The Mechanical Properties of Wood Part 6

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 100C. (212F.) 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 Arborvitae 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.)


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_ 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_ 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.]

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