305. The Strength of Timber is measured by its resistance to yielding under the influence of external force applied in any form. Timbers may be so located with reference to the load they sustain as to be strained in tension, or in compression, or in shear, or by bending; and in each case the maximum resistance which can be offered by a piece of wood will have a different value. The maximum resistance also depends upon the direction of the grain relative to the direction in which the load is applied. In general, knotty and cross-grained wood is not so strong as clear and straight-grained pieces of the same material. Large timbers usually contain more imperfections in grain than small ones which might be cut from the larger bulk, and, hence, large timbers are likely to be relatively weaker than small ones. In general, the heavier woods are the stronger.

306. Strength in Tension is measured by the resistance which is offered to a force drawing in the direction of length. In a piece of wood, this is the sum of the resistances of all the separate fibers making up the cross-section. Long-leaved, yellow pine and Washington fir will withstand about 12,000 pounds for each square inch of cross-section, while oak, Canadian white pine, and red fir withstand about 10,000 pounds, and the more common woods, such as white pine, Norway pine, spruce, hemlock, cypress, and chestnut, from 6000 to 9000 pounds. These values are remarkably large when one considers the lightness of the materials involved.

307. Strength in Compression is the resistance offered to a force which tends to reduce the dimension of a material in the direction in which the force is applied. Columns which stand upon a foundation or base of any sort, and bear a load upon the top, are in compression. In this case the individual fibers act as so many hollow columns firmly bound together. Failure under compression occurs when the fibers, by separating into small bodies and sliding over each other, cease to act as a solid mass. This action is obviously assisted by the presence of the smallest knot or the slightest irregularity in grain. When tested in the form of short columns in which the grain runs lengthwise, the common woods withstand loads in compression of from 5000 to 8000 pounds per square inch of cross-section.