Strength In Shear. A pin which holds a tenon in its mortise (Fig. 191) must resist shear when a force is applied to draw the tenon out of the mortise. Similarly, that portion of the tenon which is immediately beyond the pin is, under the condition stated, in shear. The shear upon the pin is across the grain, while that upon the tenon is with the grain. Again, in the case cited, the pin is said to be in double shear, since in giving way it would need to yield at two points in its length, while the tenon is in single shear. The resistance of wood to shear is much less than that to tension or compression. Assuming the stress to fall on a piece one square inch in section, the resistance to shear is greatest in white oak, for which the value across the grain is 2000 pounds and with the grain about 800 pounds. In other woods the resistance to shear across the grain is from 600 to 1400 pounds, and with the grain from 350 to 600 pounds.

309. Strength under Transverse Loads is shown by resistance to forces which tend to bend the piece. Closely allied to the question of strength under the conditions stated, is that of stiffness, which is often quite as important as that of strength. A green stick is only about two-thirds as stiff as one that is dry. Heavy pine is stiffer than light pine. Wood from the butt of a tree is usually stiffer than that from the upper part of the trunk. In all full-grown pine trees the heartwood is stiffer than the sapwood, but in young pines, and also in young, second-growth hard woods, the sapwood is stiffer. It is the sapwood of second-growth hickory that is prized for carriage spokes and tool handles. The load which can be withstood by a timber subjected to a bending force varies directly as its width, as the square of its depth, and inversely as the length of the span. For example, a timber 5 inches deep and 4 inches wide is twice as strong as one which is 5 inches deep and 2 inches wide; while one which is 2 inches wide and 10 inches deep is four times as strong as one which is 2 inches wide and 5 inches deep. Again, a timber which rests on supports 16 feet apart will carry but half the load which may be sustained by a similar timber which rests on supports 8 feet apart. A consideration of numerical values is difficult unless aided by mathematical preparation. Students who are interested should seek to master the theory of beams as presented in texts dealing with the strength of materials.