Timbers in a state of tension are called ties, while such as are in a state of compression are termed struts. This subject can be illustrated in the following manner:
Let A and B (Fig. 30) represent beams of timber supporting the weights W, W, and W; A having but one support, which is in the middle of its length, and B two, one at each end. To show the nature of the strains, let each beam be sawed in the middle from a to b. The effects are obvious: the cut in the beam A will open, whereas that in B will close. If the weights are heavy enough, the beam A will break at b; while the cut in B will be closed perfectly tight at a, and the beam be very little injured by it. But if, on the other hand, the cuts be made in the bottom edge of the timbers, from c to b, B will be seriously injured, while A will scarcely be affected. . By this it appears evident that, in a piece of timber subject to a pressure across the direction of its length, the fibres are exposed to contrary strains. If the timber is supported at both ends, as at B, those from the top edge down to the middle are compressed in the direction of their length, while those from the middle to the bottom edge are in a state of tension; but if the beam is supported as at A, the contrary effect is produced; while the fibres at the middle of either beam are not at all strained. The strains in a framed truss are of the same nature as those in a single beam. The truss for a roof, being supported at each end, has its tie-beam in a state of tension, while its rafters are compressed in the direction of their length. By this, it appears highly important that pieces in a state of tension should be distinguished from such as are compressed, in order that the former may be preserved continuous. A strut may be constructed of two or more pieces; yet, where there are many joints, it will not resist compression so well.