Fig. 42. A Timber Under Cross Stress, Showing Neutral Axis, and the Lines of Tension and Compression. A knot occurring in such a timber should be in the upper half, as at A.
Wood is much stronger than is commonly supposed. A hickory bar will stand more strain under tension than a wrought iron bar of the same length and weight, and a block of long-leaf pine a greater compression endwise than a block of wrought iron of the same height and weight. It approaches the strength of cast iron under the same conditions.
Strength depends on two factors: the strength of the individual fibers, and the adhesive power of the fibers to each other. So, when a piece of wood is pulled apart, some of the fibers break and some are pulled out from among their neighbors. Under compression, however, the fibers seem to act quite independently of each other, each bending over like the strands of a rope when the ends are pushed together. As a consequence, we find that wood is far stronger under tension than under compression, varying from two to four times.
Woods do not vary nearly so much under compression as under tension, the straight-grained conifers, like larch and longleaf pine, being nearly as strong under compression as the hard woods, like hickory and elm, which have entangled fibers, whereas the hard woods are nearly twice as strong as the conifers under tension.
Moisture has more effect on the strength of wood than any other extrinsic condition. In sound wood under ordinary conditions, it outweighs all other causes which affect strength. When thoroly seasoned, wood is two or three times stronger, both under compression and in bending, than when green or water soaked.[6]
The tension or pulling strength of wood is much affected by the direction of the grain, a cross-grained piece being only 1⁄10th to 1⁄20th as strong as a straight-grained piece. But under compression there is not much difference; so that if a timber is to be subjected to cross strain, that is the lower half under tension and the upper half under compression, a knot or other cross-grained portion should be in the upper half.
Fig. 43. Shearing Strength is Measured by the Adhesion of the Portion A, B, C, D or to the Wood on both sides of it.