tests on + air-seasoned structural timbers + XXII. Working unit stresses for structural timber + expressed in pounds per square inch + + + + +PART I THE MECHANICAL PROPERTIES OF WOOD + + + +INTRODUCTION + + +The mechanical properties of wood are its fitness and ability to +resist applied or external forces. By external force is meant +any force outside of a given piece of material which tends to +deform it in any manner. It is largely such properties that +determine the use of wood for structural and building purposes +and innumerable other uses of which furniture, vehicles, +implements, and tool handles are a few common examples. + +Knowledge of these properties is obtained through +experimentation either in the employment of the wood in practice +or by means of special testing apparatus in the laboratory. +Owing to the wide range of variation in wood it is necessary +that a great number of tests be made and that so far as possible +all disturbing factors be eliminated. For comparison of +different kinds or sizes a standard method of testing is +necessary and the values must be expressed in some defined +units. For these reasons laboratory experiments if properly +conducted have many advantages over any other method. + +One object of such investigation is to find unit values for +strength and stiffness, etc. These, because of the complex +structure of wood, cannot have a constant value which will be +exactly repeated in each test, even though no error be made. The +most that can be accomplished is to find average values, the +amount of variation above and below, and the laws which govern +the variation. On account of the great variability in strength +of different specimens of wood even from the same stick and +appearing to be alike, it is important to eliminate as far as +possible all extraneous factors liable to influence the results +of the tests. + +The mechanical properties of wood considered in this book are: +(1) stiffness and elasticity, (2) tensile strength, (3) +compressive or crushing strength, (4) shearing strength, (5) +transverse or bending strength, (6) toughness, (7) hardness, (8) +cleavability, (9) resilience. In connection with these, +associated properties of importance are briefly treated. + +In making use of figures indicating the strength or other +mechanical properties of wood for the purpose of comparing the +relative merits of different species, the fact should be borne +in mind that there is a considerable range in variability of +each individual material and that small differences, such as a +few hundred pounds in values of 10,000 pounds, cannot be +considered as a criterion of the quality of the timber. In +testing material of the same kind and grade, differences of 25 +per cent between individual specimens may be expected in +conifers and 50 per cent or even more in hardwoods. The figures +given in the tables should be taken as indications rather than +fixed values, and as applicable to a large number collectively +and not to individual pieces. + + + +FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS + + +Study of the mechanical properties of a material is concerned +mostly with its behavior in relation to stresses and strains, +and the factors affecting this behavior. A ~stress~ is a +distributed force and may be defined as the mutual action (1) of +one body upon another, or (2) of one part of a body upon another +part. In the first case the stress is _external_; in the other +_internal_. The same stress may be internal from one point of +view and external from another. An external force is always +balanced by the internal stresses when the body is in +equilibrium. + +If no external forces act upon a body its particles assume +certain relative positions, and it has what is called its +_natural shape and size_. If sufficient external force is +applied the natural shape and size will be changed. This +distortion or deformation of the material is known as the +~strain~. Every stress produces a corresponding strain, and +within a certain limit (see _elastic limit_, in FUNDAMENTAL +CONSIDERATIONS AND DEFINITIONS, above) the strain is directly +proportional to the stress producing it.[1] The same intensity +of stress, however, does not produce the same strain in +different materials or in different qualities of the same +material. No strain would be produced in a perfectly rigid body, +but such is not known to exist. + +[Footnote 1: This is in accordance with the discovery made in +1678 by Robert Hooke, and is known as _Hooke's law_.] + +Stress is measured in pounds (or other unit of weight or force). +A ~unit stress~ is the stress on a unit of the sectional + { P } +area. { Unit stress = --- } For instance, if a load (P) of one + { A } +hundred pounds is uniformly supported by a vertical post with a +cross-sectional area (A) of ten square inches, the unit +compressive stress is ten pounds per square inch. + +Strain is measured in inches (or other linear unit). A ~unit +strain~ is the strain per unit of length. Thus if a post 10 +inches long before compression is 9.9 inches long under the +compressive stress, the total strain is 0.1 inch, and the unit + l 0.1 +strain is --- = ----- = 0.01 inch per inch of length. + L 10 + +As the stress increases there is a corresponding increase in the +strain. This ratio may be graphically shown by means of a +diagram or curve plotted with the increments of load or stress +as ordinates and the increments of strain as abscissæ. This is +known as the ~stress-strain diagram~. Within the limit mentioned +above the diagram is a straight line. (See Fig. 1.) If the +results of similar experiments on different specimens are +plotted to the same scales, the diagrams furnish a ready means +for comparison. The greater the resistance a material offers to +deformation the steeper or nearer the vertical axis will be the +line. + +[Illustration: FIG. 1.--Stress-strain diagrams of two longleaf +pine beams. E.L. = elastic limit. The areas of the triangles +0(EL)A and 0(EL)B represent the elastic resilience of the dry +and green beams, respectively.] + +There are three kinds of internal stresses, namely, (1) +~tensile~, (2) ~compressive~, and (3) ~shearing~. When external +forces act upon a bar in a direction away from its ends or a +direct pull, the stress is a tensile stress; when toward the +ends or a direct push, compressive stress. In the first instance +the strain is an _elongation_; in the second a _shortening_. +Whenever the forces tend to cause one portion of the material to +slide upon another adjacent to it the action is called a +_shear_. The action is that of an ordinary pair of shears. When +riveted plates slide on each other the rivets are sheared off. + +These three simple stresses may act together, producing compound +stresses, as in flexure. When a bow is bent there is a +compression of the fibres on the inner or concave side and an +elongation of the fibres on the outer or convex side. There is +also a tendency of the various fibres to slide past one another +in a longitudinal direction. If the bow were made of two or more +separate pieces of equal length it would be noted on bending +that slipping occurred along the surfaces of contact, and that +the ends would no longer be even. If these pieces were securely +glued together they would no longer slip, but the tendency to do +so would exist just the same. Moreover, it would be found in the +latter case that the bow would be much harder to bend than where +the pieces were not glued together--in other words, the +_stiffness_ of the bow would be materially increased. + +~Stiffness~ is the property by means of which a body acted upon +by external forces tends to retain its natural size and shape, +or resists deformation. Thus a material that is difficult to +bend or otherwise deform is stiff; one that is easily bent or +otherwise deformed is _flexible_. Flexibility is not the exact +counterpart of stiffness, as it also involves toughness and +pliability. + +If successively larger loads are applied to a body and then +removed it will be found that at first the body completely +regains its original form upon release from the stress--in other +words, the body is ~elastic~. No substance known is perfectly +elastic, though many are practically so under small loads. +Eventually a point will be reached where the recovery of the +specimen is incomplete. This point is known as the ~elastic +limit~, which may be defined as the limit beyond which it is +impossible to carry the distortion of a body without producing a +permanent alteration in shape. After this limit has been +exceeded, the size and shape of the specimen after removal of +the load will not be the same as before, and the difference or +amount of change is known as the ~permanent set~. + +Elastic limit as measured in tests and used in design may be +defined as that unit stress at which the deformation begins to +increase in a faster ratio than the applied load. In practice +the elastic limit of a material under test is determined from +the stress-strain diagram. It is that point in the line where +the diagram begins perceptibly to curve.[2] (See Fig. 1.) + +[Footnote 2: If the straight portion does not pass through the +origin, a parallel line should be drawn through the origin, and +the load at elastic limit taken from this line. (See Fig. 32.)] + +~Resilience~ is the amount of work done upon a body in deforming +it. Within the elastic limit it is also a measure of the +potential energy stored in the material and represents the +amount of work the material would do upon being released from a +state of stress. This may be graphically represented by a +diagram in which the abscissæ represent the amount of deflection +and the ordinates the force acting. The area included between +the stress-strain curve and the initial line (which is zero) +represents the work done. (See Fig. 1.) If the unit of space is +in inches and the unit of force is in pounds the result is +inch-pounds. If the elastic limit is taken as the apex of the +triangle the area of the triangle will represent the ~elastic +resilience~ of the specimen. This amount of work can be applied +repeatedly and is perhaps the best measure of the toughness of +the wood as a working quality, though it is not synonymous with +toughness. + +Permanent set is due to the ~plasticity~ of the material. A +perfectly plastic substance would have no elasticity and the +smallest forces would cause a set. Lead and moist clay are +nearly plastic and wood possesses this property to a greater or +less extent. The plasticity of wood is increased by wetting, +heating, and especially by steaming and boiling. Were it not for +this property it would be impossible to dry wood without +destroying completely its cohesion, due to the irregularity of +shrinkage. + +A substance that can undergo little change in shape without +breaking or rupturing is ~brittle~. Chalk and glass are common +examples of brittle materials. Sometimes the word _brash_ is +used to describe this condition in wood. A brittle wood breaks +suddenly with a clean instead of a splintery fracture and +without warning. Such woods are unfitted to resist shock or +sudden application of load. + +The measure of the stiffness of wood is termed the ~modulus of +elasticity~ (or _coefficient of elasticity_). It is the ratio of +stress per unit of area to the deformation per unit of + { unit stress } +length. { E = ------------- } It is a number indicative of + { unit strain } +stiffness, not of strength, and only applies to conditions +within the elastic limit. It is nearly the same whether derived +from compression tests or from tension tests. + +A large modulus indicates a stiff material. Thus in green wood +tested in static bending it varies from 643,000 pounds per +square inch for arborvitæ to 1,662,000 pounds for longleaf pine, +and 1,769,000 pounds for pignut hickory. (See Table IX.) The +values derived from tests of small beams of dry material are +much greater, approaching 3,000,000 for some of our woods. These +values are small when compared with steel which has a modulus of +elasticity of about 30,000,000 pounds per square inch. (See +Table I.) + +|------------------------------------------------------------------------------| +| TABLE I | +|------------------------------------------------------------------------------| +| COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD | +|------------------------------------------------------------------------------| +| | Sp. | Modulus of | Tensile | Crushing | Modulus | +| MATERIAL | gr., | elasticity | strength | strength | of | +| | dry | in bending | | | rupture | +|-------------------------+----- +------------+----------+----------+----------| +| | | Lbs. per | Lbs. per | Lbs. per | Lbs. per | +| | | sq. in. | sq. in. | sq. in. | sq. in. | +| | | | | | | +| Cast iron, cold blast | | | | | | +| (Hodgkinson) | 7.1 | 17,270,000 | 16,700 | 106,000 | 38,500 | +| Bessenger steel, | | | | | | +| high grade (Fairbain) | 7.8 | 29,215,000 | 88,400 | 225,600 | | +| Longleaf pine, | | | | | | +| 3.5% moisture (U.S.) | .63 | 2,800,000 | | 13,000 | 21,000 | +| Redspruce, | | | | | | +| 3.5% moisture (U.S.) | .41 | 1,800,000 | | 8,800 | 14,500 | +| Pignut hickory, | | | | | | +| 3.5% moisture (U.S.) | .86 | 2,370,000 | | 11,130 | 24,000 | +|------------------------------------------------------------------------------| +| NOTE.--Great variation may be found in different samples of metals as well | +| as of wood. The examples given represent reasonable values. | +|------------------------------------------------------------------------------| + + + +TENSILE STRENGTH + + +~Tension~ results when a pulling force is applied to opposite +ends of a body. This external pull is communicated to the +interior, so that any portion of the material exerts a pull or +tensile force upon the remainder, the ability to do so depending +upon the property of cohesion. The result is an elongation or +stretching of the material in the direction of the applied +force. The action is the opposite of compression. + +Wood exhibits its greatest strength in tension parallel to the +grain, and it is very uncommon in practice for a specimen to be +pulled in two lengthwise. This is due to the difficulty of +making the end fastenings secure enough for the full tensile +strength to be brought into play before the fastenings shear off +longitudinally. This is not the case with metals, and as a +result they are used in almost all places where tensile strength +is particularly needed, even though the remainder of the +structure, such as sills, beams, joists, posts, and flooring, +may be of wood. Thus in a wooden truss bridge the tension +members are steel rods. + +The tensile strength of wood parallel to the grain depends upon +the strength of the fibres and is affected not only by the +nature and dimensions of the wood elements but also by their +arrangement. It is greatest in straight-grained specimens with +thick-walled fibres. Cross grain of any kind materially reduces +the tensile strength of wood, since the tensile strength at +right angles to the grain is only a small fraction of that +parallel to the grain. + +|--------------------------------------------------------------| +| TABLE II | +|--------------------------------------------------------------| +| RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION | +| (Bul. 10, U. S. Div. of Forestry, p. 44) | +|--------------------------------------------------------------| +| | Ratio: | A stick 1 square inch in | +| | | cross section. | +| | Tensile | | +| KIND OF WOOD | strength | Weight required to-- | +| | R = ----------- +----------------------------| +| | compressive | Pull apart | Crush endwise | +| | strength | | | +|---------------+-----------------+------------+---------------| +| Hickory | 3.7 | 32,000 | 8,500 | +| Elm | 3.8 | 29,000 | 7,500 | +| Larch | 2.3 | 19,400 | 8,600 | +| Longleaf Pine | 2.2 | 17,300 | 7,400 | +|--------------------------------------------------------------| +| NOTE.--Moisture condition not given. | +|--------------------------------------------------------------| + +Failure of wood in tension parallel to the grain occurs +sometimes in flexure, especially with dry material. The tension +portion of the fracture is nearly the same as though the piece +were pulled in two lengthwise. The fibre walls are torn across +obliquely and usually in a spiral direction. There is +practically no pulling apart of the fibres, that is, no +separation of the fibres along their walls, regardless of their +thickness. The nature of tension failure is apparently not +affected by the moisture condition of the specimen, at least not +so much so as the other strength values.[3] + +[Footnote 3: See Brush, Warren D.: A microscopic study of the +mechanical failure of wood. Vol. II, Rev. F.S. Investigations, +Washington, D.C., 1912, p. 35.] + +Tension at right angles to the grain is closely related to +cleavability. When wood fails in this manner the thin fibre +walls are torn in two lengthwise while the thick-walled fibres +are usually pulled apart along the primary wall. + +|--------------------------------------------| +| TABLE III | +|--------------------------------------------| +| TENSILE STRENGTH AT RIGHT ANGLES TO THE | +| GRAIN OF SMALL CLEAR PIECES OF 25 WOODS IN | +| GREEN CONDITION | +| (Forest Service Cir. 213) | +|--------------------------------------------| +| | When | When | +| COMMON NAME | surface of | surface of | +| OF SPECIES | failure is | failure is | +| | radial | tangential | +|------------------+------------+------------| +| | Lbs. per | Lbs. per | +| | sq. inch | sq. inch | +| | | | +| Hardwoods | | | +| | | | +| Ash, white | 645 | 671 | +| Basswood | 226 | 303 | +| Beech | 633 | 969 | +| Birch, yellow | 446 | 526 | +| Elm, slippery | 765 | 832 | +| Hackberry | 661 | 786 | +| Locust, honey | 1,133 | 1,445 | +| Maple, sugar | 610 | 864 | +| Oak, post | 714 | 924 | +| red | 639 | 874 | +| swamp white | 757 | 909 | +| white | 622 | 749 | +| yellow | 728 | 929 | +| Sycamore | 540 | 781 | +| Tupelo | 472 | 796 | +| | | | +| Conifers | | | +| | | | +| Arborvitæ | 241 | 235 | +| Cypress, bald | 242 | 251 | +| Fir, white | 213 | 304 | +| Hemlock | 271 | 323 | +| Pine, longleaf | 240 | 298 | +| red | 179 | 205 | +| sugar | 239 | 304 | +| western yellow | 230 | 252 | +| white | 225 | 285 | +| Tamarack | 236 | 274 | +|--------------------------------------------| + + + +COMPRESSIVE OR CRUSHING STRENGTH + + +~Compression across the grain~ is very closely related to +hardness and transverse shear. There are two ways in which wood +is subjected to stress of this kind, namely, (1) with the load +acting over the entire area of the specimen, and (2) with a load +concentrated over a portion of the area. (See Fig. 2.) The +latter is the condition more commonly met with in practice, as, +for example, where a post rests on a horizontal sill, or a rail +rests on a cross-tie. The former condition, however, gives the +true resistance of the grain to simple crushing. + +[Illustration: FIG. 2.--Compression across the grain.] + +The first effect of compression across the grain is to compact +the fibres, the load gradually but irregularly increasing as the +density of the material is increased. If the specimen lies on a +flat surface and the load is applied to only a portion of the +upper area, the bearing plate indents the wood, crushing the +upper fibres without affecting the lower part. (See Fig. 3.) As +the load increases the projecting ends sometimes split +horizontally. (See Fig. 4.) The irregularities in the load are +due to the fact that the fibres collapse a few at a time, +beginning with those with the thinnest walls. The projection of +the ends increases the strength of the material directly beneath +the compressing weight by introducing a beam action which helps +support the load. This influence is exerted for a short distance +only. + +[Illustration: FIG. 3.--Side view of failures in compression +across the grain, showing crushing of blocks under bearing +plate. Specimen at right shows splitting at ends.] + +[Illustration: FIG. 4.--End view of failures in compression +across the grain, showing splitting of the ends of the test +specimens.] + +When wood is used for columns, props, posts, and spokes, the +weight of the load tends to shorten the material endwise. This +is ~endwise compression~, or compression parallel to the grain. +In the case of long columns, that is, pieces in which the length +is very great compared with their diameter, the failure is by +sidewise bending or flexure, instead of by crushing or +splitting. (See Fig. 5.) A familiar instance of this action is +afforded by a flexible walking-stick. If downward pressure is +exerted with the hand on the upper end of the stick placed +vertically on the floor, it will be noted that a definite amount +of force must be applied in each instance before decided flexure +takes place. After this point is reached a very slight increase +of pressure very largely increases the deflection, thus +obtaining so great a leverage about the middle section as to +cause rupture. + +[Illustration: FIG. 5.--Testing a buggy spoke in endwise +compression, illustrating the failure by sidewise bending of a +long column fixed only at the lower end. _Photo by U. S. Forest +Service_] + +The lateral bending of a column produces a combination of +bending with compressive stress over the section, the +compressive stress being maximum at the section of greatest +deflection on the concave side. The convex surface is under +tension, as in an ordinary beam test. (See Fig. 6.) If the same +stick is braced in such a way that flexure is prevented, its +supporting strength is increased enormously, since the +compressive stress acts uniformly over the section, and failure +is by crushing or splitting, as in small blocks. In all columns +free to bend in any direction the deflection will be seen in the +direction in which the column is least stiff. This sidewise +bending can be overcome by making pillars and columns thicker in +the middle than at the ends, and by bracing studding, props, and +compression members of trusses. The strength of a column also +depends to a considerable extent upon whether the ends are free +to turn or are fixed. + +[Illustration: FIG. 6.--Unequal distribution of stress in a long +column due to lateral bending.] + +|-------------------------------------------------------| +| TABLE IV | +|-------------------------------------------------------| +| RESULTS OF COMPRESSION TESTS ACROSS THE GRAIN ON | +| 51 WOODS IN GREEN CONDITION, AND COMPARISON WITH | +| WHITE OAK | +| (U. S. Forest Service) | +|-------------------------------------------------------| +| | Fibre stress | Fiber stress | +| COMMON NAME | at elastic | in per cent | +| OF SPECIES | limit | of white oak, | +| | perpendicular | or 853 pounds | +| | to grain | per sq. in. | +|-----------------------+---------------+---------------| +| | Lbs. per | | +| | sq. inch | Per cent | +| | | | +| Osage orange | 2,260 | 265.0 | +| Honey locust | 1,684 | 197.5 | +| Black locust | 1,426 | 167.2 | +| Post oak | 1,148 | 134.6 | +| Pignut hickory | 1,142 | 133.9 | +| Water hickory | 1,088 | 127.5 | +| Shagbark hickory | 1,070 | 125.5 | +| Mockernut hickory | 1,012 | 118.6 | +| Big shellbark hickory | 997 | 116.9 | +| Bitternut hickory | 986 | 115.7 | +| Nutmeg hickory | 938 | 110.0 | +| Yellow oak | 857 | 100.5 | +| White oak | 853 | 100.0 | +| Bur oak | 836 | 98.0 | +| White ash | 828 | 97.1 | +| Red oak | 778 | 91.2 | +| Sugar maple | 742 | 87.0 | +| Rock elm | 696 | 81.6 | +| Beech | 607 | 71.2 | +| Slippery elm | 599 | 70.2 | +| Redwood | 578 | 67.8 | +| Bald cypress | 548 | 64.3 | +| Red maple | 531 | 62.3 | +| Hackberry | 525 | 61.6 | +| Incense cedar | 518 | 60.8 | +| Hemlock | 497 | 58.3 | +| Longleaf pine | 491 | 57.6 | +| Tamarack | 480 | 56.3 | +| Silver maple | 456 | 53.5 | +| Yellow birch | 454 | 53.2 | +| Tupelo | 451 | 52.9 | +| Black cherry | 444 | 52.1 | +| Sycamore | 433 | 50.8 | +| Douglas fir | 427 | 50.1 | +| Cucumber tree | 408 | 47.8 | +| Shortleaf pine | 400 | 46.9 | +| Red pine | 358 | 42.0 | +| Sugar pine | 353 | 41.1 | +| White elm | 351 | 41.2 | +| Western yellow pine | 348 | 40.8 | +| Lodgepole pine | 348 | 40.8 | +| Red spruce | 345 | 40.5 | +| White pine | 314 | 36.8 | +| Engelman spruce | 290 | 34.0 | +| Arborvitæ | 288 | 33.8 | +| Largetooth aspen | 269 | 31.5 | +| White spruce | 262 | 30.7 | +| Butternut | 258 | 30.3 | +| Buckeye (yellow) | 210 | 24.6 | +| Basswood | 209 | 24.5 | +| Black willow | 193 | 22.6 | +|-------------------------------------------------------| + +The complexity of the computations depends upon the way in which +the stress is applied and the manner in which the stick bends. +Ordinarily where the length of the test specimen is not greater +than four diameters and the ends are squarely faced (see Fig. +7), the force acts uniformly over each square inch of area and +the crushing strength is equal to the maximum load (P) divided + { P } +by the area of the cross-section (A). { C = --- } + { A } + +[Illustration: FIG. 7.--Endwise compression of a short column.] + +It has been demonstrated[4] that the ultimate strength in +compression parallel to the grain is very nearly the same as the +extreme fibre stress at the elastic limit in bending. (See Table +V.) In other words, the transverse strength of beams at elastic +limit is practically equal to the compressive strength of the +same material in short columns. It is accordingly possible to +calculate the approximate breaking strength of beams from the +compressive strength of short columns except when the wood is +brittle. Since tests on endwise compression are simpler, easier +to make, and less expensive than transverse bending tests, the +importance of this relation is obvious, though it does not do +away with the necessity of making beam tests. + +[Footnote 4: See Circular No. 18, U.S. Division of Forestry: +Progress in timber physics, pp. 13-18; also Bulletin 70, U.S. +Forest Service: Effect of moisture on the strength and stiffness +of wood, pp. 42, 89-90.] + +|-------------------------------------------------------------------------------| +| TABLE V | +|-------------------------------------------------------------------------------| +| RELATION OF FIBRE STRESS AT ELASTIC LIMIT (r) IN BENDING TO THE CRUSHING | +| STRENGTH (C) OF BLOCKS CUT THEREFROM, IN POUNDS PER SQUARE INCH | +| (Forest Service Bul. 70, p. 90) | +|-------------------------------------------------------------------------------| +| LONGLEAF PINE | +|-------------------------------------------------------------------------------| +| | Soaked | Green | 14 | 11.5 | 9.5 | Kiln-dry | +| MOISTURE CONDITION | 50 per | 23 per | per | per | per | 6.2 per | +| | cent | cent | cent | cent | cent | cent | +| -------------------------+--------+--------+-------+-------+-------+----------| +| Number of tests averaged | 5 | 5 | 5 | 5 | 4 | 5 | +| _r_ in bending | 4,920 | 5,944 | 6,924 | 7,852 | 9,280 | 11,550 | +| _C_ in compression | 4,668 | 5,100 | 6,466 | 7,466 | 8,985 | 10,910 | +| Per cent _r_ is in | | | | | | | +| excess of _C_ | 5.5 | 16.5 | 7.1 | 5.2 | 3.3 | 5.9 | +|-------------------------------------------------------------------------------| +| SPRUCE | +|-------------------------------------------------------------------------------| +| | Soaked | Green | 10 | 8.1 | Kiln-dry | +| MOISTURE CONDITION | 30 per | 30 per | per | per | 3.9 per | +| | cent | cent | cent | cent | cent | +|----------------------------------+--------+--------+-------+-------+----------| +| Number of tests averaged | 5 | 4 | 5 | 3 | 4 | +| _r_ in bending | 3,002 | 3,362 | 6,458 | 8,400 | 10,170 | +| _C_ in compression | 2,680 | 3,025 | 6,120 | 7,610 | 9,335 | +| Per cent _r_ | | | | | | +| is in excess of _C_ | 12.0 | 11.1 | 5.5 | 10.4 | 9.0 | +|-------------------------------------------------------------------------------| + +When a short column is compressed until it breaks, the manner of +failure depends partly upon the anatomical structure and partly +upon the degree of humidity of the wood. The fibres (tracheids +in conifers) act as hollow tubes bound closely together, and in +giving way they either (1) buckle, or (2) bend.[5] + +[Footnote 5: See Bulletin 70, _op. cit._, p. 129.] + +The first is typical of any dry thin-walled cells, as is usually +the case in seasoned white pine and spruce, and in the early +wood of hard pines, hemlock, and other species with decided +contrast between the two portions of the growth ring. As a rule +buckling of a tracheid begins at the bordered pits which form +places of least resistance in the walls. In hardwoods such as +oak, chestnut, ash, etc., buckling occurs only in the +thinnest-walled elements, such as the vessels, and not in the +true fibres. + +According to Jaccard[6] the folding of the cells is accompanied +by characteristic alterations of their walls which seem to split +them into extremely thin layers. When greatly magnified, these +layers appear in longitudinal sections as delicate threads +without any definite arrangements, while on cross section they +appear as numerous concentric strata. This may be explained on +the ground that the growth of a fibre is by successive layers +which, under the influence of compression, are sheared apart. +This is particularly the case with thick-walled cells such as +are found in late wood. + +[Footnote 6: Jaccard, P.: Étude anatomique des bois comprimés. +Mit. d. Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band, +1. Heft. Zurich, 1910, p. 66.] + +|-------------------------------------------------------| +| TABLE VI | +|-------------------------------------------------------| +| RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR | +| PIECES OF 40 WOODS IN GREEN CONDITION | +| (Forest Service Cir. 213) | +|-------------------------------------------------------| +| | Fibre | | Modulus | +| COMMON NAME | stress at | Crushing | of | +| OF SPECIES | elastic | strength | elasticity | +| | limit | | | +|-------------------+-----------+----------+------------| +| | Lbs. per | Lbs. per | Lbs. per | +| | sq. inch | sq. inch | sq. inch | +| | | | | +| Hardwoods | | | | +| | | | | +| Ash, white | 3,510 | 4,220 | 1,531,000 | +| Basswood | 780 | 1,820 | 1,016,000 | +| Beech | 2,770 | 3,480 | 1,412,000 | +| Birch, yellow | 2,570 | 3,400 | 1,915,000 | +| Elm, slippery | 3,410 | 3,990 | 1,453,000 | +| Hackberry | 2,730 | 3,310 | 1,068,000 | +| Hickory, | | | | +| big shellbark | 3,570 | 4,520 | 1,658,000 | +| bitternut | 4,330 | 4,570 | 1,616,000 | +| mockernut | 3,990 | 4,320 | 1,359,000 | +| nutmeg | 3,620 | 3,980 | 1,411,000 | +| pignut | 3,520 | 4,820 | 1,980,000 | +| shagbark | 3,730 | 4,600 | 1,943,000 | +| water | 3,240 | 4,660 | 1,926,000 | +| Locust, honey | 4,300 | 4,970 | 1,536,000 | +| Maple, sugar | 3,040 | 3,670 | 1,463,000 | +| Oak, post | 2,780 | 3,330 | 1,062,000 | +| red | 2,290 | 3,210 | 1,295,000 | +| swamp white | 3,470 | 4,360 | 1,489,000 | +| white | 2,400 | 3,520 | 946,000 | +| yellow | 2,870 | 3,700 | 1,465,000 | +| Osage orange | 3,980 | 5,810 | 1,331,000 | +| Sycamore | 2,320 | 2,790 | 1,073,000 | +| Tupelo | 2,280 | 3,550 | 1,280,000 | +| | | | | +| Conifers | | | | +| | | | | +| Arborvitæ | 1,420 | 1,990 | 754,000 | +| Cedar, incense | 2,710 | 3,030 | 868,000 | +| Cypress, bald | 3,560 | 3,960 | 1,738,000 | +| Fir, alpine | 1,660 | 2,060 | 882,000 | +| amabilis | 2,763 | 3,040 | 1,579,000 | +| Douglas | 2,390 | 2,920 | 1,440,000 | +| white | 2,610 | 2,800 | 1,332,000 | +| Hemlock | 2,110 | 2,750 | 1,054,000 | +| Pine, lodgepole | 2,290 | 2,530 | 1,219,000 | +| longleaf | 3,420 | 4,280 | 1,890,000 | +| red | 2,470 | 3,080 | 1,646,000 | +| sugar | 2,340 | 2,600 | 1,029,000 | +| western yellow | 2,100 | 2,420 | 1,271,000 | +| white | 2,370 | 2,720 | 1,318,000 | +| Redwood | 3,420 | 3,820 | 1,175,000 | +| Spruce, Engelmann | 1,880 | 2,170 | 1,021,000 | +| Tamarack | 3,010 | 3,480 | 1,596,000 | +|-------------------------------------------------------| + +The second case, where the fibres bend with more or less regular +curves instead of buckling, is characteristic of any green or +wet wood, and in dry woods where the fibres are thick-walled. In +woods in which the fibre walls show all gradations of +thickness--in other words, where the transition from the +thin-walled cells of the early wood to the thick-walled cells of +the late wood is gradual--the two kinds of failure, namely, +buckling and bending, grade into each other. In woods with very +decided contrast between early and late wood the two forms are +usually distinct. Except in the case of complete failure the +cavity of the deformed cells remains open, and in hardwoods this +is true not only of the wood fibres but also of the tube-like +vessels. In many cases longitudinal splits occur which isolate +bundles of elements by greater or less intervals. The splitting +occurs by a tearing of the fibres or rays and not by the +separation of the rays from the adjacent elements. + +[Illustration: FIG. 8.--Failures of short columns of green +spruce.] + +[Illustration: FIG. 9.--Failures of short columns of dry +chestnut.] + +Moisture in wood decreases the stiffness of the fibre walls and +enlarges the region of failure. The curve which the fibre walls +make in the region of failure is more gradual and also more +irregular than in dry wood, and the fibres are more likely to be +separated. + +In examining the lines of rupture in compression parallel to the +grain it appears that there does not exist any specific type, +that is, one that is characteristic of all woods. Test blocks +taken from different parts of the same log may show very decided +differences in the manner of failure, while blocks that are much +alike in the size, number, and distribution of the elements of +unequal resistance may behave very similarly. The direction of +rupture is, according to Jaccard, not influenced by the +distribution of the medullary rays.[7] These are curved with the +bundles of fibres to which they are attached. In any case the +failure starts at the weakest points and follows the lines of +least resistance. The plane of failure, as visible on radial +surfaces, is horizontal, and on the tangential surface it is +diagonal. + +[Footnote 7: This does not correspond exactly with the +conclusions of A. Thil, who says ("Constitution anatomique du +bois," pp. 140-141): "The sides of the medullary rays sometimes +produce planes of least resistance varying in size with the +height of the rays. The medullary rays assume a direction more +or less parallel to the lumen of the cells on which they border; +the latter curve to the right or left to make room for the ray +and then close again beyond it. If the force acts parallel to +the axis of growth, the tracheids are more likely to be +displaced if the marginal cells of the medullary rays are +provided with weak walls that are readily compressed. This +explains why on the radial surface of the test blocks the plane +of rupture passes in a direction nearly following a medullary +ray, whereas on the tangential surface the direction of the +plane of rupture is oblique--but with an obliquity varying with +the species and determined by the pitch of the spirals along +which the medullary rays are distributed in the stem." See +Jaccard, _op. cit._, pp. 57 _et seq._] + + + +SHEARING STRENGTH + + +Whenever forces act upon a body in such a way that one portion +tends to slide upon another adjacent to it the action is called +a ~shear~.[8] In wood this shearing action may be (1) ~along the +grain~, or (2) ~across the grain~. A tenon breaking out its +mortise is a familiar example of shear along the grain, while +the shoving off of the tenon itself would be shear across the +grain. The use of wood for pins or tree-nails involves +resistance to shear across the grain. Another common instance of +the latter is where the steel edge of the eye of an axe or +hammer tends to cut off the handle. In Fig. 10 the action of the +wooden strut tends to shear off along the grain the portion _AB_ +of the wooden tie rod, and it is essential that the length of +this portion be great enough to guard against it. Fig. 11 shows +characteristic failures in shear along the grain. + +[Footnote 8: Shear should not be confused with ordinary cutting +or incision.] + +[Illustration: FIG. 10.--Example of shear along the grain.] + +[Illustration: FIG. 11.--Failures of test specimens in shear +along the grain. In the block at the left the surface of failure +is radial; in the one at the right, tangential] + +|---------------------------------------------| +| TABLE VII | +|---------------------------------------------| +| SHEARING STRENGTH ALONG THE GRAIN OF SMALL | +| CLEAR PIECES OF 41 WOODS IN GREEN CONDITION | +| (Forest Service Cir. 213) | +|---------------------------------------------| +| | When | When | +| COMMON NAME | surface of | surface of | +| OF SPECIES | failure is | failure is | +| | radial | tangential | +|-------------------+------------+------------| +| | Lbs. per | Lbs. per | +| | sq. inch | sq. inch | +| | | | +| Hardwoods | | | +| | | | +| Ash, black | 876 | 832 | +| white | 1,360 | 1,312 | +| Basswood | 560 | 617 | +| Beech | 1,154 | 1,375 | +| Birch, yellow | 1,103 | 1,188 | +| Elm, slippery | 1,197 | 1,174 | +| white | 778 | 872 | +| Hackberry | 1,095 | 1,161 | +| Hickory, | | | +| big shellbark | 1,134 | 1,191 | +| bitternut | 1,134 | 1,348 | +| mockernut | 1,251 | 1,313 | +| nutmeg | 1,010 | 1,053 | +| pignut | 1,334 | 1,457 | +| shagbark | 1,230 | 1,297 | +| water | 1,390 | 1,490 | +| Locust, honey | 1,885 | 2,096 | +| Maple, red | 1,130 | 1,330 | +| sugar | 1,193 | 1,455 | +| Oak, post | 1,196 | 1,402 | +| red | 1,132 | 1,195 | +| swamp white | 1,198 | 1,394 | +| white | 1,096 | 1,292 | +| yellow | 1,162 | 1,196 | +| Sycamore | 900 | 1,102 | +| Tupelo | 978 | 1,084 | +| | | | +| Conifers | | | +| | | | +| Arborvitæ | 617 | 614 | +| Cedar, incense | 613 | 662 | +| Cypress, bald | 836 | 800 | +| Fir, alpine | 573 | 654 | +| amabilis | 517 | 639 | +| Douglas | 853 | 858 | +| white | 742 | 723 | +| Hemlock | 790 | 813 | +| Pine, lodgepole | 672 | 747 | +| longleaf | 1,060 | 953 | +| red | 812 | 741 | +| sugar | 702 | 714 | +| western yellow | 686 | 706 | +| white | 649 | 639 | +| Spruce, Engelmann | 607 | 624 | +| Tamarack | 883 | 843 | +|---------------------------------------------| + +Both shearing stresses may act at the same time. Thus the weight +carried by a beam tends to shear it off at right angles to the +axis; this stress is equal to the resultant force acting +perpendicularly at any point, and in a beam uniformly loaded and +supported at either end is maximum at the points of support and +zero at the centre. In addition there is a shearing force +tending to move the fibres of the beam past each other in a +longitudinal direction. (See Fig. 12.) This longitudinal shear +is maximum at the neutral plane and decreases toward the upper +and lower surfaces. + +[Illustration: FIG. 12.--Horizontal shear in a beam.] + +Shearing across the grain is so closely related to compression +at right angles to the grain and to hardness that there is +little to be gained by making separate tests upon it. Knowledge +of shear parallel to the grain is important, since wood +frequently fails in that way. The value of shearing stress +parallel to the grain is found by dividing the maximum load in +pounds (P) by the area of the cross section in inches (A). + + { P } + { Shear = --- } + { A } + +Oblique shearing stresses are developed in a bar when it is +subjected to direct tension or compression. The maximum shearing +stress occurs along a plane when it makes an angle of 45 degrees + P +with the axis of the specimen. In this case, shear = -----. When + 2 A +the value of the angle [Greek: theta] is less than 45 degrees, + P +the shear along the plane = --- sin [Greek: theta] cos [Greek: + A +theta]. (See Fig. 13.) The effect of oblique shear is often +visible in the failures of short columns. (See Fig. 14.) + +[Illustration: FIG. 13.--Oblique shear in a short column.] + +[Illustration: FIG. 14.--Failure of short column by oblique +shear.] + +|---------------------------------------------------------------------------| +| TABLE VIII | +|---------------------------------------------------------------------------| +| SHEARING STRENGTH ACROSS THE GRAIN OF VARIOUS AMERICAN WOODS | +| (J.C. Trautwine. Jour. Franklin Institute. Vol. 109, 1880, pp. 105-106) | +|---------------------------------------------------------------------------| +| KIND OF WOOD | Lbs. per | KIND OF WOOD | Lbs. per | +| | sq. inch | | sq. inch | +|-----------------------+----------+-----------------------------+----------| +| Ash | 6,280 | Hickory | 7,285 | +| Beech | 5,223 | Locust | 7,176 | +| Birch | 5,595 | Maple | 6,355 | +| Cedar (white) | 1,372 | Oak | 4,425 | +| Cedar (white) | 1,519 | Oak (live) | 8,480 | +| Cedar (Central Amer.) | 3,410 | Pine (white) | 2,480 | +| Cherry | 2,945 | Pine (northern yellow) | 4,340 | +| Chestnut | 1,536 | Pine (southernyellow) | 5,735 | +| Dogwood | 6,510 | Pine (very resinous yellow) | 5,053 | +| Ebony | 7,750 | Poplar | 4,418 | +| Gum | 5,890 | Spruce | 3,255 | +| Hemlock | 2,750 | Walnut (black) | 4,728 | +| Hickory | 6,045 | Walnut (common) | 2,830 | +|---------------------------------------------------------------------------| +| NOTE.--Two specimens of each were tested. All were fairly seasoned and | +| without defects. The piece sheared off was 5/8 in. The single circular | +| area of each pin was 0.322 sq. in. | +|---------------------------------------------------------------------------| + + + +TRANSVERSE OR BENDING STRENGTH: BEAMS + + +When external forces acting in the same plane are applied at +right angles to the axis of a bar so as to cause it to bend, +they occasion a shortening of the longitudinal fibres on the +concave side and an elongation of those on the convex side. +Within the elastic limit the relative stretching and contraction +of the fibres is directly[9] proportional to their distances +from a plane intermediate between them--the ~neutral plane~. +(N_{1} P in Fig. 15.) Thus the fibres half-way between the +neutral plane and the outer surface experience only half as much +shortening or elongation as the outermost or extreme fibres. +Similarly for other distances. The elements along the neutral +plane experience no tension or compression in an axial +direction. The line of intersection of this plane and the plane +of section is known as the ~neutral axis~ (N A in Fig. 15) of +the section. + +[Footnote 9: While in reality this relationship does not exactly +hold, the formulæ for beams are based on its assumption.] + +[Illustration: FIG. 15.--Diagram of a simple beam. N_{1} P = +neutral plane, N A = neutral axis of section R S.] + +If the bar is symmetrical and homogeneous the neutral plane is +located half-way between the upper and lower surfaces, so long +as the deflection does not exceed the elastic limit of the +material. Owing to the fact that the tensile strength of wood is +from two to nearly four times the compressive strength, it +follows that at rupture the neutral plane is much nearer the +convex than the concave side of the bar or beam, since the sum +of all the compressive stresses on the concave portion must +always equal the sum of the tensile stresses on the convex +portion. The neutral plane begins to change from its central +position as soon as the elastic limit has been passed. Its +location at any time is very uncertain. + +The external forces acting to bend the bar also tend to rupture +it at right angles to the neutral plane by causing one +transverse section to slip past another. This stress at any +point is equal to the resultant perpendicular to the axis of the +forces acting at this point, and is termed the ~transverse +shear~ (or in the case of beams, ~vertical shear~). + +In addition to this there is a shearing stress, tending to move +the fibres past one another in an axial direction, which is +called ~longitudinal shear~ (or in the case of beams, +~horizontal shear~). This stress must be taken into +consideration in the design of timber structures. It is maximum +at the neutral plane and decreases to zero at the outer elements +of the section. The shorter the span of a beam in proportion to +its height, the greater is the liability of failure in +horizontal shear before the ultimate strength of the beam is +reached. + + +_Beams_ + +There are three common forms of beams, as follows: + +(1) ~Simple beam~--a bar resting upon two supports, one near +each end. (See Fig. 16, No. 1.) + +(2) ~Cantilever beam~--a bar resting upon one support or +fulcrum, or that portion of any beam projecting out of a wall or +beyond a support. (See Fig. 16, No. 2.) + +(3) ~Continuous beam~--a bar resting upon more than two +supports. (See Fig. 16, No. 3.) + +[Illustration: FIG. 16.--Three common forms of beams. 1. Simple. +2. Cantilever. 3. Continuous.] + + +_Stiffness of Beams_ + +The two main requirements of a beam are stiffness and strength. +The formulæ for the _modulus of elasticity (E)_ or measure of +stiffness of a rectangular prismatic simple beam loaded at the +centre and resting freely on supports at either end is:[10] + +[Footnote 10: Only this form of beam is considered since it is +the simplest. For cantilever and continuous beams, and beams +rigidly fixed at one or both ends, as well as for different +methods of loading, different forms of cross section, etc., +other formulæ are required. See any book on mechanics.] + + P' l^{3} + E = ------------- + 4 D b h^{3} + + b = breadth or width of beam, inches. + h = height or depth of beam, inches. + l = span (length between points of supports) of beam, inches. + D = deflection produced by load P', inches. + P' = load at or below elastic limit, pounds. + +From this formulæ it is evident that for rectangular beams of +the same material, mode of support, and loading, the deflection +is affected as follows: + +(1) It is inversely proportional to the width for beams of the +same length and depth. If the width is tripled the deflection is +one-third as great. + +(2) It is inversely proportional to the cube of the depth for +beams of the same length and breadth. If the depth is tripled +the deflection is one twenty-seventh as great. + +(3) It is directly proportional to the cube of the span for +beams of the same breadth and depth. Tripling the span gives +twenty-seven times the deflection. + +The number of pounds which concentrated at the centre will +deflect a rectangular prismatic simple beam one inch may be +found from the preceding formulæ by substituting D = 1" and +solving for P'. The formulæ then becomes: + + 4 E b h^{3} + Necessary weight (P') = ------------- + l^{3} + +In this case the values for E are read from tables prepared from +data obtained by experimentation on the given material. + + +_Strength of Beams_ + +The measure of the breaking strength of a beam is expressed in +terms of unit stress by a _modulus of rupture_, which is a +purely hypothetical expression for points beyond the elastic +limit. The formulæ used in computing this modulus is as follows: + + 1.5 P l + R = --------- + b h{^2} + + b, h, l = breadth, height, and span, respectively, as in + preceding formulæ. + R = modulus of rupture, pounds per square inch. + P = maximum load, pounds. + +In calculating the fibre stress at the elastic limit the same +formulæ is used except that the load at elastic limit (P_{1}) is +substituted for the maximum load (P). + +From this formulæ it is evident that for rectangular prismatic +beams of the same material, mode of support, and loading, the +load which a given beam can support varies as follows: + +(1) It is directly proportional to the breadth for beams of the +same length and depth, as is the case with stiffness. + +(2) It is directly proportional to the square of the height for +beams of the same length and breadth, instead of as the cube of +this dimension as in stiffness. + +(3) It is inversely proportional to the span for beams of the +same breadth and depth and not to the cube of this dimension as +in stiffness. + +The fact that the strength varies as the _square_ of the height +and the stiffness as the _cube_ explains the relationship of +bending to thickness. Were the law the same for strength and +stiffness a thin piece of material such as a sheet of paper +could not be bent any further without breaking than a thick +piece, say an inch board. +|-------------------------------------------------------------------------------------| +| TABLE IX | +|-------------------------------------------------------------------------------------| +| RESULTS OF STATIC BENDING TESTS ON SMALL CLEAR BEAMS OF 49 WOODS IN GREEN CONDITION | +| (Forest Service Cir. 213) | +|-------------------------------------------------------------------------------------| +| | Fibre | | | Work in Bending | +| COMMON NAME | stress at | Modulus | Modulus |-------------------------------| +| OF SPECIES | elastic | of | of | To | To | | +| | limit | rupture | elasticity | elastic | maximum | Total | +| | | | | limit | load | | +|-----------------+-----------+----------+------------+----------+----------+---------| +| | | | | In.-lbs. | In.-lbs. | In.-lb. | +| | Lbs. per | Lbs. per | Lbs. per | per cu. | per cu. | per | +| | sq. in. | sq. in. | sq. in. | inch | inch | inch | +| | | | | | | | +| Hardwoods | | | | | | | +| | | | | | | | +| Ash, black | 2,580 | 6,000 | 960,000 | 0.41 | 13.1 | 38.9 | +| white | 5,180 | 9,920 | 1,416,000 | 1.10 | 20.0 | 43.7 | +| Basswood | 2,480 | 4,450 | 842,000 | .45 | 5.8 | 8.9 | +| Beech | 4,490 | 8,610 | 1,353,000 | .96 | 14.1 | 31.4 | +| Birch, yellow | 4,190 | 8,390 | 1,597,000 | .62 | 14.2 | 31.5 | +| Elm, rock | 4,290 | 9,430 | 1,222,000 | .90 | 19.4 | 47.4 | +| slippery | 5,560 | 9,510 | 1,314,000 | 1.32 | 11.7 | 44.2 | +| white | 2,850 | 6,940 | 1,052,000 | .44 | 11.8 | 27.4 | +| Gum, red | 3,460 | 6,450 | 1,138,000 | | | | +| Hackberry | 3,320 | 7,800 | 1,170,000 | .56 | 19.6 | 52.9 | +| Hickory, | | | | | | | +| big shellbark | 6,370 | 11,110 | 1,562,000 | 1.47 | 24.3 | 78.0 | +| bitternut | 5,470 | 10,280 | 1,399,000 | 1.22 | 20.0 | 75.5 | +| mockernut | 6,550 | 11,110 | 1,508,000 | 1.50 | 31.7 | 84.4 | +| nutmeg | 4,860 | 9,060 | 1,289,000 | 1.06 | 22.8 | 58.2 | +| pignut | 5,860 | 11,810 | 1,769,000 | 1.12 | 30.6 | 86.7 | +| shagbark | 6,120 | 11,000 | 1,752,000 | 1.22 | 18.3 | 72.3 | +| water | 5,980 | 10,740 | 1,563,000 | 1.29 | 18.8 | 52.9 | +| Locust, honey | 6,020 | 12,360 | 1,732,000 | 1.28 | 17.3 | 64.4 | +| Maple, red | 4,450 | 8,310 | 1,445,000 | .78 | 9.8 | 17.1 | +| sugar | 4,630 | 8,860 | 1,462,000 | .88 | 12.7 | 32.0 | +| Oak, post | 4,720 | 7,380 | 913,000 | 1.39 | 9.1 | 17.4 | +| red | 3,490 | 7,780 | 1,268,000 | .60 | 11.4 | 26.0 | +| swamp white | 5,380 | 9,860 | 1,593,000 | 1.05 | 14.5 | 37.6 | +| tanbark | 6,580 | 10,710 | 1,678,000 | 1.49 | | | +| white | 4,320 | 8,090 | 1,137,000 | .95 | 12.1 | 36.7 | +| yellow | 5,060 | 8,570 | 1,219,000 | 1.20 | 11.7 | 30.7 | +| Osage orange | 7,760 | 13,660 | 1,329,000 | 2.53 | 37.9 | 101.7 | +| Sycamore | 2,820 | 6,300 | 961,000 | .51 | 7.1 | 13.6 | +| Tupelo | 4,300 | 7,380 | 1,045,000 | 1.00 | 7.8 | 20.9 | +| | | | | | | | +| Conifers | | | | | | | +| | | | | | | | +| Arborvitæ | 2,600 | 4,250 | 643,000 | .60 | 5.7 | 9.5 | +| Cedar, incense | 3,950 | 6,040 | 754,000 | | | | +| Cypress, bald | 4,430 | 7,110 | 1,378,000 | .96 | 5.1 | 15.4 | +| Fir, alpine | 2,366 | 4,450 | 861,000 | .66 | 4.4 | 7.4 | +| amabilis | 4,060 | 6,570 | 1,323,000 | | | | +| Douglas | 3,570 | 6,340 | 1,242,000 | .59 | 6.6 | 13.6 | +| white | 3,880 | 5,970 | 1,131,000 | .77 | 5.2 | 14.9 | +| Hemlock | 3,410 | 5,770 | 917,000 | .73 | 6.6 | 12.9 | +| Pine, lodgepole | 3,080 | 5,130 | 1,015,000 | .54 | 5.1 | 7.4 | +| longleaf | 5,090 | 8,630 | 1,662,000 | .88 | 8.1 | 34.8 | +| red | 3,740 | 6,430 | 1,384,000 | .59 | 5.8 | 28.0 | +| shortleaf | 4,360 | 7,710 | 1,395,000 | | | | +| sugar | 3,330 | 5,270 | 966,000 | .66 | 5.0 | 11.6 | +| west, yellow | 3,180 | 5,180 | 1,111,000 | .52 | 4.3 | 15.6 | +| White | 3,410 | 5,310 | 1,073,000 | .62 | 5.9 | 13.3 | +| Redwood | 4,530 | 6,560 | 1,024,000 | | | | +| Spruce, | | | | | | | +| Engelmann | 2,740 | 4,550 | 866,000 | .50 | 4.8 | 6.1 | +| red | 3,440 | 5,820 | 1,143,000 | .62 | 6.0 | | +| white | 3,160 | 5,200 | 968,000 | .58 | 6.6 | | +| Tamarack | 4,200 | 7,170 | 1,236,000 | .84 | 7.2 | 30.0 | +|-------------------------------------------------------------------------------------| + + +_Kinds of Loads_ + +There are various ways in which beams are loaded, of which the +following are the most important: + +(1) ~Uniform load~ occurs where the load is spread evenly over +the beam. + +(2) ~Concentrated load~ occurs where the load is applied at +single point or points. + +(3) ~Live~ or ~immediate load~ is one of momentary or short +duration at any one point, such as occurs in crossing a bridge. + +(4) ~Dead~ or ~permanent load~ is one of constant and +indeterminate duration, as books on a shelf. In the case of a +bridge the weight of the structure itself is the dead load. All +large beams support a uniform dead load consisting of their own +weight. + +The effect of dead load on a wooden beam may be two or more +times that produced by an immediate load of the same weight. +Loads greater than the elastic limit are unsafe and will +generally result in rupture if continued long enough. A beam may +be considered safe under permanent load when the deflections +diminish during equal successive periods of time. A continual +increase in deflection indicates an unsafe load which is almost +certain to rupture the beam eventually. + +Variations in the humidity of the surrounding air influence the +deflection of dry wood under dead load, and increased +deflections during damp weather are cumulative and not recovered +by subsequent drying. In the case of longleaf pine, dry beams +may with safety be loaded permanently to within three-fourths of +their elastic limit as determined from ordinary static tests. +Increased moisture content, due to greater humidity of the air, +lowers the elastic limit of wood so that what was a safe load +for the dry material may become unsafe. + +When a dead load not great enough to rupture a beam has been +removed, the beam tends gradually to recover its former shape, +but the recovery is not always complete. If specimens from such +a beam are tested in the ordinary testing machine it will be +found that the application of the dead load did not affect the +stiffness, ultimate strength, or elastic limit of the material. +In other words, the deflections and recoveries produced by live +loads are the same as would have been produced had not the beam +previously been subjected to a dead load.[11] + +[Footnote 11: See Tiemann, Harry D.: Some results of dead load +bending tests of timber by means of a recording deflectometer. +Proc. Am. Soc. for Testing Materials. Phila. Vol. IX, 1909, pp. +534-548.] + +~Maximum load~ is the greatest load a material will support and +is usually greater than the load at rupture. + +~Safe load~ is the load considered safe for a material to +support in actual practice. It is always less than the load at +elastic limit and is usually taken as a certain proportion of +the ultimate or breaking load. + +The ratio of the breaking to the safe load is called the factor +of safety. (Factor of safety = ultimate strength / safe load) In +order to make due allowance for the natural variations and +imperfections in wood and in the aggregate structure, as well as +for variations in the load, the factor of safety is usually as +high as 6 or 10, especially if the safety of human life depends +upon the structure. This means that only from one-sixth to +one-tenth of the computed strength values is considered safe to +use. If the depth of timbers exceeds four times their thickness +there is a great tendency for the material to twist when loaded. +It is to overcome this tendency that floor joists are braced at +frequent intervals. Short deep pieces shear out or split before +their strength in bending can fully come into play. + + +_Application of Loads_ + +There are three[12] general methods in which loads may be +applied to beams, namely: + +[Footnote 12: A fourth might be added, namely, ~vibratory~, or +~harmonic repetition~, which is frequently serious in the case +of bridges.] + +(1) ~Static loading~ or the gradual imposition of load so that +the moving parts acquire no appreciable momentum. Loads are so +applied in the ordinary testing machine. + +(2) ~Sudden imposition of load without initial velocity.~ "Thus +in the case of placing a load on a beam, if the load be brought +into contact with the beam, but its weight sustained by external +means, as by a cord, and then this external support be +_suddenly_ (instantaneously) removed, as by quickly cutting the +cord, then, although the load is already touching the beam (and +hence there is no real impact), yet the beam is at first +offering no resistance, as it has yet suffered no deformation. +Furthermore, as the beam deflects the resistance increases, but +does not come to be equal to the load until it has attained its +normal deflection. In the meantime there has been an unbalanced +force of gravity acting, of a constantly diminishing amount, +equal at first to the entire load, at the normal deflection. But +at this instant the load and the beam are in motion, the +hitherto unbalanced force having produced an accelerated +velocity, and this velocity of the weight and beam gives to them +an energy, or _vis viva_, which must now spend itself in +overcoming an _excess_ of resistance over and above the imposed +load, and the whole mass will not stop until the deflection (as +well as the resistance) has come to be equal to _twice_ that +corresponding to the static load imposed. Hence we say the +effect of a suddenly imposed load is to produce twice the +deflection and stress of the same load statically applied. It +must be evident, however, that this case has nothing in common +with either the ordinary 'static' tests of structural materials +in testing-machines, or with impact tests."[13] + +[Footnote 13: Johnson, J.B.: The materials of construction, pp. +81-82.] + +(3) ~Impact, shock,~ or ~blow.~[14] There are various common +uses of wood where the material is subjected to sudden shocks +and jars or impact. Such is the action on the felloes and spokes +of a wagon wheel passing over a rough road; on a hammer handle +when a blow is struck; on a maul when it strikes a wedge. + +[Footnote 14: See Tiemann, Harry D.: The theory of impact and +its application to testing materials. Jour. Franklin Inst., +Oct., Nov., 1909, pp. 235-259, 336-364.] + +Resistance to impact is resistance to energy which is measured +by the product of the force into the space through which it +moves, or by the product of one-half the moving mass which +causes the shock into the square of its velocity. The work done +upon the piece at the instant the velocity is entirely removed +from the striking body is equal to the total energy of that +body. It is impossible, however, to get all of the energy of the +striking body stored in the specimen, though the greater the +mass and the shorter the space through which it moves, or, in +other words, the greater the proportion of weight and the +smaller the proportion of velocity making up the energy of the +striking body, the more energy the specimen will absorb. The +rest is lost in friction, vibrations, heat, and motion of the +anvil. + +In impact the stresses produced become very complex and +difficult to measure, especially if the velocity is high, or the +mass of the beam itself is large compared to that of the weight. + +The difficulties attending the measurement of the stresses +beyond the elastic limit are so great that commonly they are not +reckoned. Within the elastic limit the formulæ for calculating +the stresses are based on the assumption that the deflection is +proportional to the stress in this case as in static tests. + +A common method of making tests upon the resistance of wood to +shock is to support a small beam at the ends and drop a heavy +weight upon it in the middle. (See Fig. 40.) The height of the +weight is increased after each drop and records of the +deflection taken until failure. The total work done upon the +specimen is equal to the area of the stress-strain diagram plus +the effect of local inertia of the molecules at point of +contact. + +The stresses involved in impact are complicated by the fact that +there are various ways in which the energy of the striking body +may be spent: + +(_a_) It produces a local deformation of both bodies at the +surface of contact, within or beyond the elastic limit. In +testing wood the compression of the substance of the steel +striking-weight may be neglected, since the steel is very hard +in comparison with the wood. In addition to the compression of +the fibres at the surface of contact resistance is also offered +by the inertia of the particles there, the combined effect of +which is a stress at the surface of contact often entirely out +of proportion to the compression which would result from the +action of a static force of the same magnitude. It frequently +exceeds the crushing strength at the extreme surface of contact, +as in the case of the swaging action of a hammer on the head of +an iron spike, or of a locomotive wheel on the steel rail. This +is also the case when a bullet is shot through a board or a pane +of glass without breaking it as a whole. + +(_b_) It may move the struck body as a whole with an accelerated +velocity, the resistance consisting of the inertia of the body. +This effect is seen when a croquet ball is struck with a mallet. + +(_c_) It may deform a fixed body against its external supports +and resistances. In making impact tests in the laboratory the +test specimen is in reality in the nature of a cushion between +two impacting bodies, namely, the striking weight and the base +of the machine. It is important that the mass of this base be +sufficiently great that its relative velocity to that of the +common centre of gravity of itself and the striking weight may +be disregarded. + +(_d_) It may deform the struck body as a whole against the +resisting stresses developed by its own inertia, as, for +example, when a baseball bat is broken by striking the ball. + +|-------------------------------------------------------| +| TABLE X | +|-------------------------------------------------------| +| RESULTS OF IMPACT BENDING TESTS ON SMALL CLEAR BEAMS | +| OF 34 WOODS IN GREEN CONDITION | +| (Forest Service Cir. 213) | +|-------------------------------------------------------| +| | Fibre | | Work in | +| COMMON NAME | stress at | Modulus of | bending | +| OF SPECIES | elastic | elasticity | to | +| | limit | | elastic | +| | | | limit | +|-------------------+-----------+------------+----------| +| | | | In.-lbs. | +| | Lbs. per | Lbs. per | per cu. | +| | sq. in. | sq. in. | inch | +| | | | | +| Hardwoods | | | | +| | | | | +| Ash, black | 7,840 | 955,000 | 3.69 | +| white | 11,710 | 1,564,000 | 4.93 | +| Basswood | 5,480 | 917,000 | 1.84 | +| Beech | 11,760 | 1,501,000 | 5.10 | +| Birch, yellow | 11,080 | 1,812,000 | 3.79 | +| Elm, rock | 12,090 | 1,367,000 | 6.52 | +| slippery | 11,700 | 1,569,000 | 4.86 | +| white | 9,910 | 1,138,000 | 4.82 | +| Hackberry | 10,420 | 1,398,000 | 4.48 | +| Locust, honey | 13,460 | 2,114,000 | 4.76 | +| Maple, red | 11,670 | 1,411,000 | 5.45 | +| sugar | 11,680 | 1,680,000 | 4.55 | +| Oak, post | 11,260 | 1,596,000 | 4.41 | +| red | 10,580 | 1,506,000 | 4.16 | +| swamp white | 13,280 | 2,048,000 | 4.79 | +| white | 9,860 | 1,414,000 | 3.84 | +| yellow | 10,840 | 1,479,000 | 4.44 | +| Osage orange | 15,520 | 1,498,000 | 8.92 | +| Sycamore | 8,180 | 1,165,000 | 3.22 | +| Tupelo | 7,650 | 1,310,000 | 2.49 | +| | | | | +| Conifers | | | | +| | | | | +| Arborvitæ | 5,290 | 778,000 | 2.04 | +| Cypress, bald | 8,290 | 1,431,000 | 2.71 | +| Fir, alpine | 5,280 | 980,000 | 1.59 | +| Douglas | 8,870 | 1,579,000 | 2.79 | +| white | 7,230 | 1,326,000 | 2.21 | +| Hemlock | 6,330 | 1,025,000 | 2.19 | +| Pine, lodgepole | 6,870 | 1,142,000 | 2.31 | +| longleaf | 9,680 | 1,739,000 | 3.02 | +| red | 7,480 | 1,438,000 | 2.18 | +| sugar | 6,740 | 1,083,000 | 2.34 | +| western yellow | 7,070 | 1,115,000 | 2.51 | +| white | 6,490 | 1,156,000 | 2.06 | +| Spruce, Engelmann | 6,300 | 1,076,000 | 2.09 | +| Tamarack | 7,750 | 1,263,000 | 2.67 | +|-------------------------------------------------------| + +Impact testing is difficult to conduct satisfactorily and the +data obtained are of chief value in a relative sense, that is, +for comparing the shock-resisting ability of woods of which like +specimens have been subjected to exactly identical treatment. +Yet this test is one of the most important made on wood, as it +brings out properties not evident from other tests. Defects and +brittleness are revealed by impact better than by any other kind +of test. In common practice nearly all external stresses are of +the nature of impact. In fact, no two moving bodies can come +together without impact stress. Impact is therefore the +commonest form of applied stress, although the most difficult to +measure. + + +_Failures in Timber Beams_ + +If a beam is loaded too heavily it will break or fail in some +characteristic manner. These failures may be classified +according to the way in which they develop, as tension, +compression, and horizontal shear; and according to the +appearance of the broken surface, as brash, and fibrous. A +number of forms may develop if the beam is completely ruptured. + +Since the tensile strength of wood is on the average about three +times as great as the compressive strength, a beam should, +therefore, be expected to fail by the formation in the first +place of a fold on the compression side due to the crushing +action, followed by failure on the tension side. This is usually +the case in green or moist wood. In dry material the first +visible failure is not infrequently on the lower or tension +side, and various attempts have been made to explain why such is +the case.[15] + +[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912, +XXIII_{2}, pp. 12-13.] + +Within the elastic limit the elongations and shortenings are +equal, and the neutral plane lies in the middle of the beam. +(See TRANSVERSE OR BENDING STRENGTH: BEAMS, above.) Later the +top layer of fibres on the upper or compression side fail, and +on the load increasing, the next layer of fibres fail, and so +on, even though this failure may not be visible. As a result the +shortenings on the upper side of the beam become considerably +greater than the elongations on the lower side. The neutral +plane must be presumed to sink gradually toward the tension +side, and when the stresses on the outer fibres at the bottom +have become sufficiently great, the fibres are pulled in two, +the tension area being much smaller than the compression area. +The rupture is often irregular, as in direct tension tests. +Failure may occur partially in single bundles of fibres some +time before the final failure takes place. One reason why the +failure of a dry beam is different from one that is moist, is +that drying increases the stiffness of the fibres so that they +offer more resistance to crushing, while it has much less effect +upon the tensile strength. + +There is considerable variation in tension failures depending +upon the toughness or the brittleness of the wood, the +arrangement of the grain, defects, etc., making further +classification desirable. The four most common forms are: + +(1)~Simple tension,~ in which there is a direct pulling in two +of the wood on the under side of the beam due to a tensile +stress parallel to the grain, (See Fig. 17, No. 1.) This is +common in straight-grained beams, particularly when the wood is +seasoned. + +[Illustration: FIG. 17.--Characteristic failures of simple +beams.] + +(2)~Cross-grained tension,~ in which the fracture is caused by a +tensile force acting oblique to the grain. (See Fig. 17, No. 2.) +This is a common form of failure where the beam has diagonal, +spiral or other form of cross grain on its lower side. Since the +tensile strength of wood across the grain is only a small +fraction of that with the grain it is easy to see why a +cross-grained timber would fail in this manner. + +(3)~Splintering tension,~ in which the failure consists of a +considerable number of slight tension failures, producing a +ragged or splintery break on the under surface of the beam. (See +Fig. 17, No. 3.) This is common in tough woods. In this case the +surface of fracture is fibrous. + +(4)~Brittle tension,~ in which the beam fails by a clean break +extending entirely through it. (See Fig. 17, No. 4.) It is +characteristic of a brittle wood which gives way suddenly +without warning, like a piece of chalk. In this case the surface +of fracture is described as brash. + +~Compression failure~ (see Fig. 17, No. 5) has few variations +except that it appears at various distances from the neutral +plane of the beam. It is very common in green timbers. The +compressive stress parallel to the fibres causes them to buckle +or bend as in an endwise compressive test. This action usually +begins on the top side shortly after the elastic limit is +reached and extends downward, sometimes almost reaching the +neutral plane before complete failure occurs. Frequently two or +more failures develop at about the same time. + +~Horizontal shear failure,~ in which the upper and lower +portions of the beam slide along each other for a portion of +their length either at one or at both ends (see Fig. 17, No. 6), +is fairly common in air-dry material and in green material when +the ratio of the height of the beam to the span is relatively +large. It is not common in small clear specimens. It is often +due to shake or season checks, common in large timbers, which +reduce the actual area resisting the shearing action +considerably below the calculated area used in the formulæ for +horizontal shear. (See page 98 for this formulæ.) For this +reason it is unsafe, in designing large timber beams, to use +shearing stresses higher than those calculated for beams that +failed in horizontal shear. The effect of a failure in +horizontal shear is to divide the beam into two or more beams +the combined strength of which is much less than that of the +original beam. Fig. 18 shows a large beam in which two failures +in horizontal shear occurred at the same end. That the parts +behave independently is shown by the compression failure below +the original location of the neutral plane. + +[Illustration: FIG. 18.--Failure of a large beam by horizontal +shear. _Photo by U. S, Forest Service._] + +Table XI gives an analysis of the causes of first failure in 840 +large timber beams of nine different species of conifers. Of the +total number tested 165 were air-seasoned, the remainder green. +The failure occurring first signifies the point of greatest +weakness in the specimen under the particular conditions of +loading employed (in this case, third-point static loading). + +|-----------------------------------------------------------| +| TABLE XI | +|-----------------------------------------------------------| +| MANNER OF FIRST FAILURE OF LARGE BEAMS | +| (Forest Service Bul. 108, p. 56) | +|-----------------------------------------------------------| +| | Total | Per cent of total failing by | +| COMMON NAME | number |---------+-------------+-------| +| OF SPECIES | of | Tension | Compression | Shear | +| | tests | | | | +|------------------+--------+---------+-------------+-------| +| Longleaf pine: | | | | | +| green | 17 | 18 | 24 | 58 | +| dry | 9 | 22 | 22 | 56 | +| Douglas fir: | | | | | +| green | 191 | 27 | 72 | 1 | +| dry | 91 | 19 | 76 | 5 | +| Shortleaf pine: | | | | | +| green | 48 | 27 | 56 | 17 | +| dry | 13 | 54 | | 46 | +| Western larch: | | | | | +| green | 62 | 23 | 71 | 6 | +| dry | 52 | 54 | 19 | 27 | +| Loblolly pine: | | | | | +| green | 111 | 40 | 53 | 7 | +| dry | 25 | 60 | 12 | 28 | +| Tamarack: | | | | | +| green | 30 | 37 | 53 | 10 | +| dry | 9 | 45 | 22 | 33 | +| Western hemlock: | | | | | +| green | 39 | 21 | 74 | 5 | +| dry | 44 | 11 | 66 | 23 | +| Redwood: | | | | | +| green | 28 | 43 | 50 | 7 | +| dry | 12 | 83 | 17 | | +| Norway pine: | | | | | +| green | 49 | 18 | 76 | 6 | +| dry | 10 | 30 | 60 | 10 | +|-----------------------------------------------------------| +| NOTE.--These tests were made on timbers ranging in cross | +| section from 4" x 10" to 8" x 16", and with a span of 15 | +| feet. | +|-----------------------------------------------------------| + + + +TOUGHNESS: TORSION + + +Toughness is a term applied to more than one property of wood. +Thus wood that is difficult to split is said to be tough. Again, +a tough wood is one that will not rupture until it has deformed +considerably under loads at or near its maximum strength, or one +which still hangs together after it has been ruptured and may be +bent back and forth without breaking apart. Toughness includes +flexibility and is the reverse of brittleness, in that tough +woods break gradually and give warning of failure. Tough woods +offer great resistance to impact and will permit rougher +treatment in manipulations attending manufacture and use. +Toughness is dependent upon the strength, cohesion, quality, +length, and arrangement of fibre, and the pliability of the +wood. Coniferous woods as a rule are not as tough as hardwoods, +of which hickory and elm are the best examples. + +The torsion or twisting test is useful in determining the +toughness of wood. If the ends of a shaft are turned in opposite +directions, or one end is turned and the other is fixed, all of +the fibres except those at the axis tend to assume the form of +helices. (See Fig. 19.) The strain produced by torsion or +twisting is essentially shear transverse and parallel to the +fibres, combined with longitudinal tension and transverse +compression. Within the elastic limit the strains increase +directly as the distance from the axis of the specimen. The +outer elements are subjected to tensile stresses, and as they +become twisted tend to compress those near the axis. The +elongated elements also contract laterally. Cross sections which +were originally plane become warped. With increasing strain the +lateral adhesion of the outer fibres is destroyed, allowing them +to slide past each other, and reducing greatly their power of +resistance. In this way the strains on the fibres nearer the +axis are progressively increased until finally all of the +elements are sheared apart. It is only in the toughest materials +that the full effect of this action can be observed. (See Fig. +20.) Brittle woods snap off suddenly with only a small amount of +torsion, and their fracture is irregular and oblique to the axis +of the piece instead of frayed out and more nearly perpendicular +to the axis as is the case with tough woods. + +[Illustration: FIG. 19.--Torsion of a shaft.] + +[Illustration: FIG. 20.--Effect of torsion on different grades +of hickory. _Photo by U. S. Forest Service._] + + + +HARDNESS + + +The term _hardness_ is used in two senses, namely: (1) +resistance to indentation, and (2) resistance to abrasion or +scratching. In the latter sense hardness combined with toughness +is a measure of the wearing ability of wood and is an important +consideration in the use of wood for floors, paving blocks, +bearings, and rollers. While resistance to indentation is +dependent mostly upon the density of the wood, the wearing +qualities may be governed by other factors such as toughness, +and the size, cohesion, and arrangement of the fibres. In use +for floors, some woods tend to compact and wear smooth, while +others become splintery and rough. This feature is affected to +some extent by the manner in which the wood is sawed; thus +edge-grain pine flooring is much better than flat-sawn for +uniformity of wear. + +|-------------------------------------------------------------------| +| TABLE XII | +|-------------------------------------------------------------------| +| HARDNESS OF 32 WOODS IN GREEN CONDITION, | +| AS INDICATED BY THE LOAD REQUIRED TO IMBED | +| A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER | +| (Forest Service Cir. 213) | +|-------------------------------------------------------------------| +| COMMON NAME OF SPECIES | Average | End | Radial | Tangential | +| | | surface | surface | surface | +|------------------------+---------+---------+---------+------------| +| | Pounds | Pounds | Pounds | Pounds | +| | | | | | +| Hardwoods | | | | | +| | | | | | +| 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 | +| 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 | +| 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 | +| 4 White oak | 1,164 | 1,183 | 1,163 | 1,147 | +| 5 Post oak | 1,099 | 1,139 | 1,068 | 1,081 | +| 6 Black oak | 1,069 | 1,093 | 1,083 | 1,031 | +| 7 Red oak | 1,043 | 1,107 | 1,020 | 1,002 | +| 8 White ash | 1,046 | 1,121 | 1,000 | 1,017 | +| 9 Beech | 942 | 1,012 | 897 | 918 | +| 10 Sugar maple | 937 | 992 | 918 | 901 | +| 11 Rock elm | 910 | 954 | 883 | 893 | +| 12 Hackberry | 799 | 829 | 795 | 773 | +| 13 Slippery elm | 788 | 919 | 757 | 687 | +| 14 Yellow birch | 778 | 827 | 768 | 739 | +| 15 Tupelo | 738 | 814 | 666 | 733 | +| 16 Red maple | 671 | 766 | 621 | 626 | +| 17 Sycamore | 608 | 664 | 560 | 599 | +| 18 Black ash | 551 | 565 | 542 | 546 | +| 19 White elm | 496 | 536 | 456 | 497 | +| 20 Basswood | 239 | 273 | 226 | 217 | +| | | | | | +| Conifers | | | | | +| | | | | | +| 1 Longleaf pine | 532 | 574 | 502 | 521 | +| 2 Douglas fir | 410 | 415 | 399 | 416 | +| 3 Bald cypress | 390 | 460 | 355 | 354 | +| 4 Hemlock | 384 | 463 | 354 | 334 | +| 5 Tamarack | 384 | 401 | 380 | 370 | +| 6 Red pine | 347 | 355 | 345 | 340 | +| 7 White fir | 346 | 381 | 322 | 334 | +| 8 Western yellow pine | 328 | 334 | 307 | 342 | +| 9 Lodgepole pine | 318 | 316 | 318 | 319 | +| 10 White pine | 299 | 304 | 294 | 299 | +| 11 Engelmann pine | 266 | 272 | 253 | 274 | +| 12 Alpine fir | 241 | 284 | 203 | 235 | +|-------------------------------------------------------------------| +| NOTE.--Black locust and hickory are not included in this table, | +| but their position would be near the head of the list. | +|-------------------------------------------------------------------| + +Tests for either form of hardness are of comparative value only. +Tests for indentation are commonly made by penetrations of the +material with a steel punch or ball.[16] Tests for abrasion are +made by wearing down wood with sandpaper or by means of a sand +blast. + +[Footnote 16: See articles by Gabriel Janka listed in +bibliography, pages 151-152.] + + + +CLEAVABILITY + + +_Cleavability_ is the term used to denote the facility with +which wood is split. A splitting stress is one in which the +forces act normally like a wedge. (See Fig. 21.) The plane of +cleavage is parallel to the grain, either radially or +tangentially. + +[Illustration: FIG. 21.--Cleavage of highly elastic wood. The +cleft runs far ahead of the wedge.] + +This property of wood is very important in certain uses such as +firewood, fence rails, billets, and squares. Resistance to +splitting or low cleavability is desirable where wood must hold +nails or screws, as in box-making. Wood usually splits more +readily along the radius than parallel to the growth rings +though exceptions occur, as in the case of cross grain. + +Splitting involves transverse tension, but only a portion of the +fibres are under stress at a time. A wood of little stiffness +and strong cohesion across the grain is difficult to split, +while one with great stiffness, such as longleaf pine, is easily +split. The form of the grain and the presence of knots greatly +affect this quality. + +|---------------------------------------------| +| TABLE XIII | +|---------------------------------------------| +| CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF | +| 32 WOODS IN GREEN CONDITION | +| (Forest Service Cir. 213) | +|---------------------------------------------| +| | When | When | +| COMMON NAME | surface of | surface of | +| OF SPECIES | failure is | failure is | +| | radial | tangential | +|-------------------+------------+------------| +| | Lbs. per | Lbs. per | +| | sq. inch | sq. inch | +| | | | +| Hardwoods | | | +| | | | +| Ash, black | 275 | 260 | +| white | 333 | 346 | +| Bashwood | 130 | 168 | +| Beech | 339 | 527 | +| Birch, yellow | 294 | 287 | +| Elm, slippery | 401 | 424 | +| white | 210 | 270 | +| Hackberry | 422 | 436 | +| Locust, honey | 552 | 610 | +| Maple, red | 297 | 330 | +| sugar | 376 | 513 | +| Oak, post | 354 | 487 | +| red | 380 | 470 | +| swamp white | 428 | 536 | +| white | 382 | 457 | +| yellow | 379 | 470 | +| Sycamore | 265 | 425 | +| Tupelo | 277 | 380 | +| | | | +| Conifers | | | +| | | | +| Arborvitæ | 148 | 139 | +| Cypress, bald | 167 | 154 | +| Fir, alpine | 130 | 133 | +| Douglas | 139 | 127 | +| white | 145 | 187 | +| Hemlock | 168 | 151 | +| Pine, lodgepole | 142 | 140 | +| longleaf | 187 | 180 | +| red | 161 | 154 | +| sugar | 168 | 189 | +| western yellow | 162 | 187 | +| white | 144 | 160 | +| Spruce, Engelmann | 110 | 135 | +| Tamarack | 167 | 159 | +|---------------------------------------------| + + + + +PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD + + + +INTRODUCTION + + +Wood is an organic product--a structure of infinite variation of +detail and design.[17] It is on this account that no two woods +are alike--in reality no two specimens from the same log are +identical. There are certain properties that characterize each +species, but they are subject to considerable variation. Oak, +for example, is considered hard, heavy, and strong, but some +pieces, even of the same species of oak, are much harder, +heavier, and stronger than others. With hickory are associated +the properties of great strength, toughness, and resilience, but +some pieces are comparatively weak and brash and ill-suited for +the exacting demands for which good hickory is peculiarly +adapted. + +[Footnote 17: For details regarding the structure of wood see +Record, Samuel J.: Identification of the economic woods of the +United States. New York, John Wiley & Sons, 1912.] + +It follows that no definite value can be assigned to the +properties of any wood and that tables giving average results of +tests may not be directly applicable to any individual stick. +With sufficient knowledge of the intrinsic factors affecting the +results it becomes possible to infer from the appearance of +material its probable variation from the average. As yet too +little is known of the relation of structure and chemical +composition to the mechanical and physical properties to permit +more than general conclusions. + + + +RATE OF GROWTH + + +To understand the effect of variations in the rate of growth it +is first necessary to know how wood is formed. A tree increases +in diameter by the formation, between the old wood and the inner +bark, of new woody layers which envelop the entire stem, living +branches, and roots. Under ordinary conditions one layer is +formed each year and in cross section as on the end of a log +they appear as rings--often spoken of as _annual rings_. These +growth layers are made up of wood cells of various kinds, but +for the most part fibrous. In timbers like pine, spruce, +hemlock, and other coniferous or softwood species the wood cells +are mostly of one kind, and as a result the material is much +more uniform in structure than that of most hardwoods. (See +Frontispiece.) There are no vessels or pores in coniferous wood +such as one sees so prominently in oak and ash, for example. +(See Fig. 22.) + +[Illustration: FIG. 22.--Cross sections of a ring-porous +hardwood (white ash), a diffuse-porous hardwood (red gum), and a +non-porous or coniferous wood (eastern hemlock). X 30. +_Photomicrographs by the author._] + +The structure of the hardwoods is more complex. They are more or +less filled with vessels, in some cases (oak, chestnut, ash) +quite large and distinct, in others (buckeye, poplar, gum) too +small to be seen plainly without a small hand lens. In +discussing such woods it is customary to divide them into two +large classes--_ring-porous_ and _diffuse-porous_. (See Fig. +22.) In ring-porous species, such as oak, chestnut, ash, black +locust, catalpa, mulberry, hickory, and elm, the larger vessels +or pores (as cross sections of vessels are called) become +localized in one part of the growth ring, thus forming a region +of more or less open and porous tissue. The rest of the ring is +made up of smaller vessels and a much greater proportion of wood +fibres. These fibres are the elements which give strength and +toughness to wood, while the vessels are a source of weakness. + +In diffuse-porous woods the pores are scattered throughout the +growth ring instead of being collected in a band or row. +Examples of this kind of wood are gum, yellow poplar, birch, +maple, cottonwood, basswood, buckeye, and willow. Some species, +such as walnut and cherry, are on the border between the two +classes, forming a sort of intermediate group. + +If one examines the smoothly cut end of a stick of almost any +kind of wood, he will note that each growth ring is made up of +two more or less well-defined parts. That originally nearest the +centre of the tree is more open textured and almost invariably +lighter in color than that near the outer portion of the ring. +The inner portion was formed early in the season, when growth +was comparatively rapid and is known as _early wood_ (also +spring wood); the outer portion is the _late wood_, being +produced in the summer or early fall. In soft pines there is not +much contrast in the different parts of the ring, and as a +result the wood is very uniform in texture and is easy to work. +In hard pine, on the other hand, the late wood is very dense and +is deep-colored, presenting a very decided contrast to the soft, +straw-colored early wood. (See Fig. 23.) In ring-porous woods +each season's growth is always well defined, because the large +pores of the spring abut on the denser tissue of the fall +before. In the diffuse-porous, the demarcation between rings is +not always so clear and in not a few cases is almost, if not +entirely, invisible to the unaided eye. (See Fig. 22.) + +[Illustration: FIG. 23.--Cross section of longleaf pine showing +several growth rings with variations in the width of the +dark-colored late wood. Seven resin ducts are visible. X 33. +_Photomicrograph by U.S. Forest Service_] + +If one compares a heavy piece of pine with a light specimen it +will be seen at once that the heavier one contains a larger +proportion of late wood than the other, and is therefore +considerably darker. The late wood of all species is denser than +that formed early in the season, hence the greater the +proportion of late wood the greater the density and strength. +When examined under a microscope the cells of the late wood are +seen to be very thick-walled and with very small cavities, while +those formed first in the season have thin walls and large +cavities. The strength is in the walls, not the cavities. In +choosing a piece of pine where strength or stiffness is the +important consideration, the principal thing to observe is the +comparative amounts of early and late wood. The width of ring, +that is, the number per inch, is not nearly so important as the +proportion of the late wood in the ring. + +It is not only the proportion of late wood, but also its +quality, that counts. In specimens that show a very large +proportion of late wood it may be noticeably more porous and +weigh considerably less than the late wood in pieces that +contain but little. One can judge comparative density, and +therefore to some extent weight and strength, by visual +inspection. + +The conclusions of the U.S. Forest Service regarding the effect +of rate of growth on the properties of Douglas fir are +summarized as follows: + +"1. In general, rapidly grown wood (less than eight rings per +inch) is relatively weak. A study of the individual tests upon +which the average points are based shows, however, that when it +is not associated with light weight and a small proportion of +summer wood, rapid growth is not indicative of weak wood. + +"2. An average rate of growth, indicated by from 12 to 16 rings +per inch, seems to produce the best material. + +"3. In rates of growths lower than 16 rings per inch, the +average strength of the material decreases, apparently +approaching a uniform condition above 24 rings per inch. In such +slow rates of growth the texture of the wood is very uniform, +and naturally there is little variation in weight or strength. + +"An analysis of tests on large beams was made to ascertain if +average rate of growth has any relation to the mechanical +properties of the beams. The analysis indicated conclusively +that there was no such relation. Average rate of growth [without +consideration also of density], therefore, has little +significance in grading structural timber."[18] This is because +of the wide variation in the percentage of late wood in +different parts of the cross section. + +[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p. +29.] + +Experiments seem to indicate that for most species there is a +rate of growth which, in general, is associated with the +greatest strength, especially in small specimens. For eight +conifers it is as follows:[19] + +[Footnote 19: Bul. 108, U. S. Forest Service: Tests of +structural timbers, p. 37.] + + Rings per inch +Douglas fir 24 +Shortleaf pine 12 +Loblolly pine 6 +Western larch 18 +Western hemlock 14 +Tamarack 20 +Norway pine 18 +Redwood 30 + +No satisfactory explanation can as yet be given for the real +causes underlying the formation of early and late wood. Several +factors may be involved. In conifers, at least, rate of growth +alone does not determine the proportion of the two portions of +the ring, for in some cases the wood of slow growth is very hard +and heavy, while in others the opposite is true. The quality of +the site where the tree grows undoubtedly affects the character +of the wood formed, though it is not possible to formulate a +rule governing it. In general, however, it may be said that +where strength or ease of working is essential, woods of +moderate to slow growth should be chosen. But in choosing a +particular specimen it is not the width of ring, but the +proportion and character of the late wood which should govern. + +In the case of the ring-porous hardwoods there seems to exist a +pretty definite relation between the rate of growth of timber +and its properties. This may be briefly summed up in the general +statement that the more rapid the growth or the wider the rings +of growth, the heavier, harder, stronger, and stiffer the wood. +This, it must be remembered, applies only to ring-porous woods +such as oak, ash, hickory, and others of the same group, and is, +of course, subject to some exceptions and limitations. + +In ring-porous woods of good growth it is usually the middle +portion of the ring in which the thick-walled, strength-giving +fibres are most abundant. As the breadth of ring diminishes, +this middle portion is reduced so that very slow growth produces +comparatively light, porous wood composed of thin-walled vessels +and wood parenchyma. In good oak these large vessels of the +early wood occupy from 6 to 10 per cent of the volume of the +log, while in inferior material they may make up 25 per cent or +more. The late wood of good oak, except for radial grayish +patches of small pores, is dark colored and firm, and consists +of thick-walled fibres which form one-half or more of the wood. +In inferior oak, such fibre areas are much reduced both in +quantity and quality. Such variation is very largely the result +of rate of growth. + +Wide-ringed wood is often called "second-growth," because the +growth of the young timber in open stands after the old trees +have been removed is more rapid than in trees in the forest, and +in the manufacture of articles where strength is an important +consideration such "second-growth" hardwood material is +preferred. This is particularly the case in the choice of +hickory for handles and spokes. Here not only strength, but +toughness and resilience are important. The results of a series +of tests on hickory by the U.S. Forest Service show that "the +work or shock-resisting ability is greatest in wide-ringed wood +that has from 5 to 14 rings per inch, is fairly constant from 14 +to 38 rings, and decreases rapidly from 38 to 47 rings. The +strength at maximum load is not so great with the most +rapid-growing wood; it is maximum with from 14 to 20 rings per +inch, and again becomes less as the wood becomes more closely +ringed. The natural deduction is that wood of first-class +mechanical value shows from 5 to 20 rings per inch and that +slower growth yields poorer stock. Thus the inspector or buyer +of hickory should discriminate against timber that has more than +20 rings per inch. Exceptions exist, however, in the case of +normal growth upon dry situations, in which the slow-growing +material may be strong and tough."[20] + +[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.] + +The effect of rate of growth on the qualities of chestnut wood +is summarized by the same authority as follows: "When the rings +are wide, the transition from spring wood to summer wood is +gradual, while in the narrow rings the spring wood passes into +summer wood abruptly. The width of the spring wood changes but +little with the width of the annual ring, so that the narrowing +or broadening of the annual ring is always at the expense of the +summer wood. The narrow vessels of the summer wood make it +richer in wood substance than the spring wood composed of wide +vessels. Therefore, rapid-growing specimens with wide rings have +more wood substance than slow-growing trees with narrow rings. +Since the more the wood substance the greater the weight, and +the greater the weight the stronger the wood, chestnuts with +wide rings must have stronger wood than chestnuts with narrow +rings. This agrees with the accepted view that sprouts (which +always have wide rings) yield better and stronger wood than +seedling chestnuts, which grow more slowly in diameter."[21] + +[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp. +20-21.] + +In diffuse-porous woods, as has been stated, the vessels or +pores are scattered throughout the ring instead of collected in +the early wood. The effect of rate of growth is, therefore, not +the same as in the ring-porous woods, approaching more nearly +the conditions in the conifers. In general it may be stated that +such woods of medium growth afford stronger material than when +very rapidly or very slowly grown. In many uses of wood, +strength is not the main consideration. If ease of working is +prized, wood should be chosen with regard to its uniformity of +texture and straightness of grain, which will in most cases +occur when there is little contrast between the late wood of one +season's growth and the early wood of the next. + + + +HEARTWOOD AND SAPWOOD + + +Examination of the end of a log of many species reveals a +darker-colored inner portion--the _heartwood_, surrounded by a +lighter-colored zone--the _sapwood_. In some instances this +distinction in color is very marked; in others, the contrast is +slight, so that it is not always easy to tell where one leaves +off and the other begins. The color of fresh sapwood is always +light, sometimes pure white, but more often with a decided tinge +of green or brown. + +Sapwood is comparatively new wood. There is a time in the early +history of every tree when its wood is all sapwood. Its +principal functions are to conduct water from the roots to the +leaves and to store up and give back according to the season the +food prepared in the leaves. The more leaves a tree bears and +the more thrifty its growth, the larger the volume of sapwood +required, hence trees making rapid growth in the open have +thicker sapwood for their size than trees of the same species +growing in dense forests. Sometimes trees grown in the open may +become of considerable size, a foot or more in diameter, before +any heartwood begins to form, for example, in second-growth +hickory, or field-grown white and loblolly pines. + +As a tree increases in age and diameter an inner portion of the +sapwood becomes inactive and finally ceases to function. This +inert or dead portion is called heartwood, deriving its name +solely from its position and not from any vital importance to +the tree, as is shown by the fact that a tree can thrive with +its heart completely decayed. Some, species begin to form +heartwood very early in life, while in others the change comes +slowly. Thin sapwood is characteristic of such trees as +chestnut, black locust, mulberry, Osage orange, and sassafras, +while in maple, ash, gum, hickory, hackberry, beech, and +loblolly pine, thick sapwood is the rule. + +There is no definite relation between the annual rings of growth +and the amount of sapwood. Within the same species the +cross-sectional area of the sapwood is roughly proportional to +the size of the crown of the tree. If the rings are narrow, more +of them are required than where they are wide. As the tree gets +larger, the sapwood must necessarily become thinner or increase +materially in volume. Sapwood is thicker in the upper portion of +the trunk of a tree than near the base, because the age and the +diameter of the upper sections are less. + +When a tree is very young it is covered with limbs almost, if +not entirely, to the ground, but as it grows older some or all +of them will eventually die and be broken off. Subsequent growth +of wood may completely conceal the stubs which, however, will +remain as knots. No matter how smooth and clear a log is on the +outside, it is more or less knotty near the middle. Consequently +the sapwood of an old tree, and particularly of a forest-grown +tree, will be freer from knots than the heartwood. Since in most +uses of wood, knots are defects that weaken the timber and +interfere with its ease of working and other properties, it +follows that sapwood, because of its position in the tree, may +have certain advantages over heartwood. + +It is really remarkable that the inner heartwood of old trees +remains as sound as it usually does, since in many cases it is +hundreds of years, and in a few instances thousands of years, +old. Every broken limb or root, or deep wound from fire, +insects, or falling timber, may afford an entrance for decay, +which, once started, may penetrate to all parts of the trunk. +The larvæ of many insects bore into the trees and their tunnels +remain indefinitely as sources of weakness. Whatever advantages, +however, that sapwood may have in this connection are due solely +to its relative age and position. + +If a tree grows all its life in the open and the conditions of +soil and site remain unchanged, it will make its most rapid +growth in youth, and gradually decline. The annual rings of +growth are for many years quite wide, but later they become +narrower and narrower. Since each succeeding ring is laid down +on the outside of the wood previously formed, it follows that +unless a tree materially increases its production of wood from +year to year, the rings must necessarily become thinner. As a +tree reaches maturity its crown becomes more open and the annual +wood production is lessened, thereby reducing still more the +width of the growth rings. In the case of forest-grown trees so +much depends upon the competition of the trees in their struggle +for light and nourishment that periods of rapid and slow growth +may alternate. Some trees, such as southern oaks, maintain the +same width of ring for hundreds of years. Upon the whole, +however, as a tree gets larger in diameter the width of the +growth rings decreases. + +It is evident that there may be decided differences in the grain +of heartwood and sapwood cut from a large tree, particularly one +that is overmature. The relationship between width of growth +rings and the mechanical properties of wood is discussed under +Rate of Growth. In this connection, however, it may be stated +that as a general rule the wood laid on late in the life of a +tree is softer, lighter, weaker, and more even-textured than +that produced earlier. It follows that in a large log the +sapwood, because of the time in the life of the tree when it was +grown, may be inferior in hardness, strength, and toughness to +equally sound heartwood from the same log. + +After exhaustive tests on a number of different woods the U.S. +Forest Service concludes as follows: "Sapwood, except that from +old, overmature trees, is as strong as heartwood, other things +being equal, and so far as the mechanical properties go should +not be regarded as a defect."[22] Careful inspection of the +individual tests made in the investigation fails to reveal any +relation between the proportion of sapwood and the breaking +strength of timber. + +[Footnote 22: Bul. 108: Tests of structural timber, p. 35.] + +In the study of the hickories the conclusion was: "There is an +unfounded prejudice against the heartwood. Specifications place +white hickory, or sapwood, in a higher grade than red hickory, +or heartwood, though there is no inherent difference in +strength. In fact, in the case of large and old hickory trees, +the sapwood nearest the bark is comparatively weak, and the best +wood is in the heart, though in young trees of thrifty growth +the best wood is in the sap."[23] The results of tests from +selected pieces lying side by side in the same tree, and also +the average values for heartwood and sapwood in shipments of the +commercial hickories without selection, show conclusively that +"the transformation of sapwood into heartwood does not affect +either the strength or toughness of the wood.... It is true, +however, that sapwood is usually more free from latent defects +than heartwood."[24] + +[Footnote 23: Bul. 80: The commercial hickories, p. 50.] + +[Footnote 24: _Loc. cit._] + +Specifications for paving blocks often require that longleaf +pine be 90 per cent heart. This is on the belief that sapwood is +not only more subject to decay, but is also weaker than +heartwood. In reality there is no sound basis for discrimination +against sapwood on account of strength, provided other +conditions are equal. It is true that sapwood will not resist +decay as long as heartwood, if both are untreated with +preservatives. It is especially so of woods with deep-colored +heartwood, and is due to infiltrations of tannins, oils, and +resins, which make the wood more or less obnoxious to +decay-producing fungi. If, however, the timbers are to be +treated, sapwood is not a defect; in fact, because of the +relative ease with which it can be impregnated with +preservatives it may be made more desirable than heartwood.[25] + +[Footnote 25: Although the factor of heart or sapwood does not +influence the mechanical properties of the wood and there is +usually no difference in structure observable under the +microscope, nevertheless sapwood is generally decidedly +different from heartwood in its physical properties. It dries +better and more easily than heartwood, usually with less +shrinkage and little checking or honeycombing. This is +especially the case with the more refractory woods, such as +white oaks and _Eucalyptus globulus_ and _viminalis_. It is +usually much more permeable to air, even in green wood, notably +so in loblolly pine and even in white oak. As already stated, it +is much more subject to decay. The sapwood of white oak may be +impregnated with creosote with comparative ease, while the +heartwood is practically impenetrable. These facts indicate a +difference in its chemical nature.--H.D. Tiemann.] + +In specifications for structural timbers reference is sometimes +made to "boxheart," meaning the inclusion of the pith or centre +of the tree within a cross section of the timber. From numerous +experiments it appears that the position of the pith does not +bear any relation to the strength of the material. Since most +season checks, however, are radial, the position of the pith may +influence the resistance of a seasoned beam to horizontal shear, +being greatest when the pith is located in the middle half of +the section.[26] + +[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.] + + + +WEIGHT, DENSITY, AND SPECIFIC GRAVITY + + +From data obtained from a large number of tests on the strength +of different woods it appears that, other things being equal, +the crushing strength parallel to the grain, fibre stress at +elastic limit in bending, and shearing strength along the grain +of wood vary in direct proportion to the weight of dry wood per +unit of volume when green. Other strength values follow +different laws. The hardness varies in a slightly greater ratio +than the square of the density. The work to the breaking point +increases even more rapidly than the cube of density. The +modulus of rupture in bending lies between the first power and +the square of the density. This, of course, is true only in case +the greater weight is due to increase in the amount of wood +substance. A wood heavy with resin or other infiltrated +substance is not necessarily stronger than a similar specimen +free from such materials. If differences in weight are due to +degree of seasoning, in other words, to the relative amounts of +water contained, the rules given above will of course not hold, +since strength increases with dryness. But of given specimens of +pine or of oak, for example, in the green condition, the +comparative strength may be inferred from the weight. It is not +permissible, however, to compare such widely different woods as +oak and pine on a basis of their weights.[27] + +[Footnote 27: The oaks for some unknown reason fall below the +normal strength for weight, whereas the hickories rise above. +Certain other woods also are somewhat exceptional to the normal +relation of strength and density.] + +The weight of wood substance, that is, the material which +composes the walls of the fibres and other cells, is practically +the same in all species, whether pine, hickory, or cottonwood, +being a little greater than half again as heavy as water. It +varies slightly from beech sapwood, 1.50, to Douglas fir +heartwood, 1.57, averaging about 1.55 at 30° to 35° C., in terms +of water at its greatest density 4° C. The reason any wood +floats is that the air imprisoned in its cavities buoys it up. +When this is displaced by water the wood becomes water-logged +and sinks. Leaving out of consideration infiltrated substances, +the reason a cubic foot of one kind of dry wood is heavier than +that of another is because it contains a greater amount of wood +substance. ~Density~ is merely the weight of a unit of volume, +as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre. +~Specific gravity~ or relative density is the ratio of the +density of any material to the density of distilled water at 4° +C. (39.2° F.). A cubic foot of distilled water at 4° C. weighs +62.43 pounds. Hence the specific gravity of a piece of wood with +a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight +per cubic foot when the specific gravity is given, simply +multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric +system, since the weight of a cubic centimetre of pure water is +one gram, the density in grams per cubic centimetre has the same +numerical value as the specific gravity. + +Since the amount of water in wood is extremely variable it +usually is not satisfactory to refer to the density of green +wood. For scientific purposes the density of "oven-dry" wood is +used; that is, the wood is dried in an oven at a temperature of +100°C. (212°F.) until a constant weight is attained. For +commercial purposes the weight or density of air-dry or +"shipping-dry" wood is used. This is usually expressed in pounds +per thousand board feet, a board foot being considered as +one-twelfth of a cubic foot. + +Wood shrinks greatly in drying from the green to the oven-dry +condition. (See Table XIV.) Consequently a block of wood +measuring a cubic foot when green will measure considerably less +when oven-dry. It follows that the density of oven-dry wood does +not represent the weight of the dry wood substance in a cubic +foot of green wood. In other words, it is not the weight of a +cubic foot of green wood minus the weight of the water which it +contains. Since the latter is often a more convenient figure to +use and much easier to obtain than the weight of oven-dry wood, +it is commonly expressed in tables of "specific gravity or +density of dry wood." + +|----------------------------------------------------------------------------| +| TABLE XIV | +|----------------------------------------------------------------------------| +| SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS | +| (Forest Service Cir. 213) | +|----------------------------------------------------------------------------| +| | | Specific gravity | Shrinkage from green to | +| | Mois- | oven-dry, based on | oven-dry condition | +| COMMON NAME | ture |--------------------+---------------------------| +| OF SPECIES | content | Volume | Volume | In | | Tangen- | +| | | when | when | volume | Radial | tial | +| | | green | oven-dry | | | | +|-----------------+---------+---------+----------+--------+--------+---------| +| | Per | | | Per | Per | Per | +| | cent | | | cent | cent | cent | +| | | | | | | | +| Hardwoods | | | | | | | +| | | | | | | | +| Ash, black | 77 | 0.466 | | | | | +| white | 38 | .550 | 0.640 | 12.6 | 4.3 | 6.4 | +| " | 47 | .516 | .590 | 11.7 | | | +| Basswood | 110 | .315 | .374 | 14.5 | 6.2 | 8.4 | +| Beech | 61 | .556 | .669 | 16.5 | 4.6 | 10.5 | +| Birch, yellow | 72 | .545 | .661 | 17.0 | 7.9 | 9.0 | +| Elm, rock | 46 | .578 | | | | | +| slippery | 57 | .541 | .639 | 15.5 | 5.1 | 9.9 | +| white | 66 | .430 | | | | | +| Gum, red | 71 | .434 | | | | | +| Hackberry | 50 | .504 | .576 | 14.0 | 4.2 | 8.9 | +| Hickory, | | | | | | | +| big shellbark | 64 | .601 | | 17.6 | 7.4 | 11.2 | +| " " | 55 | .666 | | 20.9 | 7.9 | 14.2 | +| bitternut | 65 | .624 | | | | | +| mockernut | 64 | .606 | | 16.5 | 6.9 | 10.4 | +| " | 57 | .662 | | 18.9 | 8.4 | 11.4 | +| " | 48 | .666 | | | | | +| nutmeg | 76 | .558 | | | | | +| pignut | 59 | .627 | | 15.0 | 5.6 | 9.8 | +| " | 54 | .667 | | 15.3 | 6.3 | 9.5 | +| " | 55 | .667 | | 16.9 | 6.8 | 10.9 | +| " | 52 | .667 | | 21.2 | 8.5 | 13.8 | +| shagbark | 65 | .608 | | 16.0 | 6.5 | 10.2 | +| " | 58 | .646 | | 18.4 | 7.9 | 11.4 | +| " | 64 | .617 | | | | | +| " | 60 | .653 | | 15.5 | 6.5 | 9.7 | +| water | 74 | .630 | | | | | +| Locust, honey | 53 | .695 | .759 | 8.6 | | | +| Maple, red | 69 | .512 | | | | | +| sugar | 57 | .546 | .643 | 14.3 | 4.9 | 9.1 | +| " | 56 | .577 | | | | | +| Oak, post | 64 | .590 | .732 | 16.0 | 5.7 | 10.6 | +| red | 80 | .568 | .660 | 13.1 | 3.7 | 8.3 | +| swamp white | 74 | .637 | .792 | 17.7 | 5.5 | 10.6 | +| tanbark | 88 | .585 | | | | | +| white | 58 | .594 | .704 | 15.8 | 6.2 | 8.3 | +| " | 62 | .603 | .696 | 14.3 | 4.9 | 9.0 | +| " | 78 | .600 | .708 | 16.0 | 4.8 | 9.2 | +| yellow | 77 | .573 | .669 | 14.2 | 4.5 | 9.7 | +| " | 80 | .550 | | | | | +| Osage orange | 31 | .761 | .838 | 8.9 | | | +| Sycamore | 81 | .454 | .526 | 13.5 | 5.0 | 7.3 | +| Tupelo | 121 | .475 | .545 | 12.4 | 4.4 | 7.9 | +|----------------------------------------------------------------------------| + +|----------------------------------------------------------------------------| +| TABLE XIV (CONT.) | +|----------------------------------------------------------------------------| +| SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS | +| (Forest Service Cir. 213) | +|----------------------------------------------------------------------------| +| | | Specific gravity | Shrinkage from green to | +| | | oven-dry, based on | oven-dry condition | +| COMMON NAME | |--------------------+---------------------------| +| OF SPECIES | Mois- | Volume | Volume | In | | Tangen- | +| | ture | when | when | volume | Radial | tial | +| | content | green | oven-dry | | | | +|-----------------+---------+---------+----------+--------+--------+---------| +| | Per | | | Per | Per | Per | +| | cent | | | cent | cent | cent | +| | | | | | | | +| Conifers | | | | | | | +| | | | | | | | +| Arborvitæ | 55 | .293 | .315 | 7.0 | 2.1 | 4.9 | +| Cedar, incense | 80 | .363 | | | | | +| Cypress, bald | 79 | .452 | .513 | 11.5 | 3.8 | 6.0 | +| Fir, alpine | 47 | .306 | .321 | 9.0 | 2.5 | 7.1 | +| amabilis | 117 | .383 | | | | | +| Douglas | 32 | .418 | .458 | 10.9 | 3.7 | 6.6 | +| white | 156 | .350 | .437 | 10.2 | 3.4 | 7.0 | +| Hemlock (east.) | 129 | .340 | .394 | 9.2 | 2.3 | 5.0 | +| Pine, lodgepole | 44 | .370 | .415 | 11.3 | 4.2 | 7.1 | +| " | 58 | .371 | .407 | 10.1 | 3.6 | 5.9 | +| longleaf | 63 | .528 | .599 | 12.8 | 6.0 | 7.6 | +| red or Nor | 54 | .440 | .507 | 11.5 | 4.5 | 7.2 | +| shortleaf | 52 | .447 | | | | | +| sugar | 123 | .360 | .386 | 8.4 | 2.9 | 5.6 | +| west yellow | 98 | .353 | .395 | 9.2 | 4.1 | 6.4 | +| " " | 125 | .377 | .433 | 11.5 | 4.3 | 7.3 | +| " " | 93 | .391 | .435 | 9.9 | 3.8 | 5.8 | +| white | 74 | .363 | .391 | 7.8 | 2.2 | 5.9 | +| Redwood | 81 | .334 | | | | | +| " | 69 | .366 | | | | | +| Spruce, | | | | | | | +| Engelmann | 45 | .325 | .359 | 10.5 | 3.7 | 6.9 | +| " | 156 | .299 | .335 | 10.3 | 3.0 | 6.2 | +| red | 31 | .396 | | | | | +| white | 41 | .318 | | | | | +| Tamarack | 52 | .491 | .558 | 13.6 | 3.7 | 7.4 | +|----------------------------------------------------------------------------| + +This weight divided by 62.43 gives the specific gravity per +green volume. It is purely a fictitious quantity. To convert +this figure into actual density or specific gravity of the dry +wood, it is necessary to know the amount of shrinkage in volume. +If S is the percentage of shrinkage from the green to the +oven-dry condition, based on the green volume; D, the density of +the dry wood per cubic foot while green; and d the actual + D +density of oven-dry wood, then ---------- = d. + 1 - .0 S + +This relation becomes clearer from the following analysis: +Taking V and W as the volume and weight, respectively, when +green, and v and w as the corresponding volume and weight when + w W V - v +oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and + v V V + V - v +s = ------- X 100, in which S is the percentage of shrinkage + v +from the green to the oven-dry condition, based on the green +volume, and s the same based on the oven-dry volume. + +In tables of specific gravity or density of wood it should +always be stated whether the dry weight per unit of volume when +green or the dry weight per unit of volume when dry is intended, +since the shrinkage in volume may vary from 6 to 50 per cent, +though in conifers it is usually about 10 per cent, and in +hardwoods nearer 15 per cent. (See Table XIV.) + + + +COLOR + + +In species which show a distinct difference between heartwood +and sapwood the natural color of heartwood is invariably darker +than that of the sapwood, and very frequently the contrast is +conspicuous. This is produced by deposits in the heartwood of +various materials resulting from the process of growth, +increased possibly by oxidation and other chemical changes, +which usually have little or no appreciable effect on the +mechanical properties of the wood. (See HEARTWOOD AND SAPWOOD, +above.) Some experiments[28] on very resinous longleaf pine +specimens, however, indicate an increase in strength. This is +due to the resin which increases the strength when dry. Spruce +impregnated with crude resin and dried is greatly increased in +strength thereby. + +[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126, +appendix.] + +Since the late wood of a growth ring is usually darker in color +than the early wood, this fact may be used in judging the +density, and therefore the hardness and strength of the +material. This is particularly the case with coniferous woods. +In ring-porous woods the vessels of the early wood not +infrequently appear on a finished surface as darker than the +denser late wood, though on cross sections of heartwood the +reverse is commonly true. Except in the manner just stated the +color of wood is no indication of strength. + +Abnormal discoloration of wood often denotes a diseased +condition, indicating unsoundness. The black check in western +hemlock is the result of insect attacks.[29] The reddish-brown +streaks so common in hickory and certain other woods are mostly +the result of injury by birds.[30] The discoloration is merely +an indication of an injury, and in all probability does not of +itself affect the properties of the wood. Certain rot-producing +fungi impart to wood characteristic colors which thus become +criterions of weakness. Ordinary sap-staining is due to fungous +growth, but does not necessarily produce a weakening effect.[31] + +[Footnote 29: See Burke, H.E.: Black check in western hemlock. +Cir. No. 61, U.S. Bu. Entomology, 1905.] + +[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees +and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.] + +[Footnote 31: See Von Schrenck, Hermann: The "bluing" and the +"red rot" of the western yellow pine, with special reference to +the Black Hills forest reserve. Bul. No. 36, U.S. Bu. Plant +Industry, Washington, 1903, pp. 13-14. + +Weiss, Howard, and Barnum, Charles T.: The prevention of +sapstain in lumber. Cir. 192, U.S. Forest Service, Washington, +1911, pp. 16-17.] + + + +CROSS GRAIN + + +_Cross grain_ is a very common defect in timber. One form of it +is produced in lumber by the method of sawing and has no +reference to the natural arrangement of the wood elements. Thus +if the plane of the saw is not approximately parallel to the +axis of the log the grain of the lumber cut is not parallel to +the edges and is termed diagonal. This is likely to occur where +the logs have considerable taper, and in this case may be +produced if sawed parallel to the axis of growth instead of +parallel to the growth rings. + +Lumber and timber with diagonal grain is always weaker than +straight-grained material, the extent of the defect varying with +the degree of the angle the fibres make with the axis of the +stick. In the vicinity of large knots the grain is likely to be +cross. The defect is most serious where wood is subjected to +flexure, as in beams. + +_Spiral grain_ is a very common defect in a tree, and when +excessive renders the timber valueless for use except in the +round. It is produced by the arrangement of the wood fibres in a +spiral direction about the axis instead of exactly vertical. +Timber with spiral grain is also known as "torse wood." Spiral +grain usually cannot be detected by casual inspection of a +stick, since it does not show in the so-called visible grain of +the wood, by which is commonly meant a sectional view of the +annual rings of growth cut longitudinally. It is accordingly +very easy to allow spiral-grained material to pass inspection, +thereby introducing an element of weakness in a structure. + +There are methods for readily detecting spiral grain. The +simplest is that of splitting a small piece radially. It is +necessary, of course, that the split be radial, that is, in a +plane passing through the axis of the log, and not tangentially. +In the latter case it is quite probable that the wood would +split straight, the line of cleavage being between the growth +rings. + +In inspection, the elements to examine are the rays. In the case +of oak and certain other hardwoods these rays are so large that +they are readily seen not only on a radial surface, but on the +tangential as well. On the former they appear as flakes, on the +latter as short lines. Since these rays are between the fibres +it naturally follows that they will be vertical or inclined +according as the tree is straight-grained or spiral-grained. +While they are not conspicuous in the softwoods, they can be +seen upon close scrutiny, and particularly so if a small hand +magnifier is used. + +When wood has begun to dry and check it is very easy to see +whether or not it is straight- or spiral-grained, since the +checks will for the most part follow along the rays. If one +examines a row of telephone poles, for example, he will probably +find that most of them have checks running spirally around them. +If boards were sawed from such a pole after it was badly checked +they would fall to pieces of their own weight. The only way to +get straight material would be to split it out. + +It is for this reason that split billets and squares are +stronger than most sawed material. The presence of the spiral +grain has little, if any, effect on the timber when it is used +in the round, but in sawed material the greater the pitch of the +spiral the greater is the defect. + + + +KNOTS + + +_Knots_ are portions of branches included in the wood of the +stem or larger branch. Branches originate as a rule from the +central axis of a stem, and while living increase in size by the +addition of annual woody layers which are a continuation of +those of the stem. The included portion is irregularly conical +in shape with the tip at the pith. The direction of the fibre is +at right angles or oblique to the grain of the stem, thus +producing local cross grain. + +During the development of a tree most of the limbs, especially +the lower ones, die, but persist for a time--often for years. +Subsequent layers of growth of the stem are no longer intimately +joined with the dead limb, but are laid around it. Hence dead +branches produce knots which are nothing more than pegs in a +hole, and likely to drop out after the tree has been sawed into +lumber. In grading lumber and structural timber, knots are +classified according to their form, size, soundness, and the +firmness with which they are held in place.[32] + +[Footnote 32: See Standard classification of structural timber. +Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303. +Contains three plates showing standard defects.] + +Knots materially affect checking and warping, ease in working, +and cleavability of timber. They are defects which weaken timber +and depreciate its value for structural purposes where strength +is an important consideration. The weakening effect is much more +serious where timber is subjected to bending and tension than +where under compression. The extent to which knots affect the +strength of a beam depends upon their position, size, number, +direction of fibre, and condition. A knot on the upper side is +compressed, while one on the lower side is subjected to tension. +The knot, especially (as is often the case) if there is a season +check in it, offers little resistance to this tensile stress. +Small, knots, however, may be so located in a beam along the +neutral plane as actually to increase the strength by tending to +prevent longitudinal shearing. Knots in a board or plank are +least injurious when they extend through it at right angles to +its broadest surface. Knots which occur near the ends of a beam +do not weaken it. Sound knots which occur in the central portion +one-fourth the height of the beam from either edge are not +serious defects. + +Extensive experiments by the U.S. Forest Service[33] indicate +the following effects of knots on structural timbers: + +[Footnote 33: Bul. 108, pp. 52 _et seq._] + +(1) Knots do not materially influence the stiffness of +structural timber. + +(2) Only defects of the most serious character affect the +elastic limit of beams. Stiffness and elastic strength are more +dependent upon the quality of the wood fibre than upon defects +in the beam. + +(3) The effect of knots is to reduce the difference between the +fibre stress at elastic limit and the modulus of rupture of +beams. The breaking strength is very susceptible to defects. + +(4) Sound knots do not weaken wood when subject to compression +parallel to the grain.[34] + +[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical +properties of western hemlock, p. 20.] + + + +FROST SPLITS + + +A common defect in standing timber results from radial splits +which extend inward from the periphery of the tree, and almost, +if not always, near the base. It is most common in trees which +split readily, and those with large rays and thin bark. The +primary cause of the splitting is frost, and various theories +have been advanced to explain the action. + +R. Hartig[35] believes that freezing forces out a part of the +imbibition water of the cell walls, thereby causing the wood to +shrink, and if the interior layers have not yet been cooled, +tangential strains arise which finally produce radial clefts. + +[Footnote 35: Hartig, R.: The diseases of trees (trans. by +Somerville and Ward), London and New York, 1894, pp. 282-294.] + +Another theory holds that the water is not driven out of the +cell walls, but that difference in temperature conditions of +inner and outer layers is itself sufficient to set up the +strains, resulting in splitting. An air temperature of 14°F. or +less is considered necessary to produce frost splits. + +A still more recent theory is that of Busse[36] who considers +the mechanical action of the wind a very important factor. He +observed: (_a_) Frost splits sometimes occur at higher +temperatures than 14°F. (_b_) Most splits take place shortly +before sunrise, _i.e._, at the time of lowest air and soil +temperature; they are never heard to take place at noon, +afternoon, or evening. (_c_) They always occur between two roots +or between the collars of two roots, (_d_) They are most +frequent in old, stout-rooted, broad-crowned trees; in younger +stands it is always the stoutest members that are found with +frost splits, while in quite young stands they are altogether +absent, (_e_) Trees on wet sites are most liable to splits, due +to difference in wood structure, just as difference in wood +structure makes different species vary in this regard. (_f_) +Frost splits are most numerous less than three feet above the +ground. + +[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss. +Centralb., XXXII, 2, 1910, pp. 74-81.] + +When a tree is swayed by the wind the roots are counteracting +forces, and the wood fibres are tested in tension and +compression by the opposing forces; where the roots exercise +tension stresses most effectively the effect of compression +stresses is at a minimum; only where the pressure is in excess +of the tension, _i.e._, between the roots, can a separation of +the fibre result. Hence, when by frost a tension on the entire +periphery is established, and the wind localizes additional +strains, failure occurs. The stronger the compression and +tension, the severer the strains and the oftener failures occur. +The occurrence of reports of frost splits on wind-still days is +believed by Busse to be due to the opening of old frost splits +where the tension produced by the frost alone is sufficient. + +Frost splits may heal over temporarily, but usually open up +again during the following winter. The presence of old splits is +often indicated by a ridge of callous, the result of the +cambium's effort to occlude the wound. Frost splits not only +affect the value of lumber, but also afford an entrance into the +living tree for disease and decay. + + + +SHAKES, GALLS, PITCH POCKETS + + +_Heart shake_ occurs in nearly all overmature timber, being more +frequent in hardwoods (especially oak) than in conifers. In +typical heart shake the centre of the hole shows indications of +becoming hollow and radial clefts of varying size extend outward +from the pith, being widest inward. It frequently affects only +the butt log, but may extend to the entire hole and even the +larger branches. It usually results from a shrinkage of the +heartwood due probably to chemical changes in the wood. + +When it consists of a single cleft extending across the pith it +is termed _simple heart shake_. Shake of this character in +straight-grained trees affects only one or two central boards +when cut into lumber, but in spiral-grained timber the damage is +much greater. When shake consists of several radial clefts it is +termed _star shake_. In some instances one or more of these +clefts may extend nearly to the bark. In felled or converted +timber clefts due to heart shake may be distinguished from +seasoning cracks by the darker color of the exposed surfaces. +Such clefts, however, tend to open up more and more as the +timber seasons. + +_Cup_ or _ring shake_ results from the pulling apart of two or +more growth rings. It is one of the most serious defects to +which sound timber is subject, as it seriously reduces the +technical properties of wood. It is very common in sycamore and +in western larch, particularly in the butt portion. Its +occurrence is most frequent at the junction of two growth layers +of very unequal thickness. Consequently it is likely to occur in +trees which have grown slowly for a time, then abruptly +increased, due to improved conditions of light and food, as in +thinning. Old timber is more subject to it than young trees. The +damage is largely confined to the butt log. Cup shake is often +associated with other forms of shake, and not infrequently shows +traces of decay. + +The causes of cup shake are uncertain. The swaying action of the +wind may result in shearing apart the growth layers, especially +in trees growing in exposed places. Frost may in some instances +be responsible for cup shake or at least a contributing factor, +although trees growing in regions free from frost often have +ring shake. Shrinkage of the heartwood may be concentric as well +as radial in its action, thus producing cup shake instead of, or +in connection with, heart shake. + +A local defect somewhat similar in effect to cup shake is known +as _rind gall_. If the cambium layer is exposed by the removal +of the entire bark or rind it will die. Subsequent growth over +the damaged portion does not cohere with the wood previously +formed by the old cambium. The defect resulting is termed rind +gall. The most common causes of it are fire, gnawing, blazing, +chipping, sun scald, lightning, and abrasions. + +_Heart break_ is a term applied to areas of compression failure +along the grain found in occasional logs. Sometimes these breaks +are invisible until the wood is manufactured into the finished +article. The occurrence of this defect is mostly limited to the +dense hardwoods, such as hickory and to heavy tropical species. +It is the source of considerable loss in the fancy veneer +industry, as the veneer from valuable logs so affected drops to +pieces. + +The cause of heart break is not positively known. It is highly +probable, however, that when the tree is felled the trunk +strikes across a rock or another log, and the impact causes +actual failure in the log as in a beam. + +_Resin_ or _pitch pockets_ are of common occurrence in the wood +of larch, spruce, fir, and especially of longleaf and other hard +pines. They are due to accumulations of resin in openings +between adjacent layers of growth. They are more frequent in +trees growing alone than in those of dense stands. The pockets +are usually a few inches in greatest dimension and affect only +one or two growth layers. They are hidden until exposed by the +saw, rendering it impossible to cut lumber with reference to +their position. Often several boards are damaged by a single +pocket. In grading lumber, pitch pockets are classified as +small, standard, and large, depending upon their width and +length. + + + +INSECT INJURIES[37] + + +[Footnote 37: For detailed information regarding insect +injuries, the reader is referred to the various publications of +the U.S. Bureau of Entomology, Washington, D.C.] + +The larvæ of many insects are destructive to wood. Some attack +the wood of living trees, others only that of felled or +converted material. Every hole breaks the continuity of the +fibres and impairs the strength, and if there are very many of +them the material may be ruined for all purposes where strength +is required. + +Some of the most common insects attacking the wood of living +trees are the oak timber worm, the chestnut timber worm, +carpenter worms, ambrosia beetles, the locust borer, turpentine +beetles and turpentine borers, and the white pine weevil. + +The insect injuries to forest products may be classed according +to the stage of manufacture of the material. Thus round timber +with the bark on, such as poles, posts, mine props, and sawlogs, +is subject to serious damage by the same class of insects as +those mentioned above, particularly by the round-headed borers, +timber worms, and ambrosia beetles. Manufactured unseasoned +products are subject to damage from ambrosia beetles and other +wood borers. Seasoned hardwood lumber of all kinds, rough +handles, wagon stock, etc., made partially or entirely of +sapwood, are often reduced in value from 10 to 90 per cent by a +class of insects known as powder-post beetles. Finished hardwood +products such as handles, wagon, carriage and machinery stock, +especially if ash or hickory, are often destroyed by the +powder-post beetles. Construction timbers in buildings, bridges +and trestles, cross-ties, poles, mine props, fence posts, etc., +are sometimes seriously injured by wood-boring larvæ, termites, +black ants, carpenter bees, and powder-post beetles, and +sometimes reduced in value from 10 to 100 per cent. In tropical +countries termites are a very serious pest in this respect. + + + +MARINE WOOD-BORER INJURIES + + +Vast amounts of timber used for piles in wharves and other +marine structures are constantly being destroyed or seriously +injured by marine borers. Almost invariably they are confined to +salt water, and all the woods commonly used for piling are +subject to their attacks. There are two genera of mollusks, +_Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria, +Chelura_, and _Sphoeroma_, that do serious damage in many places +along both the Atlantic and Pacific coasts. + +These mollusks, which are popularly known as "shipworms," are +much alike in structure and mode of life. They attack the +exposed surface of the wood and immediately begin to bore. The +tunnels, often as large as a lead pencil, extend usually in a +longitudinal direction and follow a very irregular, tangled +course. Hard woods are apparently penetrated as readily as soft +woods, though in the same timber the softer parts are preferred. +The food consists of infusoria and is not obtained from the wood +substance. The sole object of boring into the wood is to obtain +shelter. + +Although shipworms can live in cold water they thrive best and +are most destructive in warm water. The length of time required +to destroy an average barked, unprotected pine pile on the +Atlantic coast south from Chesapeake Bay and along the entire +Pacific coast varies from but one to three years. + +Of the crustacean borers, _Limnoria_, or the "wood louse," is +the only one of great importance, although _Sphoeroma_ is +reported destructive in places. _Limnoria_ is about the size of +a grain of rice and tunnels into the wood for both food and +shelter. The galleries extend inward radially, side by side, in +countless numbers, to the depth of about one-half inch. The thin +wood partitions remaining are destroyed by wave action, so that +a fresh surface is exposed to attack. Both hard and soft woods +are damaged, but the rate is faster in the soft woods or softer +portions of a wood. + +Timbers seriously attacked by marine borers are badly weakened +or completely destroyed. If the original strength of the +material is to be preserved it is necessary to protect the wood +from the borers. This is sometimes accomplished by proper +injection of creosote oil, and more or less successfully by the +use of various kinds of external coatings.[38] No treatment, +however, has proved entirely satisfactory. + +[Footnote 38: See Smith, C. Stowell: Preservation of piling +against marine wood borers. Cir. 128, U.S. Forest Service, 1908, +pp. 15.] + + + +FUNGOUS INJURIES[39] + + +[Footnote 39: See Von Schrenck, H.: The decay of timber and +methods of preventing it. Bul. 14, U.S. Bu. Plant Industry, +Washington, D.C., 1902. Also Buls. 32, 114, 214, 266. + +Meineoke, E.P.: Forest tree diseases common in California and +Nevada, U.S. Forest Service, Washington, D.C., 1914. + +Hartig, R.: The diseases of trees. London and New York, 1894.] + +Fungi are responsible for almost all decay of wood. So far as +known, all decay is produced by living organisms, either fungi +or bacteria. Some species attack living trees, sometimes killing +them, or making them hollow, or in the case of pecky cypress and +incense cedar filling the wood with galleries like those of +boring insects. A much larger variety work only in felled or +dead wood, even after it is placed in buildings or manufactured +articles. In any case the process of destruction is the same. +The mycelial threads penetrate the walls of the cells in search +of food, which they find either in the cell contents (starches, +sugars, etc.), or in the cell wall itself. The breaking down of +the cell walls through the chemical action of so-called +"enzymes" secreted by the fungi follows, and the eventual +product is a rotten, moist substance crumbling readily under the +slightest pressure. Some species remove the ligneous matter and +leave almost pure cellulose, which is white, like cotton; others +dissolve the cellulose, leaving a brittle, dark brown mass of +ligno-cellulose. Fungi (such as the bluing fungus) which merely +stain wood usually do not affect its mechanical properties +unless the attacks are excessive. + +It is evident, then, that the action of rot-causing fungi is to +decrease the strength of wood, rendering it unsound, brittle, +and dangerous to use. The most dangerous kinds are the so-called +"dry-rot" fungi which work in many kinds of lumber after it is +placed in the buildings. They are particularly to be dreaded +because unseen, working as they do within the walls or inside of +casings. Several serious wrecks of large buildings have been +attributed to this cause. It is stated[40] that in the three +years (1911-1913) more than $100,000 was required to repair +damage due to dry rot. + +[Footnote 40: Dry rot in factory timbers, by Inspection Dept. +Associated Factory Mutual Fire Insurance Cos., 31 Milk Street, +Boston, 1913.] + +Dry rot develops best at 75°F. and is said to be killed by a +temperature of 110°F.[41] Fully 70 per cent humidity is +necessary in the air in which a timber is surrounded for the +growth of this fungus, and probably the wood must be quite near +its fibre saturation condition. Nevertheless _Merulius +lacrymans_ (one of the most important species) has been found to +live four years and eight months in a dry condition.[42] +Thorough kiln-drying will kill this fungus, but will not prevent +its redevelopment. Antiseptic treatment, such as creosoting, is +the best prevention. + +[Footnote 41: Falck, Richard: Die Meruliusfaüle des Bauholzes, +Hausschwammforschungen, 6. Heft., Jena, 1912.] + +[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.] + +All fungi require moisture and air[43] for their growth. +Deprived of either of these the fungus dies or ceases to +develop. Just what degree of moisture in wood is necessary for +the "dry-rot" fungus has not been determined, but it is +evidently considerably above that of thoroughly air-dry timber, +probably more than 15 per cent moisture. Hence the importance of +free circulation of air about all timbers in a building. + +[Footnote 43: A culture of fungus placed in a glass jar and the +air pumped out ceases to grow, but will start again as soon as +oxygen is admitted.] + +Warmth is also conducive to the growth of fungi, the most +favorable temperature being about 90°F. They cannot grow in +extreme cold, although no degree of cold such as occurs +naturally will kill them. On the other hand, high temperature +will kill them, but the spores may survive even the boiling +temperature. Mould fungus has been observed to develop rapidly +at 130°F. in a dry kiln in moist air, a condition under which an +animal cannot live more than a few minutes. This fungus was +killed, however, at about 140° or 145°F.[44] + +[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in +Berkeley, U.S. Forest Service.] + +The fungus (_Endothia parasitica_ And.) which causes the +chestnut blight kills the trees by girdling them and has no +direct effect upon the wood save possibly the four or five +growth rings of the sapwood.[45] + +[Footnote 45: See Anderson, Paul J.: The morphology and life +history of the chestnut blight fungus. Bul. No. 7, Penna. +Chestnut Tree Blight Com., Harrisburg, 1914, p. 17.] + + + +PARASITIC PLANT INJURIES.[46] + + +[Footnote 46: See York, Harlan H.: The anatomy and some of the +biological aspects of the "American mistletoe." Bul. 120, Sci. +Ser. No. 13, Univ. of Texas, Austin, 1909. + +Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166, +U.S. Bu. Plant Ind., Washington, 1910. + +Meinecke, E.P.: Forest tree diseases common in California and +Nevada. U.S. Forest Service, Washington, 1914, pp. 54-58.] + +The most common of the higher parasitic plants damaging timber +trees are mistletoes. Many species of deciduous trees are +attacked by the common mistletoe (_Phoradendron flavescens_). It +is very prevalent in the South and Southwest and when present in +sufficient quantity does considerable damage. There is also a +considerable number of smaller mistletoes belonging to the genus +_Razoumofskya (Arceuthobium)_ which are widely distributed +throughout the country, and several of them are common on +coniferous trees in the Rocky Mountains and along the Pacific +coast. + +One effect of the common mistletoe is the formation of large +swellings or tumors. Often the entire tree may become stunted or +distorted. The western mistletoe is most common on the branches, +where it produces "witches' broom." It frequently attacks the +trunk as well, and boards cut from such trees are filled with +long, radial holes which seriously damage or destroy the value +of the timber affected. + + + +LOCALITY OF GROWTH + + +The data available regarding the effect of the locality of +growth upon the properties of wood are not sufficient to warrant +definite conclusions. The subject has, however, been kept in +mind in many of the U.S. Forest Service timber tests and the +following quotations are assembled from various reports: + +"In both the Cuban and longleaf pine the locality where grown +appears to have but little influence on weight or strength, and +there is no reason to believe that the longleaf pine from one +State is better than that from any other, since such variations +as are claimed can be found on any 40-acre lot of timber in any +State. But with loblolly and still more with shortleaf this +seems not to be the case. Being widely distributed over many +localities different in soil and climate, the growth of the +shortleaf pine seems materially influenced by location. The wood +from the southern coast and gulf region and even Arkansas is +generally heavier than the wood from localities farther north. +Very light and fine-grained wood is seldom met near the southern +limit of the range, while it is almost the rule in Missouri, +where forms resembling the Norway pine are by no means rare. The +loblolly, occupying both wet and dry soils, varies accordingly." +Cir. No. 12, p. 6. + +" ... It is clear that as all localities have their heavy and +their light timber, so they all share in strong and weak, hard +and soft material, and the difference in quality of material is +evidently far more a matter of individual variation than of soil +or climate." _Ibid._, p.22 + +"A representative committee of the Carriage Builders' +Association had publicly declared that this important industry +could not depend upon the supplies of southern timber, as the +oak grown in the South lacked the necessary qualities demanded +in carriage construction. Without experiment this statement +could be little better than a guess, and was doubly unwarranted, +since it condemned an enormous amount of material, and one +produced under a great variety of conditions and by at least a +dozen species of trees, involving, therefore, a complexity of +problems difficult enough for the careful investigator, and +entirely beyond the few unsystematic observations of the members +of a committee on a flying trip through one of the greatest +timber regions of the world. + +"A number of samples were at once collected (part of them +supplied by the carriage builders' committee), and the fallacy +of the broad statement mentioned was fully demonstrated by a +short series of tests and a more extensive study into structure +and weight of these materials. From these tests it appears that +pieces of white oak from Arkansas excelled well-selected pieces +from Connecticut, both in stiffness and endwise compression (the +two most important forms of resistance)." Report upon the +forestry investigations of the U.S.D.A. 1877-1898, p. 331. See +also Rep. of Div. of For., 1890, p. 209. + +"In some regions there are many small, stunted hickories, which +most users will not touch. They have narrow sap, are likely to +be birdpecked, and show very slow growth. Yet five of these +trees from a steep, dry south slope in West Virginia had an +average strength fully equal to that of the pignut from the +better situation, and were superior in toughness, the work to +maximum load being 36.8 as against 31.2 for pignut. The trees +had about twice as many rings per inch as others from better +situations. + +"This, however, is not very significant, as trees of the same +species, age, and size, growing side by side under the same +conditions of soil and situation, show great variation in their +technical value. It is hard to account for this difference, but +it seems that trees growing in wet or moist situations are +rather inferior to those growing on fresher soil; also, it is +claimed by many hickory users that the wood from limestone soils +is superior to that from sandy soils. + +"One of the moot questions among hickory men is the relative +value of northern and southern hickory. The impression prevails +that southern hickory is more porous and brash than hickory from +the north. The tests ... indicate that southern hickory is as +tough and strong as northern hickory of the same age. But the +southern hickories have a greater tendency to be shaky, and this +results in much waste. In trees from southern river bottoms the +loss through shakes and grub-holes in many cases amounts to as +much as 50 per cent. + +"It is clear, therefore, that the difference in northern and +southern hickory is not due to geographic location, but rather +to the character of timber that is being cut. Nearly all of that +from southern river bottoms and from the Cumberland Mountains is +from large, old-growth trees; that from the north is from +younger trees which are grown under more favorable conditions, +and it is due simply to the greater age of the southern trees +that hickory from that region is lighter and more brash than +that from the north." Bul. 80, pp. 52-55. + + + +SEASON OF CUTTING + + +It is generally believed that winter-felled timber has decided +advantages over that cut at other seasons of the year, and to +that cause alone are frequently ascribed much greater +durability, less liability to check and split, better color, and +even increased strength and toughness. The conclusion from the +various experiments made on the subject is that while the time +of felling may, and often does, affect the properties of wood, +such result is due to the weather conditions rather than to the +condition of the wood. + +There are two phases of this question. One is concerned with the +physiological changes which might take place during the year in +the wood of a living tree. The other deals with the purely +physical results due to the weather, as differences in +temperature, humidity, moisture, and other features to be +mentioned later. + +Those who adhere to the first view maintain that wood cut in +summer is quite different in composition from that cut in +winter. One opinion is that in summer the "sap is up," while in +winter it is "down," consequently winter-felled timber is drier. +A variation of this belief is that in summer the sap contains +certain chemicals which affect the properties of wood and does +not contain them in winter. Again it is sometimes asserted that +wood is actually denser in winter than in summer, as part of the +wood substance is dissolved out in the spring and used for plant +food, being restored in the fall. + +It is obvious that such views could apply only to sapwood, since +it alone is in living condition at the time of cutting. +Heartwood is dead wood and has almost no function in the +existence of the tree other than the purely mechanical one of +support. Heartwood does undergo changes, but they are gradual +and almost entirely independent of the seasons. + +Sapwood might reasonably be expected to respond to seasonal +changes, and to some extent it does. Just beneath the bark there +is a thin layer of cells which during the growing season have +not attained their greatest density. With the exception of this +one annual ring, or portion of one, the density of the wood +substance of the sapwood is nearly the same the year round. +Slight variations may occur due to impregnation with sugar and +starch in the winter and its dissolution in the growing season. +The time of cutting can have no material effect on the inherent +strength and other mechanical properties of wood except in the +outermost annual ring of growth. + +The popular belief that sap is up in the spring and summer and +is down in the winter has not been substantiated by experiment. +There are seasonal differences in the composition of sap, but so +far as the amount of sap in a tree is concerned there is fully +as much, if not more, during the winter than in summer. +Winter-cut wood is not drier, to begin with, than +summer-felled--in reality, it is likely to be wetter.[47] + +[Footnote 47: See Record, S.J.: Sap in relation to the +properties of wood. Proc. Am. Wood Preservers' Assn., Baltimore, +Md., 1913, pp. 160-166. + +Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am. +Ry. Eng. Assn., 1913, p. 214.] + +The important consideration in regard to this question is the +series of circumstances attending the handling of the timber +after it is felled. Wood dries more rapidly in summer than in +winter, not because there is less moisture at one time than +another, but because of the higher temperature in summer. This +greater heat is often accompanied by low humidity, and +conditions are favorable for the rapid removal of moisture from +the exposed portions of wood. Wood dries by evaporation, and +other things being equal, this will proceed much faster in hot +weather than in cold. + +It is a matter of common observation that when wood dries it +shrinks, and if shrinkage is not uniform in all directions the +material pulls apart, causing season checks. (See Fig. 27.) If +evaporation proceeds more rapidly on the outside than inside, +the greater shrinkage of the outer portions is bound to result +in many checks, the number and size increasing with the degree +of inequality of drying. + +In cold weather, drying proceeds slowly but uniformly, thus +allowing the wood elements to adjust themselves with the least +amount of rupturing. In summer, drying proceeds rapidly and +irregularly, so that material seasoned at that time is more +likely to split and check. + +There is less danger of sap rot when trees are felled in winter +because the fungus does not grow in the very cold weather, and +the lumber has a chance to season to below the danger point +before the fungus gets a chance to attack it. If the logs in +each case could be cut into lumber immediately after felling and +given exactly the same treatment, for example, kiln-dried, no +difference due to the season of cutting would be noted. + + + +WATER CONTENT[48] + + +[Footnote 48: See Tiemann, H.D.: Effect of moisture upon the +strength and stiffness of wood. Bul. 70, U.S. Forest Service, +Washington, D.C., 1906; also Cir. 108, 1907.] + +Water occurs in living wood in three conditions, namely: (1) in +the cell walls, (2) in the protoplasmic contents of the cells, +and (3) as free water in the cell cavities and spaces. In +heartwood it occurs only in the first and last forms. Wood that +is thoroughly air-dried retains from 8 to 16 per cent of water +in the cell walls, and none, or practically none, in the other +forms. Even oven-dried wood retains a small percentage of +moisture, but for all except chemical purposes, may be +considered absolutely dry. + +The general effect of the water content upon the wood substance +is to render it softer and more pliable. A similar effect of +common observation is in the softening action of water on +rawhide, paper, or cloth. Within certain limits the greater the +water content the greater its softening effect. + +Drying produces a decided increase in the strength of wood, +particularly in small specimens. An extreme example is the case +of a completely dry spruce block two inches in section, which +will sustain a permanent load four times as great as that which +a green block of the same size will support. + +The greatest increase due to drying is in the ultimate crushing +strength, and strength at elastic limit in endwise compression; +these are followed by the modulus of rupture, and stress at +elastic limit in cross-bending, while the modulus of elasticity +is least affected. These ratios are shown in Table XV, but it is +to be noted that they apply only to wood in a much drier +condition than is used in practice. For air-dry wood the ratios +are considerably lower, particularly in the case of the ultimate +strength and the elastic limit. Stiffness (within the elastic +limit), while following a similar law, is less affected. In the +case of shear parallel to the grain, the general effect of +drying is to increase the strength, but this is often offset by +small splits and checks caused by shrinkage. + +|----------------------------------------------------------------------| +| TABLE XV | +|----------------------------------------------------------------------| +| EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD, SHOWN IN | +| RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM | +| THE GREEN CONDITION TO KILN-DRY (3.5 PER CENT) | +| (Forest Service Bul. 70, p. 89) | +|----------------------------------------------------------------------| +| KIND OF STRENGTH | Longleaf | Spruce | Chestnut | +| | pine | | | +|----------------------------+-------------+-------------+-------------| +| | (1) (2) | (1) (2) | (1) (2) | +| | | | | +| Crushing strength parallel | | | | +| to grain | 2.89 2.60 | 3.71 3.41 | 2.83 2.55 | +| Elastic limit in | | | | +| compression | | | | +| parallel to grain | 2.60 2.34 | 3.80 3.49 | 2.40 2.26 | +| Modulus of rupture in | | | | +| bending | 2.50 2.20 | 2.81 2.50 | 2.09 1.82 | +| Stress at elastic limit in | | | | +| bending | 2.90 2.55 | 2.90 2.58 | 2.30 2.00 | +| Crushing strength at right | | | | +| angles to grain | | 2.58 2.48 | | +| Shearing strength parallel | | | | +| to grain | 2.01 1.91 | 2.03 1.95 | 1.55 1.47 | +| Modulus of elasticity in | | | | +| compression parallel to | | | | +| grain | 1.63 1.47 | 2.26 2.08 | 1.43 1.29 | +| Modulus of elasticity in | | | | +| bending | 1.59 1.35 | 1.43 1.23 | 1.44 1.21 | +|----------------------------------------------------------------------| +| NOTE.--The figures in the first column show the relative increase in | +| strength between a green specimen and a kiln-dry specimen of equal | +| size. The figures in the second column show the relative increase of | +| strength of the same block after being dried from a green condition | +| to 3.5 per cent moisture, correction having been made for shrinkage. | +| That is, in the first column the strength values per actual unit of | +| area are used; in the second the values per unit of area of green | +| wood which shrinks to smaller size when dried. | +| | +| See also Cir. 108, Fig. 1, p. 8. | +|----------------------------------------------------------------------| + +The moisture content has a decided bearing also upon the manner +in which wood fails. In compression tests on very dry specimens +the entire piece splits suddenly into pieces before any buckling +takes place (see Fig. 9.), while with wet material the block +gives way gradually, due to the buckling or bending of the walls +of the fibres along one or more shearing planes. (See Fig. 14.) +In bending tests on wet beams, first failure occurs by +compression on top of the beam, gradually extending downward +toward the neutral axis. Finally the beam ruptures at the +bottom. In the case of very dry beams the failure is usually by +splitting or tension on the under side (see Fig. 17.), without +compression on the upper, and is often sudden and without +warning, and even while the load is still increasing. The effect +varies somewhat with different species, chestnut, for example, +becoming more brittle upon drying than do ash, hemlock, and +longleaf pine. The tensile strength of wood is least affected by +drying, as a rule. + +In drying wood no increase in strength results until the free +water is evaporated and the cell walls begin to dry[49]. This +critical point has been called the _fibre-saturation point_. +(See Fig. 24.) Conversely, after the cell walls are saturated +with water, any increase in the amount of water absorbed merely +fills the cavities and intercellular spaces, and has no effect +on the mechanical properties. Hence, soaking green wood does not +lessen its strength unless the water is heated, whereupon a +decided weakening results. + +[Footnote 49: The wood of _Eucalyptus globulus_ (blue gum) +appears to be an exception to this rule. Tiemann says: "The wood +of blue gum begins to shrink immediately from the green +condition, even at 70 to 90 per cent moisture content, instead +of from 30 or 25 per cent as in other species of hardwoods." +Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913, +p. 313.] + +[Illustration: FIG. 24.--Relation of the moisture content to the +various strength values of spruce. FSP = fibre-saturation +point.] + +The strengthening effects of drying, while very marked in the +case of small pieces, may be fully offset in structural timbers +by inherent weakening effects due to the splitting apart of the +wood elements as a result of irregular shrinkage, and in some +cases also to the slitting of the cell walls (see Fig. 25). +Consequently with large timbers in commercial use it is unsafe +to count upon any greater strength, even after seasoning, than +that of the green or fresh condition. + +[Illustration: FIG. 25.--Cross section of the wood of western +larch showing fissures in the thick-walled cells of the late +wood. Highly magnified. _Photo by U. S. Forest Service._] + +In green wood the cells are all intimately joined together and +are at their natural or normal size when saturated with water. +The cell walls may be considered as made up of little particles +with water between them. When wood is dried the films of water +between the particles become thinner and thinner until almost +entirely gone. As a result the cell walls grow thinner with loss +of moisture,--in other words, the cell shrinks. + +It is at once evident that if drying does not take place +uniformly throughout an entire piece of timber, the shrinkage as +a whole cannot be uniform. The process of drying is from the +outside inward, and if the loss of moisture at the surface is +met by a steady capillary current of water from the inside, the +shrinkage, so far as the degree of moisture affected it, would +be uniform. In the best type of dry kilns this condition is +approximated by first heating the wood thoroughly in a moist +atmosphere before allowing drying to begin. + +In air-seasoning and in ordinary dry kilns this condition too +often is not attained, and the result is that a dry shell is +formed which encloses a moist interior. (See Fig. 26.) +Subsequent drying out of the inner portion is rendered more +difficult by this "case-hardened" condition. As the outer part +dries it is prevented from shrinking by the wet interior, which +is still at its greatest volume. This outer portion must either +check open or the fibres become strained in tension. If this +outer shell dries while the fibres are thus strained they become +"set" in this condition, and are no longer in tension. Later +when the inner part dries, it tends to shrink away from the +hardened outer shell, so that the inner fibres are now strained +in tension and the outer fibres are in compression. If the +stress exceeds the cohesion, numerous cracks open up, producing +a "honey-combed" condition, or "hollow-horning," as it is +called. If such a case-hardened stick of wood be resawed, the +two halves will cup from the internal tension and external +compression, with the concave surface inward. + +[Illustration: FIG. 26.--Progress of drying throughout the +length of a chestnut beam, the black spots indicating the +presence of free water in the wood. The first section at the +left was cut one-fourth inch from the end, the next one-half +inch, the next one inch, and all the others one inch apart. The +illustration shows case-hardening very clearly. _Photo by U. S. +Forest Service._] + +For a given surface area the loss of water from wood is always +greater from the ends than from the sides, due to the fact that +the vessels and other water-carriers are cut across, allowing +ready entrance of drying air and outlet for the water vapor. +Water does not flow out of boards and timbers of its own accord, +but must be evaporated, though it may be forced out of very +sappy specimens by heat. In drying a log or pole with the bark +on, most of the water must be evaporated through the ends, but +in the case of peeled timbers and sawn boards the loss is +greatest from the surface because the area exposed is so much +greater. + +The more rapid drying of the ends causes local shrinkage, and +were the material sufficiently plastic the ends would become +bluntly tapering. The rigidity of the wood substance prevents +this and the fibres are split apart. Later, as the remainder of +the stick dries many of the checks will come together, though +some of the largest will remain and even increase in size as the +drying proceeds. (See Fig. 27.) + +[Illustration: FIG. 27.--Excessive season checking. _Photo by U. +S. Forest Service._] + +A wood cell shrinks very little lengthwise. A dry wood cell is, +therefore, practically of the same length as it was in a green +or saturated condition, but is smaller in cross section, has +thinner walls, and a larger cavity. It is at once evident that +this fact makes shrinkage more irregular, for wherever cells +cross each other at a decided angle they will tend to pull apart +upon drying. This occurs wherever pith rays and wood fibres +meet. A considerable portion of every wood is made up of these +rays, which for the most part have their cells lying in a radial +direction instead of longitudinally. (See Frontispiece.) In +pine, over 15,000 of these occur on a square inch of a +tangential section, and even in oak the very large rays which +are readily visible to the eye as flakes on quarter-sawed +material represent scarcely one per cent of the number which the +microscope reveals. + +A pith ray shrinks in height and width, that is, vertically and +tangentially as applied to the position in a standing tree, but +very little in length or radially. The other elements of the +wood shrink radially and tangentially, but almost none +lengthwise or vertically as applied to the tree. Here, then, we +find the shrinkage of the rays tending to shorten a stick of +wood, while the other cells resist it, and the tendency of a +stick to get smaller in circumference is resisted by the endwise +reaction or thrust of the rays. Only in a tangential direction, +or around the stick in direction of the annual rings of growth, +do the two forces coincide. Another factor to the same end is +that the denser bands of late wood are continuous in a +tangential direction, while radially they are separated by +alternate zones of less dense early wood. Consequently the +shrinkage along the rings (tangential) is fully twice as much as +toward the centre (radial). (See Table XIV.) This explains why +some cracks open more and more as drying advances. (See Fig. +27.) + +Although actual shrinkage in length is small, nevertheless the +tendency of the rays to shorten a stick produces strains which +are responsible for some of the splitting open of ties, posts, +and sawed timbers with box heart. At the very centre of a tree +the wood is light and weak, while farther out it becomes denser +and stronger. Longitudinal shrinkage is accordingly least at the +centre and greater toward the outside, tending to become +greatest in the sapwood. When a round or a box-heart timber +dries fast it splits radially, and as drying continues the cleft +widens partly on account of the greater tangential shrinkage and +also because the greater contraction of the outer fibres warps +the sections apart. If a small hardwood stem is split while +green for a short distance at the end and placed where it can +dry out rapidly, the sections will become bow-shaped with the +concave sides out. These various facts, taken together, explain +why, for example, an oak tie, pole, or log may split open its +entire length if drying proceeds rapidly and far enough. Initial +stresses in the living trees produce a similar effect when the +log is sawn into boards. This is especially so in _Eucalyptus +globulus_ and to a less extent with any rapidly grown wood. + +The use of S-shaped thin steel clamps to prevent large checks +and splits is now a common practice in this country with +crossties and poles as it has been for a long time in European +countries. These devices are driven into the butts of the +timbers so as to cross incipient checks and prevent their +widening. In place of the regular S-hook another of crimped iron +has been devised. (See Fig. 28.) Thin straps of iron with one +tapered edge are run between intermeshing cogs and crimped, +after which they may be cut off any length desired. The time for +driving S-irons of either form is when the cracks first appear. + +[Illustration: FIG. 28.--Control of season checking by the use +of S-irons. _Photo by U. S. Forest Service._] + +The tendency of logs to split emphasizes the importance of +converting them into planks or timbers while in a green +condition. Otherwise the presence of large checks may render +much lumber worthless which might have been cut out in good +condition. The loss would not be so great if logs were perfectly +straight-grained, but this is seldom the case, most trees +growing more or less spirally or irregularly. Large pieces crack +more than smaller ones, quartered lumber less than that sawed +through and through, thin pieces, especially veneers, less than +thicker boards. + +In order to prevent cracks at the ends of boards, small straps +of wood may be nailed on them or they may be painted. This +method is usually considered too expensive, except in the case +of valuable material. Squares used for shuttles, furniture, +gun-stocks, and tool handles should always be protected at the +ends. One of the best means is to dip them into melted +paraffine, which seals the ends and prevents loss of moisture +there. Another method is to glue paper on the ends. In some +cases abroad paper is glued on to all the surfaces of valuable +exotic balks. Other substances sometimes employed for the +purpose of sealing the wood are grease, carbolineum, wax, clay, +petroleum, linseed oil, tar, and soluble glass. In place of +solid beams, built-up material is often preferable, as the +disastrous results of season checks are thereby largely overcome +or minimized. + + + +TEMPERATURE + + +The effect of temperature on wood depends very largely upon the +moisture content of the wood and the surrounding medium. If +absolutely dry wood is heated in absolutely dry air the wood +expands. The extent of this expansion is denoted by a +coefficient corresponding to the increase in length or other +dimensions for each degree rise in temperature divided by the +original length or other dimension of the specimen. The +coefficient of linear expansion of oak has been found to be +.00000492; radial expansion, .0000544, or about eleven times the +longitudinal. Spruce expands less than oak, the ratio of radial +to longitudinal expansion being about six to one. Metals and +glass expand equally in all directions, since they are +homogeneous substances, while wood is a complicated structure. +The coefficient of expansion of iron is .0000285, or nearly six +times the coefficient of linear expansion of oak and seven times +that of spruce[50]. + +[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev. +ed.), p. 75.] + +Under ordinary conditions wood contains more or less moisture, +so that the application of heat has a drying effect which is +accompanied by shrinkage. This shrinkage completely obscures the +expansion due to the heating. + +Experiments made at the Yale Forest School revealed the effect +of temperature on the crushing strength of wet wood. In the case +of wet chestnut wood the strength decreases 0.42 per cent for +each degree the water is heated above 60° F.; in the case of +spruce the decrease is 0.32 per cent. + +The effects of high temperature on wet wood are very marked. +Boiling produces a condition of great pliability, especially in +the case of hardwoods. If wood in this condition is bent and +allowed to dry, it rigidly retains the shape of the bend, though +its strength may be somewhat reduced. Except in the case of very +dry wood the effect of cold is to increase the strength and +stiffness of wood. The freezing of any free water in the pores +of the wood will augment these conditions. + +The effect of steaming upon the strength of cross-ties was +investigated by the U.S. Forest Service in 1904. The conclusions +were summarized as follows: + +"(1) The steam at pressure up to 40 pounds applied for 4 hours, +or at a pressure of 20 pounds up to 20 hours, increases the +weight of ties. At 40 pounds' pressure applied for 4 hours and +at 20 pounds for 5 hours the wood began to be scorched. + +"(2) The steamed and saturated wood, when tested immediately +after treatment, exhibited weaknesses in proportion to the +pressure and duration of steaming. (See Table XVI.) If allowed +to air-dry subsequently the specimens regained the greater part +of their strength, provided the pressure and duration had not +exceeded those cited under (1). Subsequent immersion in water of +the steamed wood and dried specimens showed that they were +weaker than natural wood similarly dried and resoaked."[51] + +[Footnote 51: Cir. 39. Experiments on the strength of treated +timber, p. 18.] + +|------------------------------------------------------------------------------------------| +| TABLE XVI | +|------------------------------------------------------------------------------------------| +| EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE | +| (Forest Service, Cir. 39) | +|------------------------------------------------------------------------------------------| +| | Cylinder conditions | Strength | +| |---------------------------------+--------------------------------------------| +| | Steaming | Static | Impact | | +| |---------------------------------+---------------------+----------| Average | +| Treatment | | | | Bending | Compres- | Height | of the | +| | | | | modulus | sion | of drop | three | +| | Period | Pressure | Temperature | of | parallel | causing | strengths | +| | | | | rupture | to grain | complete | | +| | | | | | | failure | | +|-----------+--------+----------+-------------+----------+----------+----------+-----------| +| | | Lbs. per | | Per cent | Per cent | Per cent | Per cent | +| | Hrs. | sq. inch | °F. | | | | | +| | | | | Untreated wood = 100% | +| | | | | | | | | +| Steam, | 4 | | 230[a] | 91.3 | 79.1 | 96.4 | 88.9 | +| at | 4 | 10 | 238 | 78.2 | 93.7 | 93.3 | 88.4 | +| various | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 80.8 | +| pressures | 4 | 30 | 269 | 80.4 | 78.4 | 89.8 | 82.9 | +| | 4 | 40 | 283 | 78.1 | 74.4 | 74.0 | 75.5 | +| | 4 | 50 | 292 | 75.8 | 71.5 | 63.9 | 70.4 | +| | 4 | 100 | 337 | 41.4 | 65.0 | 55.2 | 53.9 | +|-----------+--------+----------+-------------+----------+----------+----------+-----------| +| Steam, | 1 | 20 | 257 | 100.6 | 98.6 | 86.7 | 95.3 | +| for | 2 | 20 | 267 | 88.4 | 93.0 | 107.0 | 96.1 | +| various | 3 | 20 | 260 | 90.0 | 93.6 | 84.1 | 89.2 | +| periods | 4 | 20 | 253 | 83.3 | 84.2 | 91.4 | 86.3 | +| | 5 | 20 | 253 | 85.0 | 78.1 | 84.2 | 82.4 | +| | 6 | 20 | 242 | 95.2 | 89.8 | 76.0 | 87.0 | +| | 10 | 20 | 255 | 73.7 | 82.0 | 76.0 | 77.2 | +| | 20 | 20 | 258 | 67.5 | 65.0 | 99.0 | 77.2 | +|------------------------------------------------------------------------------------------| +| [Footnote a: It will be noted that the temperature was 230°. This is the maximum | +| temperature by the maximum-temperature recording thermometer, and is due to the handling | +| of the exhaust valve. The average temperature was that of exhaust steam.] | +|------------------------------------------------------------------------------------------| + +"(3) A high degree of steaming is injurious to wood in strength +and spike-holding power. The degree of steaming at which +pronounced harm results will depend upon the quality of the wood +and its degree of seasoning, and upon the pressure (temperature) +of steam and the duration of its application. For loblolly pine +the limit of safety is certainly 30 pounds for 4 hours, or 20 +pounds for 6 hours."[52] + +[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table +5.] + +Experiments made at the Yale Forest School showed that steaming +above 30 pounds' gauge pressure reduces the strength of wood +permanently while wet from 25 to 75 per cent. + + + +PRESERVATIVES + + +The exact effects of chemical impregnation upon the mechanical +properties of wood have not been fully determined, though they +have been the subject of considerable investigation.[53] More +depends upon the method of treatment than upon the preservatives +used. Thus preliminary steaming at too high pressure or for too +long a period will materially weaken the wood, (See TEMPERATURE, +above.) + +[Footnote 53: Hatt, W. K.: Experiments on the strength of +treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.] + +The presence of zinc chloride does not weaken wood under static +loading, although the indications are that the wood becomes +brittle under impact. If the solution is too strong it will +decompose the wood. + +Soaking in creosote oil causes wood to swell, and accordingly +decreases the strength to some extent, but not nearly so much so +as soaking in water.[54] + +[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by +the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p. +7.] + +Soaking in kerosene seems to have no significant weakening +effect.[55] + +[Footnote 55: Tiemann, H.D.: Effect of moisture upon the +strength and stiffness of wood. Bul. 70, U. S. Forest Service, +1907, pp. 122-123, tables 43-44.] + + + + +PART III TIMBER TESTING[56] + + + +[Footnote 56: The methods of timber testing described here are +for the most part those employed by the U. S. Forest Service. +See Cir. 38 (rev. ed.), 1909.] + + + +WORKING PLAN + + +Preliminary to making a series of timber tests it is very +important that a working plan be prepared as a guide to the +investigation. This should embrace: (1) the purpose of the +tests; (2) kind, size, condition, and amount of material needed; +(3) full description of the system of marking the pieces; (4) +details of any special apparatus and methods employed; (5) +proposed method of analyzing the data obtained and the nature of +the final report. Great care should be taken in the preparation +of this plan in order that all problems arising may be +anticipated so far as possible and delays and unnecessary work +avoided. A comprehensive study of previous investigations along +the same or related lines should prove very helpful in outlining +the work and preparing the report. (For sample working plan see +Appendix.) + + + +FORMS OF MATERIAL TESTED + + +In general, four forms of material are tested, namely: (1) large +timbers, such as bridge stringers, car sills, large beams, and +other pieces five feet or more in length, of actual sizes and +grades in common use; (2) built-up structural forms and +fastenings, such as built-up beams, trusses, and various kind of +joints; (3) small clear pieces, such as are used in compression, +shear, cleavage, and small cross-breaking tests; (4) +manufactured articles, such as axles, spokes, shafts, +wagon-tongues, cross-arms, insulator pins, barrels, and packing +boxes. + +As the moisture content is of fundamental importance (see WATER +CONTENT, above.), all standard tests are usually made in the +green condition. Another series is also usually run in an +air-dry condition of about 12 per cent moisture. In all cases +the moisture is very carefully determined and stated with the +results in the tables. + + + +SIZE OF TEST SPECIMENS + + +The size of the test specimen must be governed largely by the +purpose for which the test is made. If the effect of a single +factor, such as moisture, is the object of experiment, it is +necessary to use small pieces of wood in order to eliminate so +far as possible all disturbing factors. If the specimens are too +large, it is impossible to secure enough perfect pieces from one +tree to form a series for various tests. Moreover, the drying +process with large timbers is very difficult and irregular, and +requires a long period of time, besides causing checks and +internal stresses which may obscure the results obtained. + +On the other hand, the smaller the dimensions of the test +specimen the greater becomes the relative effect of the inherent +factors affecting the mechanical properties. For example, the +effect of a knot of given size is more serious in a small stick +than in a large one. Moreover, the smaller the specimen the +fewer growth rings it contains, hence there is greater +opportunity for variation due to irregularities of grain. + +Tests on large timbers are considered necessary to furnish +designers data on the probable strength of the different sizes +and grades of timber on the market; their coefficients of +elasticity under bending (since the stiffness rather than the +strength often determines the size of a beam); and the manner of +failure, whether in bending fibre stress or horizontal shear. It +is believed that this information can only be obtained by direct +tests on the different grades of car sills, stringers, and other +material in common use. + +When small pieces are selected for test they very often are +clear and straight-grained, and thus of so much better grade +than the large sticks that tests upon them may not yield unit +values applicable to the larger sizes. Extensive experiments +show, however, (1) that the modulus of elasticity is +approximately the same for large timbers as for small clear +specimens cut from them, and (2) that the fibre stress at +elastic limit for large beams is, except in the weakest timbers, +practically equal to the crushing strength of small clear pieces +of the same material.[57] + +[Footnote 57: Bul. 108, U. S. Forest Service: Tests of +structural timbers, pp. 53-54.] + + + +MOISTURE DETERMINATION + + +In order for tests to be comparable, it is necessary to know the +moisture content of the specimens at the zone of failure. This +is determined from disks an inch thick cut from the timber +immediately after testing. + +In cases, as in large beams, where it is desirable to know not +only the average moisture content but also its distribution +through the timber, the disks are cut up so as to obtain an +outside, a middle, and an inner portion, of approximately equal +areas. Thus in a section 10" x 12" the outer strip would be one +inch wide, and the second one a little more than an inch and a +quarter. Moisture determinations are made for each of the three +portions separately. + +The procedure is as follows: + +(1) Immediately after sawing, loose splinters are removed and +each section is weighed. + +(2) The material is put into a drying oven at 100° C. (212° F.) +and dried until the variation in weight for a period of +twenty-four hours is less than 0.5 per cent. + +(3) The disk is again carefully weighed. + +(4) The loss in weight expressed in per cent of the dry weight +indicates the moisture content of the specimen from which the +specimen was cut. + + + +MACHINE FOR STATIC TESTS + + +The standard screw machines used for metal tests are also used +for wood, but in the case of wood tests the readings must be +taken "on the fly," and the machine operated at a uniform speed +without interruption from beginning to end of the test. This is +on account of the time factor in the strength of wood. (See +SPEED OF TESTING MACHINE, below.) + +The standard machines for static tests can be used for +transverse bending, compression, tension, shear, and cleavage. A +common form consists of three main parts, namely: (1) the +straining mechanism, (2) the weighing apparatus, and (3) the +machinery for communicating motion to the screws. + +The straining mechanism consists of two parts, one of which is a +movable crosshead operated by four (sometimes two or three) +upright steel straining screws which pass through openings in +the platform and bear upward on the bed of the machine upon +which the weighing platform rests as a fulcrum. At the lower +ends of these screws are geared nuts all rotated simultaneously +by a system of gears which cause the movable crosshead to rise +and fall as desired. + +The stationary part of the straining mechanism, which is used +only for tension and cleavage tests, consists of a steel cage +above the movable crosshead and rests directly upon the weighing +platform. The top of the cage contains a square hole into which +one end of the test specimen may be clamped, the crosshead +containing a similar clamp for the other end, in making tension +tests. + +For testing long beams a special form of machine with an +extended platform is used. (See Fig. 29.) + +The weighing platform rests upon knife edges carried by primary +levers of the weighing apparatus, the fulcrum being on the bed +of the machine, and any pressure upon it is directly transmitted +through a series of levers to the weighing beam. This beam is +adjusted by means of a poise running on a screw. In operation +the beam is kept floating by means of another poise moved back +and forth by a screw which is operated by a hand wheel or +automatically. The larger units of stress are read from the +graduations along the side of the beam, while the intermediate +smaller weights are observed on the dial on the rear end of the +beam. + +The machine is driven by power from a shaft or a motor and is so +geared that various speeds are obtainable. One man can operate +it. + +In making tests the operation of the straining screws is always +downward so as to bring pressure to bear upon the weighing +platform. For tests in tension and cleavage the specimen is +placed between the top of the stationary cage and the movable +head and subjected to a pull. For tests in transverse bending, +compression, and cleavage the specimen is placed between the +movable head and the platform, and a direct compression force +applied. + +Testing machines are usually calibrated to a portion of their +capacity before leaving the factory. The delicacy of the +weighing levers is verified by determining the number of pounds +necessary to move the beam between the stops while a load of +1,000 pounds rests on the platform. The usual requirement is +that ten pounds should accomplish this movement. + +The size of machine suitable for compression tests on 2" X 2" +sticks or for 2" X 2" beams with 26 to 36-inch span has a +capacity of 30,000 pounds. + + + +SPEED OF TESTING MACHINE + + +In instructions for making static tests the rate of application +of the stress, _i.e._, the speed of the machine, is given +because the strength of wood varies with the speed at which the +fibres are strained. The speed of the crosshead of the testing +machine is practically never constant, due to mechanical defects +of the apparatus and variations in the speed of the motor, but +so long as it does not exceed 25 per cent the results will not +be appreciably affected. In fact, a change in speed of 50 per +cent will not cause the strength of the wood to vary more than 2 +per cent.[58] + +[Footnote 58: See Tiemann, Harry Donald: The effect of the speed +of testing upon the strength and the standardization of tests +for speed. Proc. Am. Soc. for Testing Materials, Vol. VIII, +Philadelphia, 1908.] + +Following are the formulæ used in determining the speed of the +movable head of the machine in inches per minute (n): + +(1) For endwise compression n = Z l + + Z l^{2} +(2) For beams (centre loading) n = --------- + 6h + + Z l^{2} +(3) For beams (third-pointloading) n = --------- + 5.4h + + Z = rate of fibre strain per inch of fibre length. + l = span of beam or length of compression specimen. + h = height of beam. + +The values commonly used for Z are as follows: + + Bending large beams Z = 0.0007 + Bending small beams Z = 0.0015 + Endwise compression-large specimens Z = 0.0015 + Endwise compression-small " Z = 0.003 + Right-angled compression-large " Z = 0.007 + Right-angled compression-small " Z = 0.015 + Shearing parallel to the grain Z = 0.015 + +Example: At what speed should the crosshead move to give the +required rate of fibre strain in testing a small beam 2" X 2" X +30". (Span = 28".) Substituting these values in equation (2) +above: + (0.0015 X 28^2) + n = ----------------- = 0.1 inch per minute. + (6 X 2) + +In order that tests may be intelligently compared, it is +important that account be taken of the speed at which the stress +was applied. In determining the basis for a ratio between time +and strength the rate of strain, which is controllable, and not +the ratio of stress, which is circumstantial, should be used. In +other words, the rate at which the movable head of the testing +machine descends and not the rate of increase in the load is to +be regulated. This ratio, to which the name _speed-strength +modulus_ has been given, may be expressed as a coefficient +which, if multiplied into any proportional change in speed, will +give the proportional change in strength. This ratio is derived +from empirical curves. (See Table XVII.) + +|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| TABLE XVII TABLE XVII | +|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, _loc. cit._) | +| (Values in parentheses are approximate) | +|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| Rate of fibre strain. | | | | | | | | +| Ten-thousandths inch | 2/3 | 2 | 6 | 18 | 54 | 162 | 486 | +| per minute per inch | | | | | | | | +|-------------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------| +| C | Speed of crosshead. | | | | | | | | +| O | Inches per minute | 0.000383 | 0.00115 | 0.00345 | 0.0103 | 0.0310 | 0.0931 | .279 | +| M | | | | | | | | | +| P |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------| +| R | Specimens | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | +| E |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------| +| S | Relative | | | | | | | | | | | | | | | | | | | | +| S | crushing | | 100.0 | 100.0 | 100.0 | 103.4 | 100.8 | 101.5 | 107.5 | 102.7 | 103.8 | 113.9 | 105.5 | 107.9 | 121.3 | 108.3 | 116.4 | 128.8 | 110.0 |118.9 | +| I | strength | | | | | | | | | | | | | | | | | | | | +| O | | | | | | | | | | | | | | | | | | | | | +| N | Speed-strength | | 0.017 |(0.006)|(0.009)| 0.033 | 0.012 | 0.016 | 0.047 | 0.021 | 0.029 | 0.053 | 0.027 | 0.039 | 0.060 | 0.023 | 0.049 |(0.052)|(0.015)|(0.040)| +| | modulus, _T_ | | | | | | | | | | | | | | | | | | | | +|---+---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------| +| | Speed of crosshead. | | | | | | | | +| | Inches per minute | 0.0072 | 0.0216 | 0.0648 | 0.194 | 0.583 | 1.75 | 5.25 | +| B | | | | | | | | | +| E |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------| +| N | Specimens | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | Wet | Dry | All | +| D |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------| +| I | Relative | | | | | | | | | | | | | | | | | | | | | | +| N | crushing | 97.4 | 99.0 | 98.2 | 100.0 | 100.0 | 100.0 | 105.1 | 102.1 | 103.7 | 111.3 | 105.8 | 108.1 | 117.9 | 108.6 | 112.7 | 123.7 | 109.6 | 116.3 | 126.3 | 110.3 | 118.9 | +| G | strength | | | | | | | | | | | | | | | | | | | | | | +| | | | | | | | | | | | | | | | | | | | | | | | +| | Speed-strength |(0.014)|(0.005)| 0.012 | 0.033 | 0.014 | 0.026 | 0.049 | 0.026 | 0.037 | 0.053 | 0.033 | 0.038 | 0.049 | 0.014 | 0.035 | 0.038 | 0.006 | 0.025 |(0.023)|(0.004)|(0.014)| +| | modulus, _T_ | | | | | | | | | | | | | | | | | | | | | | +|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| NOTE.--The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain | +| of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively. | +|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| + + + +BENDING LARGE BEAMS + + +_Apparatus_: A static bending machine (described above), with a +special crosshead for third-point loading and a long platform +bearing knife-edge supports, is required. (See Fig. 29.) + +[Illustration: FIG. 29.--Static bending test on large beam. Note +arrangement of wire and scale for measuring deflection; also +method of applying load at "third-points."] + +_Preparing the material_: Standard sizes and grades of beams and +timbers in common use are employed. The ends are roughly squared +and the specimen weighed and measured, taking the +cross-sectional dimensions midway of the length. Weights should +be to the nearest pound, lengths to the nearest 0.1 inch, and +cross-sectional dimensions to the nearest 0.01 inch. + +_Marking and sketching_: The butt end of the beam is marked _A_ +and the top end _B_. While facing _A_, the top side is marked +_a_, the right hand _b_, the bottom _c_, the left hand _d_. +Sketches are made of each side and end, showing (1) size, +location, and condition of knots, checks, splits, and other +defects; (2) irregularities of grain; (3) distribution of +heartwood and sapwood; and on the ends: (4) the location of the +pith and the arrangement of the growth rings, (5) number of +rings per inch, and (6) the proportion of late wood. + +The number of rings per inch and the proportion of late wood +should always be determined along a radius or a line normal to +the rings. The average number of rings per inch is the total +number of rings divided by the length of the line crossing them. +The proportion of late wood is equal to the sum of the widths of +the late wood crossed by the line, divided by the length of the +line. Rings per inch should be to the nearest 0.1; late wood to +the nearest 0.1 per cent. + +Since in large beams a great variation in rate of growth and +relative amount of late wood is likely in different parts of the +section, it is advisable to consider the cross section in three +volumes, namely, the upper and lower quarters and the middle +half. The determination should be made upon each volume +separately, and the average for the entire cross section +obtained from these results. + +At the conclusion of the test the failure, as it appears on each +surface, is traced on the sketches, with the failures numbered +in the order of their occurrence. If the beam is subsequently +cut up and used for other tests an additional sketch may be +desirable to show the location of each piece. + +_Adjusting specimen in machine_: The beam is placed in the +machine with the side marked _a_ on top, and with the ends +projecting equally beyond the supports. In order to prevent +crushing of the fibre at the points where the stress is applied +it is necessary to use bearing blocks of maple or other hard +wood with a convex surface in contact with the beam. Roller +bearings should be placed between the bearing blocks and the +knife edges of the crosshead to allow for the shortening due to +flexure. (See Fig. 29.) Third-point loading is used, that is, +the load is applied at two points one-third the span of the beam +apart. (See Fig. 30.) This affords a uniform bending moment +throughout the central third of the beam. + +[Illustration: FIG. 30.--Two methods of loading a beam, namely, +third-point loading (upper), and centre loading (lower).] + +_Measuring the deflection_: The method of measuring the +deflection should be such that any compression at the points of +support or at the application of the load will not affect the +reading. This may be accomplished by driving a small nail near +each end of the beam, the exact location being on the neutral +plane and vertically above each knife-edge support. Between +these nails a fine wire is stretched free of the beam and kept +taut by means of a rubber band or coiled spring on one end. +Behind the wire at a point on the beam midway between the +supports a steel scale graduated to hundredths of an inch is +fastened vertically by means of thumb-tacks or small screws +passing through holes in it. Attachment should be made on the +neutral plane. + +The first reading is made when the scale beam is balanced at +zero load, and afterward at regular increments of the load which +is applied continuously and at a uniform speed. (See SPEED OF +TESTING MACHINE, above.) If desired, however, the load may be +read at regular increments of deflection. The deflection +readings should be to the nearest 0.01 inch. To avoid error due +to parallax, the readings may be taken by means of a reading +telescope about ten feet distant and approximately on a level +with the wire. A mirror fastened to the scale will increase the +accuracy of the readings if the telescope is not used. As in all +tests on timber, the strain must be continuous to rupture, not +intermittent, and readings must be taken "on the fly." The +weighing beam is kept balanced after the yield point is reached +and the maximum load, and at least one point beyond it, noted. + +_Log of the test_: The proper log sheet for this test consists +of a piece of cross-section paper with space at the margin for +notes. (See Fig. 32.) The load in some convenient unit (1,000 to +10,000 pounds, depending upon the dimensions of the specimen) is +entered on the ordinates, the deflection in tenths of an inch on +the abscissæ. The increments of load should be chosen so as to +furnish about ten points on the stress-strain diagram below the +elastic limit. + +As the readings of the wire on the scale are made they are +entered directly in their proper place on the cross-section +paper. In many cases a test should be continued until complete +failure results. The points where the various failures occur are +indicated on the stress-strain diagram. A brief description of +the failure is made on the margin of the log sheet, and the form +traced on the sketches. + +_Disposal of the specimen_: Two one-inch sections are cut from +the region of failure to be used in determining the moisture +content. (See MOISTURE DETERMINATION, above.) A two-inch section +may be cut for subsequent reference and identification, and +possible microscopic study. The remainder of the beam may be cut +into small beams and compression pieces. + +_Calculating the results_: The formulæ used in calculating the +results of tests on large rectangular simple beams loaded at +third points of the span are as follows: + + 0.75 P + (1) J = -------- + b h + + l (P_{1} + 0.75 W) + (2) r = -------------------- + b h^{2} + + l (P + 0.75 W) + (3) R = ---------------- + b h^{2} + + P_{1} l^{3} + (4) E = --------------- + 4.7 D b h^{3} + + 0.87 P_{1} D + (5) S = -------------- + 2 V + + b, h, l = breadth, height, and span of specimen, inches. + D = total deflection at elastic limit, inches. + P = maximum load, pounds. + P_{1} = load at elastic limit, pounds. + E = modulus of elasticity, pounds per square inch. + r = fibre stress at elastic limit, pounds per sq. inch. + R = modulus of rupture, pounds per square inch. + S = elastic resilience or work to elastic limit, inch-pounds + per cu. in. + J = greatest calculated longitudinal shear, pounds per square + inch. + V = volume of beam, cubic inches. + W = weight of the beam. + +In large beams the weight should be taken into account in +calculating the fibre stress. In (2) and (3) three-fourths of +the weight of the beam is added to the load for this reason. + + + +BENDING SMALL BEAMS + + +_Apparatus_: An ordinary static bending machine, a steel I-beam +bearing two adjustable knife-edge supports to rest on the +platform, and a special deflectometer, are required. (See Fig. +31.) + +[Illustration: FIG. 31.--Static bending test on small beam. Note +the use of the deflectometer with indicator and dial for +measuring the deflection; also roller bearings between beam and +supports.] + +_Preparing the material_: The specimens may be of any convenient +size, though beams 2" X 2" X 30" tested over a 28-inch span, are +considered best. The beams are surfaced on all four sides, care +being taken that they are not damaged by the rollers of the +surfacing machine. Material for these tests is sometimes cut +from large beams after failure. The specimens are carefully +weighed in grams, and all dimensions measured to the nearest +0.01 inch. If to be tested in a green or fresh condition the +specimens should be kept in a damp box or covered with moist +sawdust until needed. No defects should be allowed in these +specimens. + +_Marking and sketching_: Sketches are made of each end of the +specimen to show the character of the growth, and after testing, +the manner of failure is shown for all four sides. In obtaining +data regarding the rate of growth and the proportion of late +wood the same procedure is followed as with large beams. + +_Adjusting specimen in machine_: The beam should be correctly +centred in the machine and each end should have a plate with +roller bearings between it and the support. Centre loading is +used. Between the movable head of the machine and the specimen +is placed a bearing block of maple or other hard wood, the lower +surface of which is curved in a direction along the beam, the +curvature of which should be slightly less than that of the beam +at rupture, in order to prevent the edges from crushing into the +fibres of the test piece. + +_Measuring the deflection_: The method of measuring deflection +of large beams can be used for small sizes, but because of the +shortness of the span and consequent slight deformation in the +latter, it is hardly accurate enough for good work. The special +deflectometer shown in Fig. 31 allows closer reading, as it +magnifies the deflection ten times. It rests on two small nails +driven in the beam on the neutral plane and vertically above the +supports. The fine wire on the wheel at the base of the +indicator is attached to another small nail driven in the beam +on the neutral plane midway between the end nails. All three +nails should be in place before the beam is put into the +machine. The indicator is adjustable by means of a thumb-screw +at the base and is set at zero before the load is applied. +Deflections are read to the nearest 0.001 inch. For rate of +application of load see SPEED OF TESTING MACHINE, above. The +speed should be uniform from start to finish without stopping. +Readings must be made "on the fly." + +_Log of the test_: The log sheets used for small beams (see Fig. +32) are the same as for large sizes and the procedure is +practically identical. The stress-strain diagram is continued to +or beyond the maximum load, and in a portion of the tests should +be continued to six-inch deflection or until the specimen fails +to support a load of 200 pounds. Deflection readings for equal +increments of load are taken until well beyond the elastic +limit, after which the scale beam is kept balanced and the load +read for each 0.1 inch deflection. The load and deflection at +first failure, the maximum load, and any points of sudden change +should be shown on the diagram, even though they do not occur at +one of the regular points. A brief description of the failure +and the nature of any defects is entered on the log sheet. + +[Illustration: FIG. 32.--Sample log sheet, giving full details +of a transverse bending test on a small pine beam.] + +_Calculating the results_: The formulæ used in calculating the +results of tests on small rectangular simple beams are as +follows: + + 0.75 P + (1) J = -------- + b h + + 1.5 P_{1} l + (2) r = ------------- + b h^{2} + + 1.5 P l + (3) R = --------- + b h^{2} + + P_{1} l^{3} + (4) E = ------------- + 4 D b h^{3} + + P_{1} D + (5) S = --------- + 2 V + +The same legend is used as in BENDING LARGE BEAMS. The weight of +the beam itself is disregarded. + + + +ENDWISE COMPRESSION + + +_Apparatus_: An ordinary static testing machine and a +compressometer are required. (See Fig. 33.) + +[Illustration: FIG. 33.--Endwise compression test, showing +method of measuring the deformation by means of a +compressometer.] + +_Preparing the material_: Two classes of specimens are commonly +used, namely, (1) posts 24 inches in length, and (2) small clear +blocks approximately 2" X 2" X 8". The specimens are surfaced on +all four sides and both ends squared smoothly and evenly. They +are carefully weighed, measured, rate of growth and proportion +of late wood determined, as in bending tests. After the test a +moisture section is cut and weighed. Ordinarily these specimens +should be free from defects. + +_Sketching_: Sketches are made of each end of the specimens to +show the character of the growth. After testing, the manner of +failure is shown for all four sides, and the various parts of +the failure are numbered in the order of their occurrence. + +_Adjusting specimen in machine_: The compressometer collars are +adjusted, the distance between them being 20 inches for the +posts and 6 inches for the blocks. If the two ends of the blocks +are not exactly parallel a ball-and-socket block can be placed +between the upper end of the specimen and the movable head of +the machine to overcome the irregularity. If the blocks are true +they can simply be stood on end upon the platform and the +movable head allowed to press directly upon the upper end. + +_Measuring the deformation_: The deformation is measured by a +compressometer. (See Fig. 33.) The latter registers to 0.001 +inch. In the case of posts the compression between the collars +is communicated to the four points on the arms by means of brass +rods; with short blocks, as in Fig. 33, the points of the arms +are in direct contact with the collars. The operator lowers the +fulcrum of the apparatus by moving the micrometer screws at such +a rate that the set-screw in the rear end of the upper lever is +kept barely touching the fixed arm below it, being guided by a +bell operated by electric contact. + +_Log of the test_: The load is applied continuously at a uniform +rate of speed. (See SPEED OF TESTING MACHINE, above.) Readings +are taken from the scale of the compressometer at regular +increments of either load or compression. The stress-strain +diagram is continued to at least one deformation point beyond +the maximum load, and in event of sudden failure, the direction +of the curve beyond the maximum point is indicated. A brief +description of the failure is entered on the log sheet. (See +Fig. 34.) + +[Illustration: FIG. 34.--Sample log sheet of an endwise +compression test on a short pine column.] + +In short specimens the failure usually occurs in one or several +planes diagonal to the axis of the specimen. If the ends are +more moist than the middle a crushing may occur on the extreme +ends in a horizontal plane. Such a test is not valid and should +always be culled. If the grain is diagonal or the stress is +unevenly applied a diagonal shear may occur from top to bottom +of the test specimen. Such tests are also invalid and should be +culled. When the plane (or several planes) of failure occurs +through the body of the specimen the test is valid. It may +sometimes be advantageous to allow the extreme ends to dry +slightly before testing in order to bring the planes of failure +within the body. This is a perfectly legitimate procedure +provided no drying is allowed from the sides of the specimen, +and the moisture disk is cut from the region of failure. + +_Calculating the results:_ The formulæ used in calculating the +results of tests on endwise compression are as follows: + + P + (1) C = ----- + A + + P_{1} + (2) c = ------- + A + + P_{1} l + (3) E = --------- + A D + + P D + (4) S = ----- + 2 V + + C = crushing strength, pounds per square inch. + c = fibre strength at elastic limit, pounds per square inch. + A = area of cross section, square inches. + l = distance between centres of collars, inches. + D = total shortening at elastic limit, inches. + V = volume of specimen, cubic inches. + +Remainder of legend as in BENDING LARGE BEAMS, above. + + + +COMPRESSION ACROSS THE GRAIN + + +_Apparatus_: An ordinary static testing machine, a bearing +plate, and a deflectometer are required. (See Fig. 35.) + +[Illustration: FIG. 35.--Compression across the grain. Note +method of measuring the deformation by means of a +deflectomoter.] + +_Preparing the material_: Two classes of specimens are used, +namely, (1) sections of commercial sizes of ties, beams, and +other timbers, and (2) small, clear specimens with the length +several times the width. Sometimes small cubes are tested, but +the results are hardly applicable to conditions in practice. In +(2) the sides are surfaced and the ends squared. The specimens +are then carefully measured and weighed, defects noted, rate of +growth and proportion of late wood determined, as in bending +tests. (See BENDING LARGE BEAMS, above.) After the test a +moisture section is cut and weighed. + +_Sketching_: Sketches are made as in endwise compression tests. +(See ENDWISE COMPRESSION, above.) + +_Adjusting specimen in machine_: The specimen is laid +horizontally upon the platform of the machine and a steel +bearing plate placed on its upper surface immediately beneath +the centre of the movable head. For the larger specimens this +plate is six inches wide; for the smaller sizes, two inches +wide. The plate in all cases projects over the edges of the test +piece, and in no case should the length of the latter be less +than four times the width of the plate. + +_Measuring the deformation_: The compression is measured by +means of a deflectometer (see Fig. 35), which, after the first +increment of load is applied, is adjusted (by means of a small +set screw) to read zero. The actual downward motion of the +movable head (corresponding to the compression of the specimen) +is multiplied ten times on the scale from which the readings are +made. + +_Log of the test_: The load is applied continuously and at +uniform speed (see SPEED OF TESTING MACHINE, above), until well +beyond the elastic limit. The compression readings are taken at +regular load increments and entered on the cross-section paper +in the usual way. Usually there is no real maximum load in this +case, as the strength continually increases as the fibres are +crushed more compactly together. + +_Calculating the results_: Ordinarily only the fibre stress at +the elastic limit (c) is computed. It is equal to the load at +elastic limit (P_{1}) divided by the area under the plate (B). +{ P_{1} } +{ c = ------- } +{ B } + + + +SHEAR ALONG THE GRAIN + + +_Apparatus_: An ordinary static testing machine and a special +tool designed for producing single shear are required. (See +Figs. 36 and 37.) This shearing apparatus consists of a solid +steel frame with set screws for clamping the block within it +firmly in a vertical position. In the centre of the frame is a +vertical slot in which a square-edged steel plate slides freely. +When the testing block is in position, this plate impinges +squarely along the upper surface of the tenon or lip, which, as +vertical pressure is applied, shears off. + +[Illustration: Fig. 36.--Vertical section of shearing tool.] + +[Illustration: FIG. 37.--Front view of shearing tool with test +specimen and steel plate in position for testing.] + +_Preparing the material_: The specimens are usually in the form +of small, clear, straight-grained blocks with a projecting tenon +or lip to be sheared off. Two common forms and sizes are shown +in Figure 38. Part of the blocks are cut so that the shearing +surface is parallel to the growth rings, or tangential; others +at right angles to the growth rings, or radial. It is important +that the upper surface of the tenon or lip be sawed exactly +parallel to the base of the block. When the form with a tenon is +used the under cut is extended a short distance horizontally +into the block to prevent any compression from below. + +[Illustration: FIG. 38.--Two forms of shear test specimens.] + +In designing a shearing specimen it is necessary to take into +consideration the proportions of the area of shear, since, if +the length of the portion to be sheared off is too great in the +direction of the shearing face, failure would occur by +compression before the piece would shear. Inasmuch as the +endwise compressive strength is sometimes not more than five +times the shearing strength, the shearing surface should be less +than five times the surface to which the load is applied. This +condition is fulfilled in the specimens illustrated. + +Shearing specimens are frequently cut from beams after testing. +In this case the specific gravity (dry), proportion of late +wood, and rate of growth are assumed to be the same as already +recorded for the beams. In specimens not so taken, these +quantities are determined in the usual way. The sheared-off +portion is used for a moisture section. + +_Adjusting specimen in machine_: The test specimen is placed in +the shearing apparatus with the tenon or lip under the sliding +plate, which is centred under the movable head of the machine. +(See Fig. 39.) In order to reduce to a minimum the friction due +to the lateral pressure of the plate against the bearings of the +slot, the apparatus is sometimes placed upon several parallel +steel rods to form a roller base. A slight initial load is +applied to take up the lost motion of the machinery, and the +beam balanced. + +[Illustration: FIG. 39.--Making a shearing test.] + +_Log of the test_: The load is applied continuously and at a +uniform rate until failure, but no deformations are measured. +The points noted are the maximum load and the length of time +required to reach it. Sketches are made of the failure. If the +failure is not pure shear the test is culled. + +The shearing strength per square inch is found by dividing the + { P } +maximum load by the cross-sectional area. { Q = --- } + { A } + + + +IMPACT TEST + + +_Apparatus_: There are several types of impact testing +machines.[59] One of the simplest and most efficient for use +with wood is illustrated in Figure 40. The base of the machine +is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500 +pounds. Two upright columns, each 8 feet long, act as guides for +the striking head. At the top of the column is the hoisting +mechanism for raising or lowering the striking weights. The +power for operating the machine is furnished by a motor set on +the top. The hoisting-mechanism is all controlled by a single +operating lever, shown on the side of the column, whereby the +striking weight may be raised, lowered, or stopped at the will +of the operator. There is an automatic safety device for +stopping the machine when the weight reaches the top. + +[Footnote 59: For description of U.S. Forest Service automatic +and autographic impact testing machine, see Proc. Am. Soc. for +Testing Materials, Vol. VIII, 1908, pp. 538-540.] + +[Illustration: FIG. 40.--Impact testing machine.] + +The weight is lifted by a chain, one end of which passes over a +sprocket wheel in the hoisting mechanism. On the lower end of +the chain is hung an electro-magnet of sufficient magnetic +strength to support the heaviest striking weights. When it is +desired to drop the striking weight the electric current is +broken and reversed by means of an automatic switch and current +breaker. The height of drop may be regulated by setting at the +desired height on one of the columns a tripping pin which throws +the switch on the magnet and so breaks and reverses the current. + +There are four striking weights, weighing respectively 50, 100, +250, and 500 pounds, any one of which may be used, depending +upon the desired energy of blow. When used for compression tests +a flat steel head six inches in diameter is screwed into the +lower end of the weight. For transverse tests, a well-rounded +knife edge is screwed into the weight in place of the flat head. +Knife edges for supporting the ends of the specimen to be +tested, are securely bolted to the base of the machine. + +The record of the behavior of the specimen at time of impact is +traced upon a revolving drum by a pencil fixed in the striking +head. (See Fig. 41.) When a drop is made the pencil comes in +contact with the drum and is held in place by a spring. The drum +is revolved very slowly, either automatically or by hand. The +speed of the drum can be recorded by a pencil in the end of a +tuning fork which gives a known number of vibrations per second. + +[Illustration: FIG. 41.--Drum record of impact bending test.] + +One size of this machine will handle specimens for transverse +tests 9 inches wide and 6-foot span; the other, 12 inches wide +and 8-foot span. For compression tests a free fall of about 6.5 +feet may be obtained. For transverse tests the fall is a little +less, depending upon the size of the specimen. + +The machine is calibrated by dropping the hammer upon a copper +cylinder. The axial compression of the plug is noted. The energy +used in static tests to produce this axial compression under +stress in a like piece of metal is determined. The external +energy of the blow (_i.e._, the weight of the hammer X the +height of drop) is compared with the energy used in static tests +at equal amounts of compression. For instance: + +Energy delivered, impact test 35,000 inch-pounds +Energy computed from static test .26,400 " " +Efficiency of blow of hammer .75.3 per cent. + +_Preparing the material_: The material used in making impact +tests is of the same size and prepared in the same way as for +static bending and compression tests. Bending in impact tests is +more commonly used than compression, and small beams with +28-inch span are usually employed. + +_Method_: In making an impact bending test the hammer is allowed +to rest upon the specimen and a zero or datum line is drawn. The +hammer is then dropped from increasing heights and drum records +taken until first failure. The first drop is one inch and the +increase is by increments of one inch until a height of ten +inches is reached, after which increments of two inches are used +until complete failure occurs or 6-inch deflection is secured. + +The 50-pound hammer is used when with drops up to 68 inches it +is reasonably certain it will produce complete failure or 6-inch +deflection in the case of all specimens of a species; for all +other species a 100-pound hammer is used. + +_Results_: The tracing on the drum (see Fig. 41) represents the +actual deflection of the stick and the subsequent rebounds for +each drop. The distance from the lowest point in each case to +the datum line is measured and its square in tenths of a square +inch entered as an abscissa on cross-section paper, with the +height of drop in inches as the ordinate. The elastic limit is +that point on the diagram where the square of the deflection +begins to increase more rapidly than the height of drop. The +difference between the datum line and the final resting point +after each drop represents the set the material has received. + +The formulæ used in calculating the results of impact tests in +bending when the load is applied at the centre up to the elastic +limit are as follows: + + 3 W H l + (1) r = ----------- + D b h^{2} + + F S l^{2} + (2) E = ----------- + 6 D h + + W H + (3) S = ------- + l b h + + H = height of drop of hammer, including deflection, inches. + S = modulus of elastic resilience, inch-pounds per cubic inch. + W = weight of hammer, pounds. + +Remainder of legend as in BENDING LARGE BEAMS, above. + + + +HARDNESS TEST: ABRASION AND INDENTATION + + +_Abrasion_: The machine used by the U.S. Forest Service is a +modified form of the Dorry abrasion machine. (See Fig. 42.) Upon +the revolving horizontal disk is glued a commercial sandpaper, +known as garnet paper, which is commonly employed in factories +in finishing wood. + +[Illustration: FIG. 42.--Abrasion machine for testing the +wearing qualities of woods.] + +A small block of the wood to be tested is fixed in one clamp and +a similar block of some wood chosen as a standard, as sugar +maple, at 10 per cent moisture, in the opposite, and held +against the same zone of sandpaper by a weight of 26 pounds +each. The size of the section under abrasion for each specimen +is 2" X 2". The conditions for wear are the same for both +specimens. The speed of rotation is 68 revolutions a minute. + +The test is continued until the standard specimen is worn a +specified amount, which varies with the kind of wood under test. +A comparison of the wear of the two blocks affords a fair idea +of their relative resistance to abrasion. + +Another method makes use of a sand blast to abrade the woods and +is the one employed in New South Wales.[60] The apparatus +consists essentially of a nozzle through which sand can be +propelled at a high velocity against the test specimen by means +of a steam jet. + +[Footnote 60: See Warren, W.H.: The strength, elasticity, and +other properties of New South Wales hardwood timbers. Dept. +For., N.S.W., Sydney, 1911, pp. 88-95.] + +The wood to be tested is cut into blocks 3" X 3" X 1', and these +are weighed to the nearest grain just before placing in the +apparatus. Steam from the boiler at a pressure of about 43 +pounds per square inch is ejected from a nozzle in such a way +that particles of fine quartz sand are caught up and thrown +violently against the block which is being rotated. Only +superheated steam strikes the block, thus leaving the wood dry. +The test is continued for two minutes, after which the specimen +is removed and immediately weighed. + +By comparison with the original weight the loss from abrasion is +determined, and by comparison with a certain wood chosen as a +standard, a coefficient of wear-resistance can be obtained. The +amount of wear will vary more or less according to the surface +exposed, and in these tests quarter-sawed material was used with +the edge grain to the blast. + +_Indentation_: The tool used for this test consists of a punch +with a hemispherical end or steel ball having a diameter of +0.444 inch, giving a surface area of one-fourth square inch. It +is fitted with a guard plate, which works loosely until the +penetration has progressed to a depth of 0.222 inch, whereupon +it tightens. (See Fig. 43.) The effect is that of sinking a ball +half its diameter into the specimen. This apparatus is fitted +into the movable head of the static testing machine. + +[Illustration: FIG. 43.--Design of tool for testing the hardness +of woods by indentation.] + +The wood to be tested is cut square with the grain into +rectangular blocks measuring 2" X 2" X 6". A block is placed on +the platform and the end of the punch forced into the wood at +the rate of 0.25 inch per minute. The operator keeps moving the +small handle of the guard plate back and forth until it +tightens. At this instant the load is read and recorded. + +Two penetrations each are made on the tangential and radial +surfaces, and one on each end of every specimen tested. + +In choosing the places on the block for the indentations, effort +should be made to get a fair average of heartwood and sapwood, +fine and coarse grain, early and late wood. + +Another method of testing by indentation involves the use of a +right-angled cone instead of a ball. For details of this test as +used in New South Wales see _loc. cit._, pp. 86-87. + + + +CLEAVAGE TEST + + +A static testing machine and a special cleavage testing device +are required. (See Fig. 44.) The latter consists essentially of +two hooks, one of which is suspended from the centre of the top +of the cage, the other extended above the movable head. + +[Illustration: FIG. 44.--Design of tool for cleavage test.] + +The specimens are 2" X 2" X 3.75". At one end a one-inch hole is +bored, with its centre equidistant from the two sides and 0.25 +inch from the end. (See Fig. 45.) This makes the cross section +to be tested 2" X 3". Some of the blocks are cut radially and +some tangentially, as indicated in the figure. + +[Illustration: FIG. 45.--Design of cleavage test specimen.] + +The free ends of the hooks are fitted into the notch in the end +of the specimen. The movable head of the machine is then made to +descend at the rate of 0.25 inch per minute, pulling apart the +hooks and splitting the block. The maximum load only is taken +and the result expressed in pounds per square inch of width. A +piece one-half inch thick is split off parallel to the failure +and used for moisture determination. + + + +TENSION TEST PARALLEL TO THE GRAIN + + +Since the tensile strength of wood parallel to the grain is +greater than the compressive strength, and exceedingly greater +than the shearing strength, it is very difficult to make +satisfactory tension tests, as the head and shoulders of the +test specimen (which is subjected to both compression and shear) +must be stronger than the portion subjected to a pure tensile +stress. + +Various designs of test specimens have been made. The one first +employed by the Division of Forestry[61] was prepared as +follows: Sticks were cut measuring 1.5" X 2.5" X 16". The +thickness at the centre was then reduced to three-eighths of an +inch by cutting out circular segments with a band saw. This left +a breaking section of 2.5" X 0.375". Care was taken to cut the +specimen as nearly parallel to the grain as possible, so that +its failure would occur in a condition of pure tension. The +specimen was then placed between the plane wedge-shaped steel +grips of the cage and the movable head of the static machine and +pulled in two. Only the maximum load was recorded. (See Fig. 46, +No. 1.) + +[Illustration: FIG. 46.--Designs of tension test specimens used +in United States.] + +[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.] + +The difficulty of making such tests compared with the minor +importance of the results is so great that they are at present +omitted by the U.S. Forest Service. A form of specimen is +suggested, however, and is as follows: "A rod of wood about one +inch in diameter is bored by a hollow drill from the stick to be +tested. The ends of this rod are inserted and glued in +corresponding holes in permanent hardwood wedges. The specimen +is then submitted to the ordinary tension test. The broken ends +are punched from the wedges."[62] (See Fig. 46, No. 2.) + +[Footnote 62: Cir. 38: Instructions to engineers of timber +tests, 1906, p. 24.] + +The form used by the Department of Forestry of New South +Wales[63] is as shown in Fig. 47. The specimen has a total +length of 41 inches and is circular in cross section. On each +end is a head 4 inches in diameter and 7 inches long. Below each +head is a shoulder 8.5 inches long, which tapers from a diameter +of 2.75 inches to 1.25 inches. In the middle is a cylindrical +portion 1.25 inches in diameter and 10 inches long. + +[Illustration: FIG. 47.--Design of tension test specimen used in +New South Wales.] + +[Footnote 63: Warren, W.H.: The strength, elasticity, and other +properties of New South Wales hardwood timbers, 1911, pp. +58-62.] + +In making the test the specimen is fitted in the machine, and an +extensometer attached to the middle portion and arranged to +record the extension between the gauge points 8 inches apart. +The area of the cross section then is 1.226 square inches, and +the tensile strength is equal to the total breaking load applied +divided by this area. + + + +TENSION TEST AT RIGHT ANGLES TO THE GRAIN + + +A static testing machine and a special testing device (see Fig. +48) are required. The latter consists essentially of two double +hooks or clamps, one of which is suspended from the centre of +the top of the cage, the other extended above the movable head. +The specimens are 2" X 2" X 2.5". At each end a one-inch hole is +bored with its centre equidistant from the two sides and 0.25 +inch from the ends. This makes the cross section to be tested 1" +X 2". + +[Illustration: FIG. 48.--Design of tool and specimen for testing +tension at right angles to the grain.] + +The free ends of the clamps are fitted into the notches in the +ends of the specimen. The movable head of the machine is then +made to descend at the rate of 0.25 inch per minute, pulling the +specimen in two at right angles to the grain. The maximum load +only is taken and the result expressed in pounds per inch of +width. A piece one-half inch thick is split off parallel to the +failure and used for moisture determination. + + + +TORSION TEST[64] + + +[Footnote 64: Wood is so seldom subjected to a pure stress of +this kind that the torsion test is usually omitted.] + +_Apparatus_: The torsion test is made in a Riehle-Miller +torsional testing machine or its equivalent. (See Fig. 49.) + +[Illustration: FIG. 49.--Making a torsion test on hickory.] + +_Preparation of material_: The test pieces are cylindrical, 1.5 +inches in diameter and 18 inches gauge length, with squared ends +4 inches long joined to the cylindrical portion with a fillet. +The dimensions are carefully measured, and the usual data +obtained in regard to the rate of growth, proportion of late +wood, location and kind of defects. The weight of the +cylindrical portion of the specimen is obtained after the test. + +_Making the test_: After the specimen is fitted in the machine +the load is applied continuously at the rate of 22° per minute. +A troptometer is used in measuring the deformation. Readings are +made until failure occurs, the points being entered on the +cross-section paper. The character of the failure is described. +Moisture determinations are made by the disk method. + +_Results_: The conditions of ultimate rupture due to torsion +appear not to be governed by definite mathematical laws; but +where the material is not overstrained, laws may be assumed +which are sufficiently exact for practical cases. The formulæ +commonly used for computations are as follows: + + 5.1 M + (1) T = ------- + c^{3} + + 114.6 T f + (2) G = ----------- + a c + + a = angle measured by troptometer at elastic limit, in + degrees. + c = diameter of specimen, inches. + f = gauge length of specimen, inches. _G_ = modulus of + elasticity in shear across the grain, pounds per square + inch. + M = moment of torsion at elastic limit, inch-pounds. + T = outer fibre torsional stress at elastic limit, pounds per + square inch. + + + +SPECIAL TESTS + + +_Spike-pulling Test_ + +Spike-pulling tests apply to problems of railroad maintenance, +and the results are used to compare the spike-holding powers of +various woods, both untreated and treated with different +preservatives, and the efficiency of various forms of spikes. +Special tests are also made in which the spike is subjected to a +transverse load applied repetitively by a blow. + +For details of tests and results see: + +Cir. 38, U.S.F.S.: Instructions to engineers of timber tests, +p. 26. Cir. 46, U.S.F.S.: Holding force of railroad spikes in +wooden ties. Bul. 118, U.S.F.S,: Prolonging the life of +cross-ties, pp. 37-40. + + +_Packing Boxes_ + +Special tests on the strength of packing boxes of various woods +have been made by the U.S. Forest Service to determine the +merits of different kinds of woods as box material with the view +of substituting new kinds for the more expensive ones now in +use. The methods of tests consisted in applying a load along the +diagonal of a box, an action similar to that which occurs when a +box is dropped on one of its corners. The load was measured at +each one-fourth inch in deflection, and notes were made of the +primary and subsequent failures. + +For details of tests and results, see: + +Cir. 47, U.S.F.S.: Strength of packing boxes of various woods. +Cir. 214, U.S.F.S.: Tests of packing boxes of various forms. + + +_Vehicle and Implement Woods_ + +Tests were made by the U.S. Forest Service to obtain a better +knowledge of the mechanical properties of the woods at present +used in the manufacture of vehicles and implements and of those +which might be substituted for them. Tests were made upon the +following materials: hickory buggy spokes (see Fig. 5); hickory +and red oak buggy shafts; wagon tongues; Douglas fir and +southern pine cultivator poles. + +Details of the tests and results may be found in: + +Cir. 142, U.S.F.S.: Tests on vehicle and implement woods. + + +_Cross-arms_ + +In tests by the U.S. Forest Service on cross-arms a special +apparatus was devised in which the load was distributed along +the arm as in actual practice. The load was applied by rods +passing through the pinholes in the arms. Nuts on these rods +pulled down on the wooden bearing-blocks shaped to fit the upper +side of the arm. The lower ends of these rods were attached to a +system of equalizing levers, so arranged that the load at each +pinhole would be the same. In all the tests the load was applied +vertically by means of the static machine. + +See Cir. 204, U.S.F.S.: Strength tests of cross-arms. + + +_Other Tests_ + +Many other kinds of tests are made as occasion demands. One kind +consists of barrels and liquid containers, match-boxes, and +explosive containers. These articles are subjected to shocks +such as they would receive in transit and in handling, and also +to hydraulic pressure. + +One of the most important tests from a practical standpoint is +that of built-up structures such as compounded beams composed of +small pieces bolted together, mortised joints, wooden trusses, +etc. Tests of this kind can best be worked out according to the +specific requirements in each case. + + + + +APPENDIX + + + +SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE + +MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES + +Working Plan No. 124 + + +PURPOSE OF WORK + +It is the general purpose of the work here outlined to provide: + +(_a_) Reliable data for comparing the mechanical properties of +various species; + +(_b_) Data for the establishment of correct strength functions +or working stresses; + +(_c_) Data upon which may be based analyses of the influence on +the mechanical properties of such factors as: + +Locality; + +Distance of timber from the pith of the tree; + +Height of timber in the tree; + +Change from the green to the air-dried condition, etc. + +The mechanical properties which will be considered and the +principal tests used to determine them are as follows: + +Strength and stiffness-- + Static bending; + Compression parallel to grain; + Compression perpendicular to grain; + Shear. + +Toughness-- + Impact bending; + Static bending; + Work to maximum load and total work. + +Cleavability-- + Cleavage test. + +Hardness-- + Modification of Janka ball test for surface hardness. + + +MATERIAL + + +_Selection and Number of Trees_ + +The material will be from trees selected in the forest by one +qualified to determine the species. From each locality, three to +five dominant trees of merchantable size and approximately +average age will be so chosen as to be representative of the +dominant trees of the species. Each species will eventually be +represented by trees from five to ten localities. These +localities will be so chosen as to be representative of the +commercial range of the species. Trees from one to three +localities will be used to represent each species until most of +the important species have been tested. + +The 16-foot butt log will be taken from each tree selected and +the entire merchantable hole of one average tree for each +species. + + +_Field Notes and Shipping Instructions_ + +Field notes as outlined in Form--_a_ Shipment Description, +Manual of the Branch of Products, will be fully and carefully +made by the collector. The age of each tree selected will be +recorded and any other information likely to be of interest or +importance will also be made a part of these field notes. Each +log will have the bark left on. It will be plainly marked in +accordance with directions given under Detailed Instructions. +All material will be shipped to the laboratory immediately after +being cut. No trees will be cut until the collector is notified +that the laboratory is ready to receive the material. + + +DETAILED INSTRUCTIONS + + +_Part of Tree to be Tested_ + +(_a_) For determining the value of tree and locality and the +influence on the mechanical properties of distance from the +pith, a 4-foot bolt will be cut from the top end of each 16-foot +butt log. + +(_b_) For investigating the variation of properties with the +height of timber in the tree, all the logs from one average tree +will be used. + +(_c_) For investigating the effect of drying the wood, the bolt +next below that provided for in (_a_) will be used in the case +of one tree from each locality. + + +_Marking and Grouping of Material_ + +The marking will be standard except as noted. Each log will be +considered a "piece." The piece numbers will be plainly marked +upon the butt end of each log by the collector. The north side +of each log will also be marked. + +When only one bolt from a tree is used it will be designated by +the number of the log from which it is cut. Whenever more than +one bolt is taken from a tree, each 4-foot bolt or length of +trunk will be given a letter (mark), _a, b, c,_ etc., beginning +at the stump. + +All bolts will be sawed into 2-1/2" X 2-1/2" sticks and the +sticks marked according to the sketch, Fig. 50. The letters _N, +E, S,_ and _W_ indicate the cardinal points when known; when +these are unknown, _H, K, L,_ and _M_ will be used. Thus, _N5, +K8, S7, M4_ are stick numbers, the letter being a part of the +stick number. + +[Illustration: FIG. 50.--Method of cutting and marking test +specimens.] + +Only straight-grained specimens, free from defects which will +affect their strength, will be tested. + + +_Care of Material_ + +No material will be kept in the bolt or log long enough to be +damaged or disfigured by checks, rot, or stains. + +_Green material_: The material to be tested green will be kept +in a green state by being submerged in water until near the time +of test. It will then be surfaced, sawed to length, and stored +in damp sawdust at a temperature of 70°F. (as nearly as +practicable) until time of test. Care should be taken to avoid +as much as possible the storage of green material in any form. + +_Air-dry material_: The material to be air-dried will be cut +into sticks 2-1/2" X 2-1/2" X 4'. The ends of these sticks will +be paraffined to prevent checking. This material will be so +piled as to leave an air space of at least one-half inch on each +side of each stick, and in such a place that it will be +protected from sunshine, rain, snow, and moisture from the +ground. The sticks will be surfaced and cut to length just +previous to test. + + +_Order of Tests_ + +The order of tests in all cases will be such as to eliminate so +far as possible from the comparisons the effect of changes of +condition of the specimens due to such factors as storage and +weather conditions. + +The material used for determining the effect of height in tree +will be tested in such order that the average time elapsing from +time of cutting to time of test will be approximately the same +for all bolts from any one tree. + + +_Tests on Green Material_ + +The tests on all bolts, except those from which a comparison of +green and dry timber is to be gotten, will be as follows: + +_Static bending_: One stick from each pair. A pair consists of +two adjacent sticks equidistant from the pith, as _N_7 and _N_8, +or _H_5 and _H_6. + +_Impact bending_: Four sticks; one to be taken from near the +pith; one from near the periphery; and two representative of the +cross section. + +_Compression parallel to grain_: One specimen from each stick. +These will be marked "1" in addition to the number of the stick +from which they are taken. + +_Compression perpendicular to grain_: One specimen from each of +50 per cent of the static bending sticks. These will be marked +"2" in addition to the number of the stick from which they are +cut. + +_Hardness_: One specimen from each of the other 50 per cent of +the static bending sticks. These specimens will be marked "4." + +_Shear_: Six specimens from sticks not tested in bending or from +the ends cut off in preparing the bending specimens. Two +specimens will be taken from near the pith; two from near the +periphery; and two that are representative of the average +growth. One of each two will be tested in radial shear and the +other in tangential shear. These specimens will have the mark +"3." + +_Cleavage_: Six specimens chosen and divided just as those for +shearing. These specimens will have the mark "5." (For sketches +showing radial and tangential cleavage, see Fig. 45.) + +When it is impossible to secure clear specimens for all of the +above tests, tests will have precedence in the order in which +they are named. + + +_Tests to Determine the Effect of Air-drying_ + +These tests will be made on material from the adjacent bolts +mentioned in "_c_" under Part of Tree to be Tested. Both bolts +will be cut as outlined above. One-half the sticks from each +bolt will be tested green, the other half will be air-dried and +tested. The division of green and air-dry will be according to +the following scheme: + + STICK NUMBERS + +Lower bolt, 1, 4, 5, 8, 9, } Tested + etc. } green +Upper bolt, 2, 3, 6, 7, 10, } + +Lower bolt, 2, 3, 6, 7, 10, } Air-dried + etc. } and +Upper bolt, 1, 4, 5, 8, 9, } tested + +All green sticks from these two bolts will be tested as if they +were from the same bolt and according to the plan previously +outlined for green material from single bolts. The tests on the +air-dried material will be the same as on the green except for +the difference of seasoning. + +The material will be tested at as near 12 per cent moisture as +is practicable. The approximate weight of the air-dried +specimens at 12 per cent moisture will be determined by +measuring while green 20 per cent of the sticks to be air-dried +and assuming their dry gravity to be the same as that of the +specimens tested green. This 20 per cent will be weighed as +often as is necessary to determine the proper time of test. + + +_Methods of Test_ + +All tests will be made according to Circular 38 except in case +of conflict with the instructions given below: + +_Static bending_: The tests will be on specimens 2" X 2" X 30" +on 28-inch span. Load will be applied at the centre. + +In all tests the load-deflection curve will be carried to or +beyond the maximum load. In one-third of the tests the +load-deflection curve will be continued to 6-inch deflection, or +till the specimen fails to support a 200-pound load. Deflection +readings for equal increments of load will be taken until well +past the elastic limit, after which the scale beam will be kept +balanced and the load read for each 0.1-inch deflection. The +load and deflection at first failure, maximum load and points of +sudden change, will be shown on the curve sheet even if they do +not occur at one of the regular load or deflection increments. + +_Impact bending_: The impact bending tests will be on specimens +of the same size as those used in static bending. The span will +be 28 inches. + +The tests will be by increment drop. The first drop will be 1 +inch and the increase will be by increments of 1 inch till a +height of 10 inches is reached, after which increments of 2 +inches will be used until complete failure occurs or 6-inch +deflection is secured. + +A 50-pound hammer will be used when with drops up to 68 inches +it is practically certain that it will produce complete failure +or 6-inch deflection in the case of all specimens of a species. +For all other species, a 100-pound hammer will be used. + +In all cases drum records will be made until first failure. Also +the height of drop causing complete failure or 6-inch deflection +will be noted. + +_Compression parallel to grain_: This test will be on specimens +2" X 2" X 8" in size. On 20 per cent of these tests +load-compression curves for a 6-inch centrally located gauge +length will be taken. Readings will be continued until the +elastic limit is well passed. The other 80 per cent of the tests +will be made for the purpose of obtaining the maximum load only. + +_Compression perpendicular to grain_: This test will be on +specimens 2" X 2" X 6" in size. The bearing plates will be 2 +inches wide. The rate of descent of the moving head will be +0.024 inch per minute. The load-compression curve will be +plotted to 0.1 inch compression and the test will then be +discontinued. + +_Hardness_: The tool shown in Fig. 43 (an adaptation of the +apparatus used by the German investigator, Janka) will be used. +The rate of descent of the moving head will be 0.25 inch per +minute. When the penetration has progressed to the point at +which the plate "_a_" becomes tight, due to being pressed +against the wood, the load will be read and recorded. + +Two penetrations will be made on a tangential surface, two on a +radial, and one on each end of each specimen tested. The choice +between the two radial and between the two tangential surfaces +and the distribution of the penetrations over the surfaces will +be so made as to get a fair average of heart and sap, slow and +fast growth, and spring and summer wood. Specimens will be 2" X +2" X 6". + +_Shear_: The tests will be made with a tool slightly modified +from that shown in Circular 38. The speed of descent of head +will be 0.015 inch per minute. The only measurements to be made +are those of the shearing area. The offset will be 1/8 inch. +Specimens will be 2" X 2" X 2-1/2" in size. (For definition of +offset and form of test specimen, see Fig. 38.) + +_Cleavage_: The cleavage tests will be made on specimens of the +form and size shown in Fig. 45. The apparatus will be as shown +in Fig. 44. The maximum load only will be taken and the result +expressed in pounds per inch of width. The speed of the moving +head will be 0.25 inch per minute. + + +_Moisture Determinations_ + +Moisture determinations will be made on all specimens tested +except those to be photographed or kept for exhibit. A 1-inch +disk will be cut from near the point of failure of bending and +compression parallel specimens, from the portion under the plate +in the case of the compression perpendicular specimens, and from +the centre of the hardness test specimens. The beads from the +shear specimens will be used as moisture disks. In the case of +the cleavage specimens a piece 1/2 inch thick will be split off +parallel to the failure and used as a moisture disk. + + +RECORDS + + +All records will be standard. + + +PHOTOGRAPHS + + +_Cross Sections_ + +Just before cutting into sticks, the freshly cut end of at least +one bolt from each tree will be photographed. A scale of inches +will be shown in this photograph. + + +_Specimens_ + +Three photographs will be made of a group consisting of four 2" +X 2" X 30" specimens chosen from the material from each +locality. Two of these specimens will be representative of +average growth, one of fast and one of slow growth. These +photographs will show radial, tangential, and end surfaces for +each specimen. + + +_Failures_ + +Typical and abnormal failures of material from each site will be +photographed. + + +_Disposition of Material_ + +The specimens photographed to show typical and abnormal failures +will be saved for purposes of exhibit until deemed by the person +in charge of the laboratory to be of no further value. + + + +SHRINKAGE AND SPECIFIC GRAVITY + +Appendix to Working Plan 124 + + +PURPOSE OF WORK + + +It is the purpose of this work to secure data on the shrinkage +and specific gravity of woods tested under Project 124. The +figures to be obtained are for use as average working values +rather than as the basis for a detailed study of the principles +involved. + + +MATERIAL + + +The material will be taken from that provided for mechanical +tests. + + +RADIAL AND TANGENTIAL SHRINKAGE + + +_Specimens_ + +_Preparation_: Two specimens 1 inch thick, 4 inches wide, and 1 +inch long will be obtained from near the periphery of each "_d_" +bolt. These will be cut from the sector-shaped sections left +after securing the material for the mechanical tests or from +disks cut from near the end of the bolt. They will be taken from +adjoining pieces chosen so that the results will be comparable +for use in determining radial and tangential shrinkage. (When a +disk is used, care must be taken that it is green and has not +been affected by the shrinkage and checking near the end of the +bolt.) + +One of these specimens will be cut with its width in the radial +direction and will be used for the determination of radial +shrinkage. The other will have its width in the tangential +direction and will be used for tangential shrinkage. These +specimens will not be surfaced. + +_Marking_: The shrinkage specimens will retain the shipment and +piece numbers and marks of the bolts from which they are taken, +and will have the additional mark _7_R or _7_T according as +their widths are in the radial or tangential direction. + + +_Shrinkage measurements_: The shrinkage specimens will be +carefully weighed and measured soon after cutting. Rings per +inch, per cent sap, and per cent summer wood will be measured. +They will then be air-dried in the laboratory to constant +weight, and afterward oven-dried at 100°C. (212°F.), when they +will again be weighed and measured. + + +VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY + + +_Specimens_ + +_Selection and preparation_: Four 2" X 2" X 6" specimens will be +cut from the mechanical test sticks of each "_d_" bolt; also +from each of the composite bolts used in getting a comparison of +green and air-dry. One of these specimens will be taken from +near the pith and one from near the periphery; the other two +will be representative of the average growth of the bolt. The +sides of these specimens will be surfaced and the ends smooth +sawn. + +_Marking_: Each specimen will retain the shipment, piece, and +stick numbers and mark of the stick from which it is cut, and +will have the additional mark "_S_." + +_Manipulation_: Soon after cutting, each specimen will be +weighed and its volume will be determined by the method +described below. The rings per inch and per cent summer wood, +where possible, will be determined, and a carbon impression of +the end of the specimen made. It will then be air-dried in the +laboratory to a constant weight and afterward oven-dried at +100°C. When dry, the specimen will be taken from the oven, +weighed, and a carbon impression of its end made. While still +warm the specimen will be dipped in hot paraffine. The volume +will then be determined by the following method: + +On one pan of a pair of balances is placed a container having in +it water enough for the complete submersion of the test +specimen. This container and water is balanced by weights placed +on the other scale pan. The specimen is then held completely +submerged and not touching the container while the scales are +again balanced. The weight required to balance is the weight of +water displaced by the specimen, and hence if in grams is +numerically equal to the volume of the specimen in cubic +centimetres. A diagrammatic sketch of the arrangement of this +apparatus is shown in Fig. 51. + +[Illustration: FIG. 51.--Diagram of specific gravity apparatus, +showing a balance with container (_c_) filled with water in +which the test block (_b_) is held submerged by a light rod +(_a_) which is adjustable vertically and provided with a sharp +point to be driven into the specimen.] + +Air-dry specimens will be dipped in water and then wiped dry +after the first weighing and just before being immersed for +weighing their displacement. All displacement determinations +will be made as quickly as possible in order to minimize the +absorption of water by the specimen. + + + +STRENGTH VALUES FOR STRUCTURAL TIMBERS + +(From Cir. 189, U.S. Forest Service) + + +The following tables bring together in condensed form the +average strength values resulting from a large number of tests +made by the Forest Service on the principal structural timbers +of the United States. These results are more completely +discussed in other publications of the Service, a list of which +is given in BIBLIOGRAPHY, PART III. + +The tests were made at the laboratories of the U.S. Forest +Service, in cooperation with the following institutions: Yale +Forest School, Purdue University, University of California, +University of Oregon, University of Washington, University of +Colorado, and University of Wisconsin. + +Tables XVIII and XIX give the average results obtained from +tests on green material, while Tables XX and XXI give average +results from tests on air-seasoned material. The small +specimens, which were invariably 2" X 2" in cross section, were +free from defects such as knots, checks, and cross grain; all +other specimens were representative of material secured in the +open market. The relation of stresses developed in different +structural forms to those developed in the small clear specimens +is shown for each factor in the column headed "Ratio to 2" X +2"." Tests to determine the mechanical properties of different +species are often confined to small, clear specimens. The ratios +included in the tables may be applied to such results in order +to approximate the strength of the species in structural sizes, +and containing the defects usually encountered, when tests on +such forms are not available. + +A comparison of the results of tests on seasoned material with +those from tests on green material shows that, without +exception, the strength of the 2" X 2" specimens is increased by +lowering the moisture content, but that increase in strength of +other sizes is much more erratic. Some specimens, in fact, show +an apparent loss in strength due to seasoning. If structural +timbers are seasoned slowly, in order to avoid excessive +checking, there should be an increase in their strength. In the +light of these facts it is not safe to base working stresses on +results secured from any but green material. For a discussion of +factors of safety and safe working stresses for structural +timbers see the Manual of the American Railway Engineering +Association, Chicago, 1911. A table from that publication, +giving working unit stresses for structural timber, is +reproduced in this book, see Table XXII. + +|-----------------------------------------------------------------------------------------------------------------------------------| +| TABLE XVIII TABLE XVIII | +|-----------------------------------------------------------------------------------------------------------------------------------| +| BENDING TESTS ON GREEN MATERIAL | +|-----------------------------------------------------------------------------------------------------------------------------------| +| | Sizes | | | | F.S. at E.L. | M. of R. | M. of E. | Calculated | +| |-----------------| Num- | Per | Rings | | | | shear | +| Species | | | ber | cent | per |-----------------+-----------------+-----------------+-----------------| +| | Cross | Span | of | mois- | inch | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio | +| | Section | | tests | ture | | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | +| | | | | | | inch | by 2" | inch | by 2" | inch | by 2" | inch | by 2" | +|-----------------+----------+------+-------+-------+-------+---------+-------+---------+-------+---------+-------+---------+-------| +| | | | | | | | | | | 1,000 | | | | +| | Inches | Ins. | | | | Lbs. | | Lbs. | | lbs. | | Lbs. | | +| | | | | | | | | | | | | | | +| Longleaf pine | 12 by 12 | 138 | 4 | 28.6 | 9.7 | 4,029 | 0.83 | 6,710 | 0.74 | 1,523 | 0.99 | 261 | 0.86 | +| | 10 by 16 | 168 | 4 | 26.8 | 16.7 | 6,453 | .85 | 6,453 | .71 | 1,626 | 1.05 | 306 | 1.01 | +| | 8 by 16 | 156 | 7 | 28.4 | 14.6 | 3,147 | .64 | 5,439 | .60 | 1,368 | .89 | 390 | 1.29 | +| | 6 by 16 | 132 | 1 | 40.3 | 21.8 | 4,120 | .83 | 6,460 | .71 | 1,190 | .77 | 378 | 1.25 | +| | 6 by 10 | 180 | 1 | 31.0 | 6.2 | 3,580 | .72 | 6,500 | .72 | 1,412 | .92 | 175 | .58 | +| | 6 by 8 | 180 | 2 | 27.0 | 8.2 | 3,735 | .75 | 5,745 | .63 | 1,282 | .83 | 121 | .40 | +| | 2 by 2 | 30 | 15 | 33.9 | 14.1 | 4,950 | 1.00 | 9,070 | 1.00 | 1,540 | 1:00 | 303 | 1.00 | +| Douglas fir | 8 by 16 | 180 | 191 | 31.5 | 11.0 | 3,968 | .76 | 5,983 | .72 | 1,517 | .95 | 269 | .81 | +| | 5 by 8 | 180 | 84 | 30.1 | 10.8 | 3,693 | .71 | 5,178 | .63 | 1,533 | .96 | 172 | .52 | +| | 2 by 12 | 180 | 27 | 35.7 | 20.3 | 3,721 | .71 | 5,276 | .64 | 1,642 | 1.03 | 256 | .77 | +| | 2 by 10 | 180 | 26 | 32.9 | 21.6 | 3,160 | .60 | 4,699 | .57 | 1,593 | 1.00 | 189 | .57 | +| | 2 by 8 | 180 | 29 | 33.6 | 17.6 | 3,593 | .69 | 5,352 | .65 | 1,607 | 1.01 | 171 | .51 | +| | 2 by 2 | 24 | 568 | 30.4 | 11.6 | 5,227 | 1.00 | 9,070 | 1.00 | 1,540 | 1.00 | 303 | 1.00 | +| Douglas fir | | | | | | | | | | | | | | +| (fire-killed) | 8 by 16 | 180 | 30 | 36.8 | 10.9 | 3,503 | .80 | 4,994 | .64 | 1,531 | .94 | 330 | 1.19 | +| | 2 by 12 | 180 | 32 | 34.2 | 17.7 | 3,489 | .80 | 5,085 | .66 | 1,624 | .99 | 247 | .89 | +| | 2 by 10 | 180 | 32 | 38.9 | 18.1 | 3,851 | .88 | 5,359 | .69 | 1,716 | 1.05 | 216 | .78 | +| | 2 by 8 | 180 | 31 | 37.0 | 15.7 | 3,403 | .78 | 5,305 | .68 | 1,676 | 1.02 | 169 | .61 | +| | 2 by 2 | 30 | 290 | 33.2 | 17.2 | 4,360 | 1.00 | 7,752 | 1.00 | 1,636 | 1.00 | 277 | 1.00 | +| Shortleaf pine | 8 by 16 | 180 | 12 | 39.5 | 12.1 | 3,185 | .73 | 5,407 | .70 | 1,438 | 1.03 | 362 | 1.40 | +| | 8 by 14 | 180 | 12 | 45.8 | 12.7 | 3,234 | .74 | 5,781 | .75 | 1,494 | 1.07 | 338 | 1.31 | +| | 8 by 12 | 180 | 24 | 52.2 | 11.8 | 3,265 | .75 | 5,503 | .71 | 1,480 | 1.06 | 277 | 1.07 | +| | 5 by 8 | 180 | 24 | 47.8 | 11.5 | 3,519 | .81 | 5,732 | .74 | 1,485 | 1.06 | 185 | .72 | +| | 2 by 2 | 30 | 254 | 51.7 | 13.6 | 4,350 | 1.00 | 7,710 | 1.00 | 1,395 | 1.00 | 258 | 1.00 | +| Western larch | 8 by 16 | 180 | 32 | 51.0 | 25.3 | 3,276 | .77 | 4,632 | .64 | 1,272 | .97 | 298 | 1.11 | +| | 8 by 12 | 180 | 30 | 50.3 | 23.2 | 3,376 | .79 | 5,286 | .73 | 1,331 | 1.02 | 254 | .94 | +| | 5 by 8 | 180 | 14 | 56.0 | 25.6 | 3,528 | .83 | 5,331 | .74 | 1,432 | 1.09 | 169 | .63 | +| | 2 by 2 | 28 | 189 | 46.2 | 26.2 | 4,274 | 1.00 | 7,251 | 1.00 | 1,310 | 1.00 | 269 | 1.00 | +| Loblolly pine | 8 by 16 | 180 | 17 | 15.8 | 6.1 | 3,094 | .75 | 5,394 | .69 | 1,406 | .98 | 383 | 1.44 | +| | 5 by 12 | 180 | 94 | 60.9 | 5.9 | 3,030 | .74 | 5,028 | .64 | 1,383 | .96 | 221 | .83 | +| | 2 by 2 | 30 | 44 | 70.9 | 5.4 | 4,100 | 1.00 | 7,870 | 1.00 | 1,440 | 1.00 | 265 | 1.00 | +| Tamarack | 6 by 12 | 162 | 15 | 57.6 | 16.6 | 2,914 | .75 | 4,500 | .66 | 1,202 | 1.05 | 255 | 1.11 | +| | 4 by 10 | 162 | 15 | 43.5 | 11.4 | 2,712 | .70 | 4,611 | .68 | 1,238 | 1.08 | 209 | .91 | +| | 2 by 2 | 30 | 82 | 38.8 | 14.0 | 3,875 | 1.00 | 6,820 | 1.00 | 1,141 | 1.00 | 229 | 1.00 | +| Western hemlock | 8 by 16 | 180 | 39 | 42.5 | 15.6 | 3,516 | .80 | 5,296 | .73 | 1,445 | 1.01 | 261 | .92 | +| | 2 by 2 | 28 | 52 | 51.8 | 12.1 | 4.406 | 1.00 | 7,294 | 1.00 | 1,428 | 1.00 | 284 | 1.00 | +| Redwood | 8 by 16 | 180 | 14 | 86.5 | 19.9 | 3,734 | .79 | 4,492 | .64 | 1,016 | .96 | 300 | 1.21 | +| | 6 by 12 | 180 | 14 | 87.3 | 17.8 | 3,787 | .80 | 4,451 | .64 | 1,068 | 1.00 | 224 | .90 | +| | 7 by 9 | 180 | 14 | 79.8 | 16.7 | 4,412 | .93 | 5,279 | .76 | 1,324 | 1.25 | 199 | .80 | +| | 3 by 14 | 180 | 13 | 86.1 | 23.7 | 3,506 | .74 | 4,364 | .62 | 947 | .89 | 255 | 1.03 | +| | 2 by 12 | 180 | 12 | 70.9 | 18.6 | 3,100 | .65 | 3,753 | .54 | 1,052 | .99 | 187 | .75 | +| | 2 by 10 | 180 | 13 | 55.8 | 20.0 | 3,285 | .69 | 4,079 | .58 | 1,107 | 1.04 | 169 | .68 | +| | 2 by 8 | 180 | 13 | 63.8 | 21.5 | 2,989 | .63 | 4,063 | .58 | 1,141 | 1.08 | 134 | .54 | +| | 2 by 2 | 28 | 157 | 75.5 | 19.1 | 4,750 | 1.00 | 6,980 | 1.00 | 1,061 | 1.00 | 248 | 1.00 | +| Norway pine | 6 by 12 | 162 | 15 | 50.3 | 12.5 | 2,305 | .82 | 3,572 | .69 | 987 | 1.03 | 201 | 1.17 | +| | 4 by 12 | 162 | 18 | 47.9 | 14.7 | 2,648 | .94 | 4,107 | .79 | 1,255 | 1.31 | 238 | 1.38 | +| | 4 by 10 | 162 | 16 | 45.7 | 13.3 | 2,674 | .95 | 4,205 | .81 | 1,306 | 1.36 | 198 | 1.15 | +| | 2 by 2 | 30 | 133 | 32.3 | 11.4 | 2,808 | 1.00 | 5,173 | 1.00 | 960 | 1.00 | 172 | 1.00 | +| Red spruce | 2 by 10 | 144 | 14 | 32.5 | 21.9 | 2,394 | .66 | 3,566 | .60 | 1,180 | 1.02 | 181 | .80 | +| | 2 by 2 | 26 | 60 | 37.3 | 21.3 | 3,627 | 1.00 | 5,900 | 1.00 | 1,157 | 1.00 | 227 | 1.00 | +| White spruce | 2 by 10 | 144 | 16 | 40.7 | 9.3 | 2,239 | .72 | 3,288 | .63 | 1,081 | 1.08 | 166 | .83 | +| | 2 by 2 | 26 | 83 | 58.3 | 10.2 | 3.090 | 1.00 | 5,185 | 1.00 | 998 | 1.00 | 199 | 1.00 | +|-----------------------------------------------------------------------------------------------------------------------------------| +| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_ | +| F.S. at E.L. = Fiber stress at elastic limit. | +| M. of E. = Modulus of elasticity. | +| M. of R. = Modulus of rupture. | +| Cr. str. at E.L. = Crushing strength at elastic limit. | +| Cr. str. at max. ld. = Crushing strength at maximum load. | +|-----------------------------------------------------------------------------------------------------------------------------------| + +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| TABLE XIX TABLE XIX | +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL | +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| | Compression | Compression | Shear | +| | parallel to grain | perpendicular to grain | | +| |------------------------------------------------------+-------------------------------------------+--------------------------| +| | | | | Cr. | | Cr. | | | | | Cr. | | | | +| Species | | | Per | str. | M. of | str. | | | | Per | str. | | Per | | +| | Size of | No. | cent | at | E. | at max. | Stress | | No. | cent | at max. | No. | cent | Shear | +| | specimen | of | of | E. L. | per | ld.,. | area | Height | of | of | ld., | of | of | strength | +| | | tests | mois- | per | square | per | | | tests | mois- | per | tests | mois- | | +| | | | ture | square | inch | square | | | | ture | square | | ture | | +| | | | | inch | | inch | | | | | inch | | | | +|-----------------+----------+-------+-------+--------+--------+---------+--------+--------+-------+ ------+---------+-------+-------+----------| +| | | | | | 1,000 | | | | | | | | | | +| | Inches | | | Lbs. | lbs. | Lbs. | Inches | Inches | | | Lbs. | | | Lbs. | +| | | | | | | | | | | | | | | | +| Longleaf pine | 4 by 4 | 46 | 26.3 | 3,480 | | 4,800 | 4 by 4 | 4 | 22 | 25.3 | 568 | 44 | 21.8 | 973 | +| | 2 by 2 | 14 | 34.7 | | | 4,400 | | | | | | | | | +| Douglas fir | 6 by 6 | 515 | 30.7 | 2,780 | 1,181 | 3,500 | 4 by 8 | 16 | 259 | 30.3 | 570 | 531 | 29.7 | 765 | +| | 5 by 6 | 170 | 30.9 | 2,720 | 2,123 | 3,490 | | | | | | | | | +| | 2 by 2 | 902 | 29.8 | 3,500 | 1,925 | 4,030 | | | | | | | | | +| Douglas fir | | | | | | | | | | | | | | | +| (fire-killed) | 6 by 6 | 108 | 34.8 | 2,620 | 1,801 | 3,290 | 6 by 8 | 16 | 24 | 33.7 | 368 | 77 | 35.8 | 631 | +| | 2 by 2 | 204 | 37.9 | | | 3,430 | | | | | | | | | +| Shortleaf pine | 6 by 6 | 95 | 41.2 | 2,514 | 1,565 | 3,436 | 5 by 8 | 16 | 12 | 37.7 | 361 | 179 | 47.0 | 704 | +| | 5 by 8 | 23 | 43.5 | 2,241 | 1,529 | 3,423 | 5 by 8 | 14 | 12 | 42.8 | 366 | | | | +| | 2 by 2 | 281 | 51.4 | | | 3,570 | 5 by 8 | 12 | 24 | 53.0 | 325 | | | | +| | | | | | | | 5 by 5 | 8 | 24 | 47.0 | 344 | | | | +| | | | | | | | 2 by 2 | 2 | 277 | 48.5 | 400 | | | | +| Western larch | 6 by 6 | 107 | 49.1 | 2,675 | 1,575 | 3,510 | 6 by 8 | 16 | 22 | 43.6 | 417 | 179 | 40.7 | 700 | +| | 2 by 2 | 491 | 50.6 | 3,026 | 1,545 | 3,696 | 6 by 8 | 12 | 20 | 40.2 | 416 | | | | +| | | | | | | | 4 by 6 | 6 | 53 | 52.8 | 478 | | | | +| | | | | | | | 4 by 4 | 4 | 30 | 50.4 | 472 | | | | +| Loblolly pine | 8 by 8 | 14 | 63.4 | 1,560 | 365 | 2,140 | 8 by 4 | 8 | 16 | 67.2 | 392 | 121 | 83.2 | 630 | +| | 4 by 8 | 18 | 60.0 | 2,430 | 691 | 3,560 | 4 by 4 | 8 | 38 | 44.6 | 546 | | | | +| | 2 by 2 | 53 | 74.0 | | | 3,240 | | | | | | | | | +| Tamarack | 6 by 7 | 4 | 49.9 | 2,332 | 1,432 | 3,032 | | | | | | 24 | 39.2 | 668 | +| | 4 by 7 | 6 | 27.7 | 2,444 | 1,334 | 3,360 | | | | | | | | | +| | 2 by 2 | 165 | 36.8 | | | 3,190 | | | | | | | | | +| Western hemlock | 6 by 6 | 82 | 46.6 | 2,905 | 1,617 | 3,355 | 6 by 4 | 6 | 30 | 48.7 | 434 | 54 | 65.7 | 630 | +| | 2 by 2 | 131 | 55.6 | 2,938 | 1,737 | 3,392 | | | | | | | | | +| Redwood | 6 by 6 | 34 | 83.6 | 3,194 | 1,240 | 3,882 | 6 by 8 | 16 | 13 | 86.7 | 473 | 148 | 84.2 | 742 | +| | 2 by 2 | 143 | 36.8 | 3,490 | 1,222 | 3,980 | 6 by 6 | 12 | 14 | 83.0 | 424 | | | | +| | | | | | | | 6 by 7 | 9 | 13 | 74.7 | 477 | | | | +| | | | | | | | 6 by 3 | 14 | 13 | 75.6 | 411 | | | | +| | | | | | | | 6 by 2 | 12 | 12 | 66.5 | 430 | | | | +| | | | | | | | 6 by 2 | 10 | 11 | 55.0 | 423 | | | | +| | | | | | | | 6 by 2 | 8 | 12 | 56.7 | 396 | | | | +| | | | | | | | 2 by 2 | 2 | 186 | 75.5 | 569 | | | | +| Norway pine | 6 by 7 | 5 | 29.0 | 1,928 | 905 | 2,404 | | | | | | 20 | 26.7 | 589 | +| | 4 by 7 | 8 | 28.4 | 2,154 | 1,063 | 2,652 | | | | | | | | | +| | 2 by 2 | 178 | 26.8 | | | 2,504 | | | | | | | | | +| Red spruce | 2 by 2 | 58 | 35.4 | | | 2,750 | 2 by 2 | 2 | 43 | 31.8 | 310 | 30 | 32.0 | 758 | +| White spruce | 2 by 2 | 84 | 61.0 | | | 2,370 | 2 by 2 | 2 | 46 | 50.4 | 270 | 40 | 58.0 | 651 | +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_ | +| F.S. at E.L. = Fiber stress at elastic limit. | +| M. of E. = Modulus of elasticity. | +| M. of R. = Modulus of rupture. | +| Cr. str. at E.L. = Crushing strength at elastic limit. | +| Cr. str. at max. ld. = Crushing strength at maximum load. | +|-----------------------------------------------------------------------------------------------------------------------------------------------| + +|-----------------------------------------------------------------------------------------------------------------------------------| +| TABLE XX TABLE XX | +|-----------------------------------------------------------------------------------------------------------------------------------| +| BENDING TESTS ON AIR-SEASONED MATERIAL | +|-----------------------------------------------------------------------------------------------------------------------------------| +| | Sizes | | | | F.S. at E.L. | M. of R. | M. of E. | Calculated | +| |-----------------| Num- | Per | Rings | | | | shear | +| Species | | | ber | cent | per |-----------------+-----------------+-----------------+-----------------| +| | Cross | Span | of | mois- | inch | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio | +| | Section | | tests | ture | | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | +| | | | | | | inch | by 2" | inch | by 2" | inch | by 2" | inch | by 2" | +|-----------------+----------+------+-------+-------+-------+---------+-------+---------+-------+---------+-------+---------+-------| +| | | | | | | | | | | 1,000 | | | | +| | Inches | Ins. | | | | Lbs. | | Lbs. | | lbs. | | Lbs. | | +| | | | | | | | | | | | | | | +| Longleaf pine | 8 by 16 | 180 | 5 | 22.2 | 16.0 | 3,390 | 0.50 | 4,274 | 0.37 | 1,747 | 1.00 | 288 | 0.75 | +| | 6 by 16 | 132 | 1 | 23.4 | 17.1 | 3,470 | .51 | 6,610 | .57 | 1,501 | .86 | 388 | 1.01 | +| | 6 by 10 | 177 | 2 | 19.0 | 8.8 | 4,560 | .68 | 7,880 | .68 | 1,722 | .99 | 214 | .56 | +| | 4 by 11 | 180 | 1 | 18.4 | 23.9 | 3,078 | .46 | 8,000 | .69 | 1,660 | .95 | 251 | .66 | +| | 6 by 8 | 177 | 6 | 20.0 | 13.7 | 4,227 | .63 | 8,196 | .71 | 1,634 | .94 | 177 | .46 | +| | 2 by 2 | 30 | 17 | 15.9 | 13.9 | 6,750 | 1.00 | 11,520 | 1.00 | 1,740 | 1.00 | 383 | 1.00 | +| Douglas fir | 8 by 16 | 180 | 91 | 20.8 | 13.1 | 4,563 | .68 | 6,372 | .61 | 1,549 | .91 | 269 | .64 | +| | 5 by 8 | 180 | 30 | 14.9 | 12.2 | 5,065 | .76 | 6,777 | .65 | 1,853 | 1.09 | 218 | .52 | +| | 2 by 2 | 24 | 211 | 19.0 | 16.4 | 6,686 | 1.00 | 10,378 | 1.00 | 1,695 | 1.00 | 419 | 1.00 | +| Shortleaf pine | 8 by 16 | 180 | 3 | 17.0 | 12.3 | 4,220 | .54 | 6,030 | .50 | 1,517 | .85 | 398 | .98 | +| | 8 by 14 | 180 | 3 | 16.0 | 12.3 | 4,253 | .55 | 5,347 | .44 | 1,757 | .98 | 307 | .76 | +| | 8 by 12 | 180 | 7 | 16.0 | 12.4 | 5,051 | .65 | 7,331 | .60 | 1,803 | 1.01 | 361 | .89 | +| | 5 by 8 | 180 | 6 | 12.2 | 22.5 | 7,123 | .92 | 9,373 | .77 | 1,985 | 1.11 | 301 | .74 | +| | 2 by 2 | 30 | 67 | 14.2 | 13.7 | 7,780 | 1.00 | 12,120 | 1.00 | 1,792 | 1.00 | 404 | 1.00 | +| Western larch | 8 by 16 | 180 | 23 | 18.3 | 21.9 | 3,343 | .57 | 5,440 | .53 | 1,409 | .90 | 349 | .96 | +| | 8 by 12 | 180 | 29 | 17.8 | 23.4 | 3,631 | .62 | 6,186 | .60 | 1,549 | .99 | 295 | .81 | +| | 5 by 8 | 180 | 10 | 13.6 | 27.6 | 4,730 | .80 | 7,258 | .71 | 1,620 | 1.04 | 221 | .61 | +| | 2 by 2 | 30 | 240 | 16.1 | 26.8 | 5,880 | 1.00 | 10,254 | 1.00 | 1,564 | 1.00 | 364 | 1.00 | +| Loblolly pine | 8 by 16 | 180 | 14 | 20.5 | 7.4 | 4,195 | .81 | 6,734 | .72 | 1,619 | 1.10 | 462 | 1.45 | +| | 6 by 16 | 126 | 4 | 20.2 | 5.0 | 2,432 | .47 | 4,295 | .46 | 1,324 | .90 | 266 | .84 | +| | 6 by 10 | 174 | 3 | 21.3 | 4.7 | 3,100 | .60 | 6,167 | .66 | 1,449 | .99 | 173 | .54 | +| | 4 by 12 | 174 | 4 | 19.8 | 4.7 | 2,713 | .52 | 5,745 | .61 | 1,249 | .85 | 185 | .58 | +| | 8 by 8 | 180 | 9 | 22.9 | 4.9 | 2,903 | .56 | 4,557 | .48 | 1,136 | .77 | 93 | .29 | +| | 6 by 7 | 144 | 2 | 21.1 | 5.0 | 2,990 | .58 | 4,968 | .53 | 1,286 | .88 | 116 | .36 | +| | 4 by 8 | 132 | 8 | 19.5 | 9.1 | 3,384 | .65 | 6,194 | .66 | 1,200 | .82 | 196 | .62 | +| | 2 by 2 | 30 | 123 | 17.6 | 6.6 | 5,170 | 1.00 | 9,400 | 1.00 | 1,467 | 1.00 | 318 | 1.00 | +| Tamarack | 6 by 12 | 162 | 5 | 23.0 | 15.1 | 3,434 | .45 | 5,640 | .43 | 1,330 | .82 | 318 | .75 | +| | 4 by 10 | 162 | 4 | 14.4 | 9.7 | 4,100 | .54 | 5,320 | .41 | 1,386 | .84 | 252 | .59 | +| | 2 by 2 | 30 | 47 | 11.3 | 16.2 | 7,630 | 1.00 | 13,080 | 1.00 | 1,620 | 1.00 | 425 | 1.00 | +| Western hemlock | 8 by 16 | 180 | 44 | 17.7 | 17.8 | 4,398 | .69 | 6,420 | .62 | 1,737 | 1.04 | 406 | 1.06 | +| | 2 by 2 | 28 | 311 | 17.9 | 19.4 | 6,333 | 1.00 | 10,369 | 1.00 | 1,666 | 1.00 | 382 | 1.00 | +| Redwood | 8 by 16 | 180 | 6 | 26.3 | 22.4 | 3,797 | .79 | 4,428 | .57 | 1,107 | .96 | 294 | 1.05 | +| | 6 by 12 | 180 | 6 | 16.1 | 17.7 | 3,175 | .66 | 3,353 | .43 | 728 | .64 | 167 | .60 | +| | 7 by 9 | 180 | 6 | 15.9 | 15.2 | 3,280 | .69 | 4,002 | .51 | 1,104 | .96 | 147 | .53 | +| | 3 by 14 | 180 | 6 | 13.1 | 24.4 | | | 5,033 | .64 | | | 291 | 1.04 | +| | 2 by 12 | 180 | 5 | 13.8 | 14.4 | 3,928 | .82 | 5,336 | .68 | 1,249 | 1.09 | 260 | .93 | +| | 2 by 10 | 180 | 5 | 13.8 | 24.8 | 3,757 | .79 | 4,606 | .59 | 1,198 | 1.05 | 186 | .67 | +| | 2 by 8 | 180 | 6 | 13.7 | 20.7 | 4,314 | .90 | 5,050 | .65 | 1,313 | 1.15 | 166 | .60 | +| | 2 by 2 | 28 | 122 | 15.2 | 18.8 | 4,777 | 1.00 | 7,798 | 1.00 | 1,146 | 1.00 | 279 | 1.00 | +| Norway pine | 6 by 12 | 162 | 5 | 16.7 | 8.1 | 2,968 | .56 | 5,204 | .61 | 1,123 | .97 | 286 | 1.02 | +| | 4 by 10 | 162 | 5 | 13.7 | 12.0 | 5,170 | .98 | 6,904 | .82 | 1,712 | 1.48 | 317 | 1.13 | +| | 2 by 2 | 30 | 60 | 14.9 | 11.2 | 5,280 | 1.00 | 8,470 | 1.00 | 1,158 | 1.00 | 281 | 1.00 | +|-----------------------------------------------------------------------------------------------------------------------------------| +| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_ | +| F.S. at E.L. = Fiber stress at elastic limit. | +| M. of E. = Modulus of elasticity. | +| M. of R. = Modulus of rupture. | +| Cr. str. at E.L. = Crushing strength at elastic limit. | +| Cr. str. at max. ld. = Crushing strength at maximum load. | +|-----------------------------------------------------------------------------------------------------------------------------------| + +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| TABLE XXI TABLE XXI | +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL | +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| | Compression | Compression | Shear | +| | parallel to grain | perpendicular to grain | | +| |------------------------------------------------------+-------------------------------------------+--------------------------| +| | | | | Cr. | | Cr. | | | | | Cr. | | | | +| Species | | | Per | str. | M. of | str. | | | | Per | str. | | Per | | +| | Size of | No. | cent | at | E. | at max. | Stress | | No. | cent | at max. | No. | cent | Shear | +| | specimen | of | of | E. L. | per | ld.,. | area | Height | of | of | ld., | of | of | strength | +| | | tests | mois- | per | square | per | | | tests | mois- | per | tests | mois- | | +| | | | ture | square | inch | square | | | | ture | square | | ture | | +| | | | | inch | | inch | | | | | inch | | | | +|-----------------+----------+-------+-------+--------+--------+---------+--------+--------+-------+ ------+---------+-------+-------+----------| +| | | | | | 1,000 | | | | | | | | | | +| | Inches | | | Lbs. | lbs. | Lbs. | Inches | Inches | | | Lbs. | | | Lbs. | +| | | | | | | | | | | | | | | | +| Longleaf pine | 4 by 5 | 46 | 26.3 | 3,480 | | 4,800 | 4 by 5 | 4 | 22 | 25.1 | 572 | 52 | 20.2 | 984 | +| Douglas fir | 6 by 6 | 259 | 20.3 | 3,271 | 1,038 | 4,258 | 4 by 8 | 16 | 44 | 20.8 | 732 | 465 | 22.1 | 822 | +| | 2 by 2 | 247 | 18.7 | 3,842 | 1,084 | 5,002 | 4 by 8 | 10 | 32 | 18.1 | 584 | | | | +| | | | | | | | 4 by 4 | 8 | 51 | 20.2 | 638 | | | | +| | | | | | | | 4 by 4 | 6 | 49 | 24.0 | 613 | | | | +| | | | | | | | 4 by 4 | 4 | 29 | 24.8 | 603 | | | | +| Shortleaf pine | 6 by 6 | 29 | 15.7 | 4,070 | 1,951 | 6,030 | 8 by 5 | 16 | 4 | 17.8 | 725 | 85 | | 1,135 | +| | 2 by 2 | 57 | 14.2 | | | 6,380 | 8 by 5 | 14 | 3 | 16.3 | 757 | | | | +| | | | | | | | 8 by 5 | 12 | 5 | 15.1 | 730 | | | | +| | | | | | | | 5 by 5 | 8 | 6 | 13.0 | 918 | | | | +| | | | | | | | 2 by 2 | 2 | 57 | 13.9 | 926 | | | | +| Western larch | 6 by 6 | 112 | 16.0 | | | 5,445 | 8 by 6 | 16 | 17 | 18.8 | 491 | 193 | 15.0 | 905 | +| | 4 by 4 | 81 | 14.7 | | | 6,161 | 8 by 6 | 12 | 18 | 17.6 | 526 | | | | +| | 2 by 2 | 270 | 14.8 | | | 5,934 | 5 by 4 | 8 | 22 | 13.3 | 735 | | | | +| Loblolly pine | 6 by 6 | 23 | | 3,357 | 1,693 | 5,005 | 8 by 5 | 16 | 12 | 19.8 | 602 | 156 | 11.3 | 1,115 | +| | 5 by 5 | 10 | 22.4 | 2,217 | 545 | 2,950 | 8 by 5 | 8 | 7 | 22.9 | 679 | | | | +| | 4 by 8 | 8 | 19.4 | 3,010 | 633 | 3,920 | 4 by 5 | 8 | 8 | 19.5 | 715 | | | | +| | 2 by 2 | 69 | | | | 5,547 | | | | | | | | | +| Tamarack | 6 by 7 | 3 | 15.7 | 2,257 | 1,042 | 3,323 | 2 by 2 | 2 | 57 | 16.2 | 697 | 60 | 14.0 | 879 | +| | 4 by 7 | 3 | 13.6 | 3,780 | 1,301 | 4,823 | | | | | | | | | +| | 4 by 4 | 57 | 14.9 | 3,386 | 1,353 | 4,346 | | | | | | | | | +| | 2 by 2 | 66 | 14.6 | | | 4,790 | | | | | | | | | +| Western hemlock | 6 by 6 | 102 | 18.6 | 4,840 | 2,140 | 5,814 | 7 by 6 | 15 | 25 | 18.2 | 514 | 131 | 17.7 | 924 | +| | 2 by 2 | 463 | 17.0 | 4,560 | 1,923 | 5,403 | 6 by 6 | 6 | 26 | 16.8 | 431 | | | | +| | | | | | | | 4 by 4 | 4 | 6 | 15.9 | 488 | | | | +| Redwood | 6 by 6 | 18 | 16.9 | | | 4,276 | 8 by 6 | 16 | 5 | 25.4 | 548 | 95 | 12.4 | 671 | +| | 2 by 2 | 115 | 14.6 | | | 5,119 | 6 by 6 | 12 | 6 | 14.7 | 610 | | | | +| | | | | | | | 7 by 6 | 9 | 5 | 14.8 | 500 | | | | +| | | | | | | | 3 by 6 | 14 | 2 | 12.6 | 470 | | | | +| | | | | | | | 2 by 6 | 12 | 2 | 16.2 | 498 | | | | +| | | | | | | | 2 by 6 | 10 | 4 | 14.3 | 511 | | | | +| | | | | | | | 2 by 6 | 8 | 2 | 13.2 | 429 | | | | +| | | | | | | | 2 by 2 | 2 | 145 | 13.8 | 564 | | | | +| Norway pine | 6 by 7 | 4 | 15.2 | 2,670 | 1,182 | 4,212 | 2 by 2 | 2 | 36 | 10.0 | 924 | 44 | 11.9 | 1,145 | +| | 4 by 7 | 2 | 22.2 | 3,275 | 1,724 | 4,575 | | | | | | | | | +| | 4 by 4 | 55 | 16.6 | 3,048 | 1,367 | 4,217 | | | | | | | | | +| | 2 by 2 | 44 | 11.2 | | | 7,550 | | | | | | | | | +|-----------------------------------------------------------------------------------------------------------------------------------------------| +| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_ | +| F.S. at E.L. = Fiber stress at elastic limit. | +| M. of E. = Modulus of elasticity. | +| M. of R. = Modulus of rupture. | +| Cr. str. at E.L. = Crushing strength at elastic limit. | +| Cr. str. at max. ld. = Crushing strength at maximum load. | +|-----------------------------------------------------------------------------------------------------------------------------------------------| + +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| TABLE XXII TABLE XXII | +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| [b]WORKING UNIT-STRESSES FOR STRUCTURAL TIMBER[c] | +| EXPRESSED IN POUNDS PER SQUARE INCH | +| (From Manual of the American Railway Engineering Assn., 1911, p. 153) | +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| NOTE.--The working unit-stresses given in the table are intended for railroad bridges and trestles. For highway bridges and trestles the unit-stresses may be increased | +| twenty-five (25) per cent. For buildings and similar structures, in which the timber is protected from the weather and practically free from impact, the unit-stresses may be | +| increased fifty (50) per cent. To compute the deflection of a beam under long-continued loading instead of that when the load is first applied, only fifty (50) per cent of the | +| corresponding modulus of elasticity given in the table is to be employed. | +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| | BENDING | SHEARING | COMPRESSION | | +| |---------------------------------+-----------------------------------------+-------------------------------------------------------------------------| Ratio | +| | Extreme | Modulus of | Parallel to | Longitudinal | Perpendicular | Parallel to | For | Formulæ for | of | +| | fibre | elasticity | the grain | shear in | to the grain | the grain | columns | working stress in | length | +| KIND OF | stress | | | beams | | | under 15 | long columns over | of | +| TIMBER |--------------------+------------+--------------------+--------------------+--------------------+--------------------| diameters | 15 diameters | stringer | +| | Average | Working | | Average | Working | Elastic | Working | Elastic | Working | Average | Working | working | | to | +| | ultimate | stress | Average | ultimate | stress | limit | stress | limit | stress | ultimate | stress | stress | | depth | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Douglas fir | 6100 | 1200 | 1,510,000 | 690 | 170 | 270 | 110 | 630 | 310 | 3600 | 1200 | 900 | 1200(1-_l_/60_d_) | 10 | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Longleaf pine | 6500 | 1300 | 1,610,000 | 720 | 180 | 300 | 120 | 520 | 260 | 3800 | 1300 | 980 | 1300(1-_l_/60_d_) | 10 | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Shortleaf pine | 5600 | 1100 | 1,480,000 | 710 | 170 | 330 | 130 | 340 | 170 | 3400 | 1100 | 830 | 1100(1-_l_/60_d_) | 10 | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| White pine | 4400 | 900 | 1,130,000 | 400 | 100 | 180 | 70 | 290 | 150 | 3000 | 1000 | 750 | 1000(1-_l_/60_d_) | 10 | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Spruce | 4800 | 1000 | 1,310,000 | 600 | 150 | 170 | 70 | 370 | 180 | 3200 | 1100 | 830 | 1100(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Norway pine | 4200 | 800 | 1,190,000 | 590[d] | 130 | 250 | 100 | | 150 | 2600[d] | 800 | 600 | 800(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Tamarack | 4600 | 900 | 1,220,000 | 670 | 170 | 260 | 100 | | 220 | 3200[d] | 1000 | 750 | 1000(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Western hemlock | 5800 | 1100 | 1,480,000 | 630 | 160 | 270[d] | 100 | 440 | 220 | 3500 | 1200 | 900 | 1200(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Redwood | 5000 | 900 | 800,000 | 300 | 80 | | | 400 | 150 | 3300 | 900 | 680 | 900(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Bald cypress | 4800 | 900 | 1,150,000 | 500 | 120 | | | 340 | 170 | 3900 | 1100 | 830 | 1100(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| Red cedar | 4200 | 800 | 800,000 | | | | | 470 | 230 | 2800 | 900 | 680 | 900(1-_l_/60_d_) | | +|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------| +| White oak | 5700 | 1100 | 1,150,000 | 840 | 210 | 270 | 110 | 920 | 450 | 3500 | 1300 | 980 | 1300(1-_l_/60_d_) | 12 | +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +| These unit-stresses are for a green condition of timber and are _l_ = Length in inches. | +| to be used without increasing the live load stresses for impact. _d_ = Least side in inches. | +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| +|[Footnote b: Adopted, Vol. 1909, pp. 537, 564, 609-611.] | +|[Footnote c: Green timber in exposed work.] | +|[Footnote d: Partially air-dry] | +|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| + + + + +BIBLIOGRAPHY + + + + Part I: Some general works on mechanics, materials of + construction, and testing of materials. + + Part II: Publications and articles on the mechanical properties + of wood, and timber testing. + +Part III: Publications of the U.S. Government on the mechanical + properties of wood, and timber testing. + + + +PART 1. SOME GENERAL WORKS ON MECHANICS, MATERIALS OF + CONSTRUCTION, AND TESTING OF MATERIALS + + +ALLAN, WILLIAM: Strength of beams under transverse loads. New +York, 1893. + +ANDERSON, SIR JOHN: The strength of materials and structures. +London, 1902. + +BARLOW, PETER: Strength of materials, 1st ed. 1817; rev. 1867. + +BURR, WILLIAM H.: The elasticity and resistance of the materials +of engineering. New York, 1911. + +CHURCH, IRVING P.: Mechanics of engineering. New York, 1911. + +HATFIELD, R.G.: Theory of transverse strain. 1877. + +HATT, W.K., and SCOFIELD, H.H.: Laboratory manual of testing +materials. New York, 1913. + +JAMESON, J.M.: Exercises in mechanics. (Wiley technical series.) +New York, 1913. + +JAMIESON, ANDREW: Strength of materials. (Applied mechanics and +mechanical engineering, Vol. II.) London, 1911. + +JOHNSON, J.B.: The materials of construction. New York, 1910. + +KENT, WILLIAM: The strength of materials. New York, 1890. + +KOTTCAMP, J.P.: Exercises for the applied mechanics laboratory. +(Wiley technical series.) New York, 1913. + +LANZA, GAETANO: Applied mechanics. New York, 1901. + +MERRIMAN, MANSFIELD: Mechanics of materials. New York, 1912. + +MURDOCK, H.E.: Strength of materials. New York, 1911. + +RANKINE, WILLIAM J.M.: A manual of applied mechanics. London, +1901. + +THIL, A.: Conclusion de l'étude présentée à la Commission des +méthodes d'essai des matériaux de construction. Paris, 1900. + +THURSTON, ROBERT H.: A treatise on non-metallic materials of +engineering: stone, timber, fuel, lubricants, etc. (Materials of +engineering, Part I.) New York, 1899. + +UNWIN, WILLIAM C.: The testing of materials of construction. +London, 1899. + +WATERBURY, L.A.: Laboratory manual for testing materials of +construction. New York, 1912. + +WOOD, DEVOLSON: A treatise on the resistance of materials. New +York, 1897. + + + +PART II. PUBLICATIONS AND ARTICLES ON THE MECHANICAL PROPERTIES + OF WOOD, AND TIMBER TESTING + + +ABBOT, ARTHUR V.: Testing machines, their history, construction +and use. Van Nostrand's Eng. Mag., Vol. XXX, 1884, pp. 204-214; +325-344; 382-397; 477-490. + +ADAMS, E.E.: Tests to determine the strength of bolted timber +joints. Cal. Jour, of Technology, Sept., 1904. + +ALVAREZ, ARTHUR C.: The strength of long seasoned Douglas fir +and redwood. Univ. of Cal. Pub. in Eng., Vol. I, No. 2, +Berkeley, 1913, pp. 11-20. + +BARLOW, PETER: An essay on the strength and stress of timber. +London, 1817; 3d ed., 1826. + +----: Experiments on the strength of different kinds of wood +made in the carriage department, Royal Arsenal, Woolwich. Jour. +Franklin Inst., Vol. X, 1832, pp. 49-52. Reprinted from +Philosophical Mag. and Annals of Philos., No. 63, Mch., 1832. + +BATES, ONWARD: Pine stringers and floorbeams for bridges. Trans. +Am. Soc. C.E., Vol. XXIII. + +BAUSCHINGER, JOHANN: Untersuchungen über die Elasticität und +Festigkeit von Fichten- und Kiefernbauhölzern. Mitt. a. d. +mech.-tech. Laboratorium d. k. techn. Hochschule in München, 9. +Hft., München, 1883. + +----: Verhandlungen der Münchener Conferenz und der von ihr +gewählten ständigen Commission zur Vereinbarung einheitlicher +Prüfungsmethoden für Bau- und Constructions-material. _Ibid._, +14. Hft., 1886. + +----: Untersuchungen über die Elasticität und Festigkeit +verschiedener Nadelhölzer. _Ibid._, 16. Hft., 1887. + +BEARE, T. HUDSON: Timber: its strength and how to test it. +Engineering, London, Dec. 9, 1904. + +BEAUVERIE, J.: Le bois. I. Paris, 1905, pp. 105-185. + +----: Les bois industriels. Paris, 1910, pp. 55-77. Bending +tests with wood, executed at the Danish State Testing +Laboratory, Copenhagen. Proc. Int. Assn. Test. Mat., 1912, +XXIII_{2}, pp. 17. See also Eng. Record, Vol. LXVI, 1912, p. +269. + +BERG, WALTER G.: Berg's complete timber test record. Chicago, +1899. Reprint from Am. By. Bridges and Buildings. BOULGER, G.S.: +Wood. London, 1908, pp. 112-121. + +BOUNICEAU,--: Note et expériences sur la torsion des bois. +[N.p., n.d.] + +BOVEY, HENRY T.: Results of experiments at McGill University, +Montreal, on the strength of Canadian Douglas fir, red pine, +white pine, and spruce. Trans. Can. Soc. C.E., Vol. IX, Part I, +1895, pp. 69-236. + +BREUIL, M. PIERRE: Contribution to the discussion on the testing +of wood. Proc. Int. Assn. Test. Mat., 1906, Disc, 1_e_, pp. 2. + +BROWN, T.S.: An Account of some experiments made by order of +Col. Totten, at Fort Adams, Newport, R.I., to ascertain the +relative stiffness and strength of the following kinds of +timber, viz.: white pine (_Pinus strobus_), spruce (_Abies +nigra_), and southern pine (_Pinus australis_), also called +long-leaved pine. Jour. Franklin Inst., Vol. VII (n.s.), 1831, +pp. 230-238. + +BUCHANAN, C.P.: Some tests of old timber. Eng. News, Vol. LXIV, +No. 23, 1910, p. 67. + +BUSGEN, M.: Zur Bestimmung der Holzhärten. Zeitschrift f. Forst- +und Jagdwesen. Berlin, 1904, pp. 543-562. + +CHEVANDIER, E., et WERTHEIM, G.: Mémoire sur les propriétés +mécaniques du bois. Paris, 1846. + +CIESLAR, A.: Studien über die Qualität rasch erwachsenen +Fichtenholzes. Centralblatt f. d. ges. Forstwesen, Wien, 1902, +pp. 337-403. + +CLINE, McGARVEY: Forest Service investigations of American woods +with special reference to investigations of mechanical +properties. Proc. Int. Assn. Test. Mat., 1912, XXIII_{5}, pp. +17. + +----: Forest Service tests to determine the influence of +different methods and rates of loading on the strength and +stiffness of timber. Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, +pp. 535-540. + +----: The Forest Products Laboratory: its purpose and work. +Proc. Am. Soc. Test. Mat., Vol. X, 1910, pp. 477-489. + +----: Specifications and grading rules for Douglas fir timber: +an analysis of Forest Service tests on structural timbers. Proc. +Am. Soc. Test. Mat., Vol. XI, 1911, pp. 744-766. + +Comparative strength and resistance of various tie timbers. +Elec. Traction Weekly, Chicago, June 15, 1912. + +DAY, FRANK M.: Microscopic examination of timber with regard to +its strength. 1883, pp. 6. + +DEWELL, H.D.: Tests of some joints used in heavy timber framing. +Eng. News, Mch. 19, 1914, pp. 594-598; _et seq._ + +DÖRR, KARL: Die Festigkeit von Fichten- und Kiefernholz. +Deutsche Bauzeitung, Berlin, Aug. 17, 1910. See also Zeitschrift +d. ver. deutsch. Ing., Bd. 54, Nr. 36, 1910, p. 1503. + +DUPIN, CHARLES: Expériences sur la flexibilité, la force, et +l'élasticité des bois. Jour, de l'École Polytechnique, Vol. X, +1815. + +DUPONT, ADOLPHE, et BOUQUET DE LA GRYE: Les bois indigènes et +étrangers. Paris, 1875, pp. 273-352. + +ESTRADA, ESTEBAN DUQUE: On the strength and other properties of +Cuban woods. Van Nostrand's Eng. Mag., Vol. XXIX, 1883, pp. +417-426; 443-449. + +EVERETT, W.H.: Memorandum on mechanical tests of some Indian +timbers. Govt. Bul. No. 6 (o.s.), Calcutta. + +EXNER, WILHELM FRANZ: Die mechanische Technologie des Holzes. +Wien, 1871. (A translation and revision of Chevandier and +Wertheim's Mémoire sur les propriétés mécaniques du bois.) + +----: Die technischen Eigenschaften der Hölzer. Lorey's Handbuch +der Forstwissenschaft, II. Bd., 6. Kap., Tübingen, 1903. + +FERNOW, B.E.: Scientific timber testing. Digest of Physical +Tests, Vol. I, No. 2, 1896, pp. 87-95. + +FOWKE, FRANCIS: Experiments on British colonial and other woods. +1867. + +GARDNER, ROLAND: I. Mechanical tests, properties, and uses of +thirty Philippine woods. II. Philippine sawmills, lumber market +and prices. Bul. 4, Bu. For., P.I., 1906. (2d ed., 1907, +contains tests of 34 woods.) + +GAYER, KARL: Forest utilization. (Vol. V, Schlich's Manual of +Forestry. Translation of Die Forstbenutzung, Berlin, 1894.) +London, 1908. + +GOLLNER, H.: Ueber die Festigkeit des Schwarzföhrenholzes. Mitt. +a. d. forstl. Versuchswesen Oesterreichs. II. Bd., 3. Hft., +Wien, 1881. + +GOTTGETREU, RUDOLPH: Physische und chemische Beschaffenheit der +Baumaterialien. 3d ed., Berlin, 1880. + +GREEN, A.O.: Tasmanian timbers: their qualities and uses. +Hobart, Tasmania, 1903, pp. 63. + +GREGORY, W.B.: Tests of creosoted timber. Trans. Am. Soc. C.E., +Vol. LXXVI, 1913, pp. 1192-1203. See also _ibid._, Vol. LXX, p. +37. + +GRISARD, JULES, et VANDENBERGHE, MAXIMILIEN: Les bois +industriels, indigènes et exotiques; synonymie et description +des espèces, propriétés physiques des bois, qualités, défauts, +usages et emplois. Paris, 189-. From Bul. de la Société +nationale d'acclimatation de France, Vols. XXXVIII-XL. + +Hardwoods of Western Australia. Engineering, Vol. LXXXIII, Jan. +11, 1907, pp. 35-37. + +HATT, WILLIAM KENDRICK: A Preliminary program for the timber +test work to be undertaken by the Bureau of Forestry, United +States Department of Agriculture. Proc. Am. Soc. Test. Mat., +Vol. III, 1903, pp. 308-343. Appendix I: Method of determining +the effect of the rate of application of load on the strength of +timber, pp. 325-327; App. II: A discussion on the effect of +moisture on strength and stiffness of timber, together with a +plan of procedure for future tests, pp. 328-334. + +HATT, WILLIAM KENDRICK: Relation of timber tests to forest +products. Proc. Int. Assn. Test. Mat., 1906, C 2 _e_, pp. 6. + +----: Structural timber. Proc. Western Ry. Club, St. Louis, Mch. +17, 1908. + +----: Abstract of report on the present status of timber tests +in the Forest Service, United States Department of Agriculture. +Proc. Int. Assn. Test. Mat., 1909, XVL, pp. 10. + +---- and TURNER, W.P.: The Purdue University impact machine. +Proc. Am. Soc. Test. Mat., Vol. VI, 1906, pp. 462-475. + +HAUPT, HERMAN: formulæ for the strain upon timber. Center of +gravity of an ungula and semi-cylinder. Jour. Franklin Inst., +Vol. XIX, 3d series, 1850, pp. 408-413. + +HEARDING, W.H.: Report upon experiments ... upon the compressive +power of pine and hemlock timber. Washington, 1872, pp. 12. + +HOWE, MALVERD A.: Wood in compression; bearing values for +inclined cuts. Eng. News, Vol. LXVIII, 1912, pp. 190-191. + +HOYER, EGBERT: Lehrbuch der vergleichenden mechanischen +Technologie. 1878. + +IHLSENG, MANGUS C.: On the modulus of elasticity of some +American woods as determined by vibration. Van Nostrand's Eng. +Mag., Vol. XIX, 1878, pp. 8-9. + +----: On a mode of measuring the velocity of sounds in woods. +Am. Jour. Sci. and Arts, Vol. XVII, 1879. + +JACCARD, P.: Étude anatomique des bois comprimés. Mitt. d. Schw. +Centralanstalt f. d. forst. Versuchswesen. X. Bd., 1. Hft., +Zurich, 1910, pp. 53-101. + +JANKA, GABRIEL: Untersuchungen über die Elasticität und +Festigkeit der österreichischen Bauhölzer. I. Fichte Südtirols; +II. Fichte von Nordtirol vom Wienerwalde und Erzgebirge; III. +Fichte aus den Karpaten, aus dem Böhmerwalde, Ternovanerwalde +und den Zentralalpen. Technische Qualität des Fichtenholzes im +allgemeinen; IV. Lärche aus dem Wienerwalde, aus Schlesien, +Nord- und Südtirol. Mitt. a. d. forst. Untersuchungswesen +Oesterreichs, Wien, 1900-13. + +----: Untersuchungen über Holzqualität. Centralblatt f. d. ges. +Forstwesen. Wien, 1904, pp. 95-115. + +----: Ueber neuere holztechnologische Untersuchungen. Oesterr. +Vierteljahresschrift für Forstwesen, Wien, 1906, pp. 248-269. + +----: Die Härte des Holzes. Centralblatt f. d. ges. Forstwesen, +Wien, 1906, pp. 193-202; 241-260. + +JANKA, GABRIEL: Die Einwirkung von Süss- und Salzwässern auf die +gewerblichen Eigenschaften der Hauptholzarten. I. Teil. +Untersuchungen u. Ergebnisse in mechanisch-technischer Hinsicht. +Mitt. a. d. forst. Versuchswesen Oesterreichs, 33. Hft., Wien, +1907. + +----: Results of trials with timber carried out at the Austrian +forestry testing-station at Mariabrunn. Proc. Int. Assn. Test. +Mat., 1906, Disc. 2 _e_, pp. 7. + +----: Ueber die an der k. k. forstlichen Versuchsanstalt +Mariabrunnen gewonnenen Resultate der Holzfestigkeitsprüfungen. +Zeitschrift d. Oesterr. Ing. u. Arch. Ver., Wien, Aug. 9, 1907. + +----: Ueber Holzhärteprufüng. Centralblatt f. d. ges. +Forstwesen, Wien, 1908, pp. 443-456. + +----: Testing the hardness of wood by means of the ball test. +Proc. Int. Assn. Test. Mat., 1912, XXIII_{3}. + +JENNY, K.: Untersuchungen über die Festigkeit der Hölzer aus den +Ländern der ungarischen Krone. Budapest, 1873. + +JOHNSON, J.B.: Time tests of timber in endwise compression. +Paper before Section D, Am. Assn. for Adv. of Sci., Aug., 1898. + +JOHNSON, WALTER B.: Experiments on the adhesion of iron spikes +of various forms when driven into different species of timbers. +Jour. Franklin Inst., Vol. XIX (n.s.), 1837, pp. 281-292. + +JULIUS, G.A.: Western Australia timber tests, 1906. The physical +characteristics of the hardwoods of Western Australia. Perth, +1906, pp. 36. + +----: Supplement to the Western Australia timber tests, 1906. +The hardwoods of Australia. Perth, 1907, pp. 6. + +KARMARSH, CARL: Handbuch der mechanischen Technologie. I. Aufl., +1837; V. Aufl., 1875; verm. von H. Fisher, 1888. + +KIDDER, F.E.: Experiments on the transverse strength of southern +and white pine. Van Nostrand's Eng. Mag., Vol. XXII, 1880, pp. +166-168. + +----: Experiments on the strength and stiffness of small spruce +beams. _Ibid._, Vol. XXIV, 1881, pp. 473-477. + +----: Experiments on the fatigue of small spruce beams. Jour. +Franklin Inst., Vol. CXIV, 1882, pp. 261-279. + +KIDWELL, EDGAR: The efficiency of built-up wooden beams. Trans. +Am. Inst. Min. Eng., Feb., June, 1898. + +KIRKALDY, WM. G.: Illustrations of David Kirkaldy's system of +mechanical testing. London, 1891. + +KUMMER, FREDERICK A.: The effects of preservative treatment on +the strength of timber. Proc. Am. Soc. Test. Mat., Vol. IV, +1904, pp. 434-438. + +LABORDÈRE, P., and ANSTETT, F.: Contribution to the study of +means for improving the strength of wood for pavements. Proc. +Int. Assn. Test. Mat., 1912, XXIII_{4}, pp. 12. + +LANZA, GAETANO: An account of certain tests on the transverse +strength and stiffness of large spruce beams. Trans. Am. Soc. +Mech. Eng., Vol. IV, 1882, pp. 119-135. See also Jour. Franklin +Inst., Vol. XCV, 1883, pp. 81-94. + +LASLETT, T.: Properties and characteristics of timber. Chatham, +1867. + +----: Timber and timber trees, native and foreign. (2d ed. +revised and enlarged by H. Marshall Ward.) London and New York, +1894. + +LEA, W.: Tables of strength and deflection of timber. London, +1861. + +LEDEBUR, A.: Die Verarbeitung des Holzes auf mechanischem Wege. +1881. + +LORENZ, N. VON: Analytische Untersuchung des Begriffes der +Holzhärte. Centralblatt f. d. ges. Forstwesen, Wien, 1909, pp. +348-387. + +LUDWIG, PAUL: Die Regelprobe. Ein neues Verfahren zur +Härtebestimmung von Materialien. Berlin. 1908. + +MACFARLAND, H.B.: Tests of longleaf pine bridge timbers. Bul. +149, Am. Ry. Eng. Assn., Sept., 1912. See also Eng. News, Dec. +12, 1912, p. 1035. + +McKAY, DONALD: On the weight and strength of American +ship-timber. Jour. Franklin Inst., Vol. XXXIX (3d series), 1860, +p. 322. + +MALETTE, J.: Essais des bois de construction. Revue Technique, +Apr. 25, 1905. + +MANN, JAMES: Australian timber: its strength, durability, and +identification. Melbourne, 1900. + +MARTIN, CLARENCE A.: Tests on the relation between cross-bending +and direct compressive strength in timber. Railroad Gazette, +Mch. 13, 1903. + +Methods of testing metals and alloys ... Recommended by the +Fourth Congress of the International Association for Testing +Materials, held at Brussels, Sept. 3-6, 1906. London, 1907, pp. +54. Methods of testing wood, pp. 39-49. + +MIKOLASCHEK, CARL: Untersuchungen über die Elasticität und +Festigkeit der wichtigsten Bau- und Nutzhölzer. Mitt. a. d. +forstl. Versuchswesen Oesterreiches, II. Bd., 1. Hft., Wien, +1879. + +MOELLER, JOSEPH: Die Rohstoffe des Tischler- und +Drechslergewerbes. I. Theil: Das Holz. Kassel, 1883, pp. 68-122. + +MOLESWORTH, G.L.: Graphic diagrams of strength of teak beams. +Roorke, 1881. + +MORGAN, J.J.: Bending strength of yellow pine timber. Eng. +Record, Vol. LXVII, 1913, pp. 608-609. + +MOROTO, K.: Untersuchungen über die Biegungselasticität und +-Festigkeit der japanischen Bauhölzer. Centralblatt f. d. ges. +Forstwesen, Wien, 1908, pp. 346-355. + +NORDLINGER, H.: Die technischen Eigenschaften der Hölzer für +Forst- und Baubeamte, Technologen und Gewerbetreibende. +Stuttgart, 1860. + +----: Druckfestigkeit des Holzes. 1882. + +----: Die gewerblichen Eigenschaften der Hölzer. Stuttgart, +1890. + +NORTH, A.T.: The grading of timber on the strength basis. +Address before Western Society of Engineers. Lumber World +Review, May 25, 1914, pp. 27-29. + +NORTON, W.A.: Results of experiments on the set of bars of wood, +iron, and steel, after a transverse stress. Van Nostrand's Eng. +Mag., Vol. XVII, 1877, pp. 531-535. + +PACCINOTTI E PERI: [Investigations into the elasticity of +timbers.] Il Cimento, Vol. LVIII, 1845. + +PALACIO, E.: Tensile tests of timber. La Ingenieria, Buenos +Aires, May 31, 1903, _et seq._ + +PARENT,--: Expériences sur la résistance des bois de chêne et de +sapin. Mémoires de l'Académie des Sciences, 1707-08. + +Propositions relatives à l'établissement d'un precédé uniforme +pour l'essai des qualités techniques des bois. Proc. Int. Assn. +Test. Mat., 1901, Annexe, pp. 13-28. + +ROGERS, CHARLES G.: A manual of forest engineering for India. +Vol. I, Calcutta, 1900, pp. 50-91. + +RUDELOFF, M.: Der heutige Stand der Holzuntersuchungen. Mitt. a. +d. königlichen tech. Versuchsanstalt, Berlin, IV, 1899. + +----: Principles of a standard method of testing wood. Proc. +Int. Assn. Test. Mat., 1906, 23 C, pp. 16. + +----: Large _vs._ small test-pieces in testing wood. Proc. Int. +Soc. Test. Mat., 1912, XXIII_{1}, pp. 7. + +SARGENT, CHARLES SPRAGUE: Woods of the United States, with an +account of their structure, qualities, and uses. New York, 1885. + +SCHNEIDER, A.: Zusammengesetzte Träger. Zeitschrift d. Oesterr. +Ing. u. Arch. Ver., Nov. 24; Dec. 9, 1899. + +SCHWAPPACH, A.F.: Beiträge zur Kenntniss der Qualität des +Rotbuchenholzes. Zeitschrift f. Forst- und Jagdwesen, Berlin, +1894, pp. 513-539. + +----: Untersuchungen über Raumgewicht und Druckfestigkeit des +Holzes wichtiger Waldbäume. Berlin, 1897-98. + +----: Etablissement de méthodes uniformes pour l'essai á la +compression des bois. Proc. Int. Assn. Test. Mat., 1901, Rapport +23, pp. 28. + +SEBERT, H.: Notice sur les bois de la Nouvelle Calédonie suivie +de considerations génerates sur les propriétés mécaniques des +bois et sur les precédés employés pour les mesurer. Paris. + +SHERMAN, EDWARD C.: Crushing tests on water-soaked timbers. Eng. +News, Vol. LXII, 1909, p. 22. + +SNOW, CHARLES H.: The principal species of wood: their +characteristic properties. New York, 1908. + +STAUFFER, OTTMAR: Untersuchungen über specifisches +Trockengewicht, sowie anatomisches Verhalten des Holzes der +Birke. München, 1892. + +STENS, D.: Ueber die Eigenschaftenimprägnierter Grubenholzer, +insbesondere über ihre Festigkeit. Glückauf, Essen, Mch. 6, +1907. + +Strength of wood for pavements. Can. Eng., Toronto, Sept. 12, +1912. + +STÜBSCHEN-KISCHNER: Karmarsch-Heerins technisches Wröterbuch. 3. +Aufl., 1886. + +TALBOT, ARTHUR N.: Tests of timber beams. Bul. 41, Eng. Exp. +Sta., Univ. of Ill., Urbana, 1910. + +Tests of wooden beams made at the Massachusetts Institute of +Technology on spruce, white pine, yellow pine, and oak beams of +commercial sizes. Technology Quarterly, Boston, Vol. VII, 1894. + +TETMAJER, L. v.: Zur Frage der Knickungsfestigkeit der +Bauhölzer. Schweizerische Bauzeitung, Bd. 11, Nr. 17. + +----: Methoden und Resultate der Prüfung der schweizerischen +Bauhölzer. Mitt. d. Anstalt z. Prüfung v. Baumaterialien am +eidgenössischen Polytechnicum in Zürich. 2. Hft., 1884. + +----: Methoden und Resultate der Prüfung der schweizerischen +Bauhölzer. Mitt. d. Materialprüfungs-Anstalt am Schweiz. +Polytechnikum in Zürich. Landesaustellungs-Ausgabe, 2. Hft., +Zürich, 1896. + +THELEN, ROLF: The structural timbers of the Pacific Coast. +Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, pp. 558-567. + +THURSTON, R.H.: Torsional resistance of materials determined by +a new apparatus with automatic registry. Jour. Franklin Inst., +Vol. LXV, 1873, pp. 254-260. + +----: On the strength of American timber. _Ibid._, Vol. LXXVIII, +1879, pp. 217-235. + +----: Experiments on the strength of yellow pine. _Ibid._, Vol. +LXXIX, 1880, pp. 157-163. + +----: Influence of time on bending strength and elasticity. +Proc. Am. Assn. for Adv. Sci., 1881. Also Proc. Inst. C.E., Vol. +LXXI. + +----: On the effect of prolonged stress upon the strength and +elasticity of pine timber. Jour. Franklin Inst., Vol. LXXX, +1881, pp. 161-169. THURSTON, R.H.: On Flint's investigations of +Nicaraguan woods. _Ibid._, Vol. XCIV, 1887, pp. 289-315. + +TIEMANN, HARRY DONALD: The effect of moisture and other +extrinsic factors upon the strength of wood. Proc. Am. Soc. +Test. Mat., Vol. VII, 1907, pp. 582-594. + +----: The effect of the speed of testing upon the strength of +wood and the standardization of tests for speed. _Ibid._, Vol. +VIII, 1908, pp. 541-557. + +----: The theory of impact and its application to testing +materials. Jour. Franklin Inst., Vol. CLXVIII, 1909, pp. +235-259; 336-364. + +----: Some results of dead load bending tests of timber by means +of a recording deflectometer. Proc. Am. Soc. Test. Mat., Vol. +IX, 1909, pp. 534-548. + +TJADEN, M.E.H.: Het Indrukken van Paalkoppen in Kespen. De +Ingenieur, Sept. 11, 1909. + +----: Weerstand van Hout loodrecht op de Vezelrichting. _Ibid._, +May, 1911. + +----: Buigvastheid van Hout. _Ibid._, May 31, 1913. + +TRAUTWINE, JOHN C.: Shearing strength of some American woods. +Jour. Franklin Inst., Vol. CIX, 1880, pp. 105-106. + +TREDGOLD, THOMAS: Elementary principles of carpentry. London, +1870. + +TURNBULL, W.: A practical treatise on the strength and stiffness +of timber. London, 1833. + +Untersuchungen über den Einfluss des Blauwerdens auf die +Festigkeit von Kiefernholz. Mitt. a. d. könig. techn. +Versuchsanstalten, I, 1897. + +Verfahren zur Prüfung v. Metallen und Legierungen, von +hydraulischen Bindemitteln, von Holz, von Ton-, Steinzeug- und +Zementröhren. Empfohlen v. dem in Brüssel v. 3-6, IX, 1906, +abgeh. IV. Kongress des internationalen Verbandes f. die +Materialprüfungen der Technik. Wien, 1907. + +WARREN, W.H.: Australian timbers. Sydney, 1892. + +----: The strength, elasticity, and other properties of New +South Wales hardwood timbers. Sydney, 1911. + +----: The strength, elasticity, and other properties of New +South Wales hardwood timbers. Proc. Int. Assn. Test. Mat., 1912, +XXIII_{6}, pp. 9. + +----: The properties of New South Wales hardwood timbers. +Builder, London, Nov. 1, 1912. + +----: The hardwood timbers of New South Wales, Australia. Jour. +Soc. of Arts, London, Dec. 6, 1912. + +WELLINGTON, A.M.: Experiments on impregnated timber. Railroad +Gazette, 1880. + +WIJKANDER, ----: Untersuchung der Festigkeitseigenschaften +schwedischer Holzarten in der Materialprüfungsanstalt des +Chalmers'schen Institutes ausgeführt. 1897. + +WING, CHARLES B.: Transverse strength of the Douglas fir. Eng. +News, Vol. XXXIII, Mch. 14, 1895. + + + +PART III. PUBLICATIONS OF THE U.S. GOVERNMENT ON THE MECHANICAL + PROPERTIES OF WOOD, AND TIMBER TESTING + + +MISCELLANEOUS + + +House Misc. Doc. 42, pt. 9, 47th Cong., 2d sess., 1884. (Vol. +IX, Tenth Census report.) Report on the forests of North America +(exclusive of Mexico). Part II, The Woods of the United States. + +House Report No. 1442, 53d Cong., 2d sess. Investigations and +tests of American timber. 1894, pp. 4. + +War Dept. Doc. 1. Resolutions of the conventions held at Munich, +Dresden, Berlin, and Vienna, for the purpose of adopting uniform +methods for testing construction materials with regard to their +mechanical properties. By J. Bauschinger. Translated by O.M. +Carter and E.A. Gieseler. 1896, pp. 44. + +War Dept. Doc. 11. On tests of construction materials. +Translations from the French and from the German. By O.M. Carter +and E.A. Gieseler. 1896, pp. 84. + +House Doc. No. 181, 55th Cong., 3d sess. Report upon the +forestry investigations of the U.S. Department of Agriculture, +1877-1898. By B.E. Fernow, 1899, pp. 401. Contains chapter on +The work in timber physics in the Division of Forestry, by +Filibert Roth, pp. 330-395. + + +FOREST SERVICE + + +Cir. 7--The Government timber tests [189-], pp. 4. + +Cir. 8--Strength of "boxed" or "turpentine" timber. 1892, pp. 4. + +Bul. 6--Timber Physics. Pt. I. Preliminary report. 1. Need of +the investigation. 2. Scope and historical development of the +science of "timber physics." 3. Organization and methods of +timber examinations in the Division of Forestry. By B.E. Fernow, +1892, pp. 57. + +Unnumbered Cir.--Instructions for the collection of test pieces +of pines for timber investigations [1893], pp. 4. + +Cir. 9--Effect of turpentine gathering on the timber of longleaf +pine. By B.E. Fernow [1893], p. 1. + +Bul. 8--Timber physics. Pt. II. Progress report. Results of +investigations on longleaf pine. 1893, pp. 92. + +Bul. 10--Timber: an elementary discussion of the characteristics +and properties of wood. By Filibert Roth. 1895, pp. 88. + +Bul. 12--Economical designing of timber trestle bridges. By A.L. +Johnson, 1896, pp. 57. + +Cir. 12--Southern pine, mechanical and physical properties. +1896, pp. 12. + +Cir. 15--Summary of mechanical tests on thirty-two species of +American woods. 1897, pp. 12. + +Cir. 18--Progress in timber physics. 1898, pp. 20. + +Cir. 19--Progress in timber physics: Bald cypress (_Taxodium +distichum_). By Filibert Roth, 1898, pp. 24. + +Y.B. Extr. 288--Tests on the physical properties of woods. By +F.E. Olmstead, 1902, pp. 533-538. + +Unnumbered Cir.--Timber tests. [1903], pp. 15. + +Unnumbered Cir.--Timber preservation and timber testing at the +Louisiana Purchase Exposition. 1904, pp. 6. + +Cir. 32--Progress report on the strength of structural timber. +By W.K. Hatt, 1904, pp. 28. + +Bul. 58--The red gum. By Alfred Chittenden. Includes a +discussion of The mechanical properties of red gum wood, by W.K. +Hatt. 1905, pp. 56. + +Cir. 38--Instructions to engineers of timber tests. By W.K. +Hatt, 1906, pp. 55. Revised edition, 1909, pp. 56. + +Cir. 39--Experiments on the strength of treated timber. By W.K. +Hatt, 1906, pp. 31. Revised edition, 1908. + +Bul. 70--Effect of moisture upon the strength and stiffness of +wood. By H.D. Tiemann, 1906, pp. 144. + +Cir. 46--Holding force of railroad spikes in wooden ties. By +W.K. Hatt, 1906, pp. 7. + +Cir. 47--Strength of packing boxes of various woods. By W.K. +Hatt, 1906, pp. 7. + +Cir. 108--The strength of wood as influenced by moisture. By +H.D. Tiemann, 1907, pp. 42. + +Cir. 115--Second progress report on the strength of structural +timber. By W.K. Hatt, 1907, pp. 39. + +Cir. 142--Tests of vehicle and implement woods. By H.B. Holroyd +and H.S. Betts, 1908, pp. 29. + +Cir. 146--Experiments with railway cross-ties. By H.B. Eastman, +1908, pp. 32. + +Cir. 179--Utilization of California eucalypts. By H.S. Betts and +C. Stowell Smith, 1910, pp. 30. + +Bul. 75--California tanbark oak. Part II, Utilization of the +wood of tanbark oak, by H.S. Betts, 1911, pp. 24-32. + +Bul. 88--Properties and uses of Douglas fir. By McGarvey Cline +and J.B. Knapp, 1911, pp. 75. + +Cir. 189--Strength values for structural timbers. By McGarvey +Cline, 1912, pp. 8. + +Cir. 193--Mechanical properties of redwood. By A.L. Heim, 1912, +pp. 32. + +Bul. 108--Tests of structural timbers. By McGarvey Cline and +A.L. Heim, 1912, pp. 1231. + +Bul. 112--Fire-killed Douglas fir: a study of its rate of +deterioration, usability, and strength. By J.B. Knapp, 1912, pp. +18. + +Bul. 115--Mechanical properties of western hemlock. By O.P.M. +Goss, 1913, pp. 45. + +Bul. 122--Mechanical properties of western larch. By O.P.M. +Goss, 1913, pp. 45. + +Cir. 213--Mechanical properties of woods grown in the United +States. 1913, pp. 4. + +Cir. 214--Tests of packing boxes of various forms. By John A. +Newlin, 1913, pp. 23. + +Review Forest Service Investigations. 1913. [Outline of +investigations.] Vol. I, pp. 17-21. A microscopic study of the +mechanical failure of wood, by Warren D. Brush. Vol. II, pp. +33-38. + +Bul. 67, U.S.D.A.--Tests of Rocky Mountain woods for telephone +poles. By Norman deW. Betts and A.L. Heim, 1914, pp. 28. + +Bul. 77, U.S.D.A.--Rocky Mountain mine timbers. By Norman deW. +Betts, 1914, pp. 34. + +Bul. 86, U.S.D.A.--Tests of wooden barrels. By J.A. Newlin, +1914, pp. 12. + + +REPORTS OF TESTS ON THE STRENGTH OF STRUCTURAL MATERIAL, MADE AT +THE WATERTOWN ARSENAL, MASS. + + +House Ex. Doc. No. 12, 47th Cong., 1st sess., 1882. Strength of +wood grown on the Pacific slope, pp. 19-93. + +Senate Ex. Doc. No. 1, 47th Cong., 2d sess., 1883. Resistance of +white and yellow pines to forces of compression in the direction +of the fibers, as used for columns, or posts, pp. 239-395. + +Senate Ex. Doc. No. 5, 48th Cong., 1st sess., 1884. Tests of +California laurel wood by compression, indentation, shearing, +transverse tension, pp. 223-236. Tests of North American woods +(under supervision of Prof. C.S. Sargent in charge of the +forestry division of the Tenth Census), with 16 photographs of +fractures of American woods, pp. 237-347. + +Senate Ex. Doc. No. 35, 49th Cong., 1st sess., 1885. Adhesion of +nails, spikes, and screws in various woods. Experiments on the +resistance of cut nails, wire nails (steel), wood screws, lag +screws in white pine, yellow pine, chestnut, white oak, and +laurel, pp. 448-471. + +House Ex. Doc. No. 14, 51st Cong., 1st sess., 1890. Adhesion of +spikes and bolts in railroad ties, pp. 595-617. + +House Ex. Doc. No. 161, 52d Cong., 1st sess., 1892. Adhesion of +nails in wood, pp. 744-745. + +House Ex. Doc. No. 92, 53d Cong., 3d sess., 1895. +Woods--compression tests (endwise compression), pp. 471-476. + +House Doc. No. 54, 54th Cong., 1st sess., 1896. Compression +tests on Douglas fir wood, pp. 536-563. Expansion and +contraction of oak and pine wood, pp. 567-574. + +House Doc. No. 164, 55th Cong., 2d sess., 1898. Compression +tests of timber posts, pp. 405-411. New posts of yellow pine and +spruce, pp. 413-450; Old yellow pine posts from Boston Fire +Brick Co. building, No. 394 Federal St., Boston, Mass., pp. +451-473. + +House Doc. No. 143, 55th Cong., 3d sess., 1899. Fire-proofed +wood (endwise and transverse tests), pp. 676-681. + +House Doc. No. 190, 56th Cong., 2d sess., 1901. Cypress wood for +United States Engineer Corps; compression and transverse tests, +pp. 1121-1126. Old white pine and red oak from roof trusses of +Old South Church, Boston, Mass., pp. 1127-1130. Compression of +rubber, balata, and wood buffers, pp. 1149-1158. + +House Doc. No. 335, 57th Cong., 2d sess., 1903. Douglas fir and +white oak woods. Transverse and shearing tests; also +observations on heat conductivity of sticks over wood fires and +a stick exposed to low temperature. Expansion crosswise the +grain of wood after submersion, pp. 519-561. Adhesion of lag +screws and bolts in wood, pp. 563-578. + + + + + +End of the Project Gutenberg EBook of The Mechanical Properties of Wood +by Samuel J. Record + +*** END OF THE PROJECT GUTENBERG EBOOK 12299 *** |