TRUSS CONSTRUCTION.
The principle of a truss is theoretically a num ber of straight bars joined near their ends by flexible joints, and arranged so that all their internal stresses are sustained by its members, and only the vertical pressures (the weights of the truss and its load) are transmitted to its abutments. Trusses differ from solid beams inasmuch as the weight of the truss and its load may be regarded as divided into portions which are concentrated at the joints between the mem bers, and which act through the centers of gravity of their cross-sections. So placed, the stresses caused by them could not act trans versely of the members, as in a beam, causing secondary stresses, but must act longitudinally of the members, and must be uniformly distrib uted over their entire cross-sectional areas. This is the distinguishing feature of all trusses; while in a solid beam, when it bends under its load or its own weight, all the fibers above the neutral axis are compressed, and all those below are extended, the resulting change of length in each fiber being proportional to the distance of the fiber from the neutral axis.
Most of the trusses in common use consist of two long members, called chords, extending the entire length of the span, and connected by web members, which are sometimes all inclined and sometimes alternately vertical and inclined.
Inclined web members are called diagonals, such web members being known as ties and struts. A member sustaining tension is called a rod or tie; and one sustaining compression is called a strut or post; while one capable of sustaining both tension and compression is called a tie strut.
The simplest form of truss consists of a sin gle triangle (Fig. 170). Truss a is in corn pression in the rafters, and in tension in the chord or tie-rod; and truss b is in compression in the chord, and in tension in the tie-rods—the reverse of a. This, of course, is in common use for roofs of small span, as in dwellings, and. in practice, is loaded along the rafter, and not alone at the apex as in a; but in calculating the stresses in the members, we commonly first assume that the loads are concentrated at the intersection of the truss members, and the effect of actual distribution along the members is then determined, separately treating the members as beams. Other more elaborate and complicated trusses are all built up on this principle of the simple triangle.