The most important qualities of building material necessary to consider are its strength and breaking strain or breaking weight, and the amount of pressure which can be safely laid upon it in accordance with its form, thickness, position, etc. It has been ascertained by actual experiments that the strength of a beam or girder of timber, and hence of any piece of timber, whether large or small, increases directly as the width, and as the square of the depth. Thus, if a piece of wood measuring three inches in breadth and three inches in depth that is to say, nine inches square in section—will bear a certain weight, a beam six inches broad and three inches deep will bear twice the weight; but a beam three inches broad and six inches deep will bear four times the weight. The strength is also inversely as the length. If two beams of equal breadth and depth be taken, but one of them be twice as long as the other, the longer beam will only bear half the breaking weight that the shorter one will sustain, or, in other words, will be only half as strong.

It will now be clear why, in laying joists to sustain a floor, the timbers are so placed as to have considerable depth from top to bottom, while the breadth is comparatively narrow.

A continued strain tends to weaken the power of resistance in a beam, and the power will be lessened still more when the weight is variable, or is a rolling instead of a dead weight. The nature of the wood must also be taken into account: thus, some in which the fibre is long and the grain straight will bend to a very great degree, while others in which the grain is short and close will scarcely bend at all, but break suddenly. In framing timber, as the carpenter is called upon to do, all these points must be taken into consideration.

The instantaneous breaking weight of any kind of wood is the weight under which it will give way and break when loaded with the weight in the centre. It has been said that the load with which a beam may be weighted without risk should never exceed more than one-third of the breaking weight; but it is better and safer never to let the load exceed one-fourth of the breaking weight. Indeed, it is argued that timber is permanently injured if more than this "is applied to it. The best authorities on carpentry say that a load cannot be looked on as safe if it exceeds one-fifth part of the breaking weight.

It is by no means a difficult thing to find the breaking weight of every piece of timber, and, this being known, the load that it will sustain without injury; this as it has just been shown, estimated by different authorities at from one-fifth to one-third of the breaking weight. The following is a general rule for finding the breaking weight in the middle for girders of wood supported at both ends:

Rule.

Multiply the breadth in inches by the square of the depth in inches, and divide by the length of bearing in feet. The result obtained, when multiplied by a certain constant or invariable quantity, for the kind of timber under consideration, gives the breaking weight in the centre in hundredweights.

This constant or invariable quantity, which has been determined by a series of experiments, is stated by Barlow to be: For Ash, 6; for Oak, 5; for Pitch Pine, 5; for Red Pine, 4; for White Pine, 3.