TIMBER TRUSSES In the discussion of roofs and roof framing which has already been given here, only those roofs have been considered which were of so short a span that they could easily be covered with a frame work of ordinary rafters, spaced from 1 to 2 feet apart, between centers, but it is very often necessary to build roofs of larger span, for which ordinary rafters, even if supported by dwarf walls and collar beams, are not sufficiently strong. In this case a different method of framing must be employed.
Instead of a number of rafters spaced fairly close together, and all of equal strength, we will have a few heavy "trusses," placed at intervals of 10 or more feet, and spanning the entire distance between the two side walls. On top of the trusses are laid "purlins," running parallel to the walls, which in their turn support the common rafters, running perpendicular to the side walls, as in the case of simple rafters in an ordinary roof. There may be one or more purlins in each slope of the roof, depending upon the size of the span, since the purlins must be spaced near enough together so that a small rafter can span the distance between them. Usually there will be a pur lin at each joint of the truss and the joints will be determined by the safe span for the rafters.
This arrangement is shown in plan in Fig. 236, in which A are trusses, B are the purlins, C are the common rafters, and D is the ridge.
There are many different kinds of trusses in common use for various kinds of buildings, which differ from each other chiefly in the arrangement of the tension and compression pieces of which every truss is built up. Some trusses are built entirely of timber, while in others timber is employed only for the compression pieces, and wrought iron and steel for the tension pieces.
King=Post Truss. Fig. 237 shows what is known as a king-post truss. Its distinguishing feature is the member A called a king post, B are the purlins, and E are the rafters resting on them. As will be seen by a study of the figure, the members of the truss are so arranged as to divide it up into a series of triangles, or rather into a series of triangular open spaces, bounded by the various members of the framework. This is an essential characteristic of a good and
efficient truss. Such a framework may fail by overloading in such a way as to be crushed or broken, but it can not be distorted, that is, none of the triangular spaces can change their shape without some member of the truss being either lengthened or shortened, which means that some member of the framework must fail by either tension or compression before the truss can be distorted, or can fail to carry its load by reason of the failure of the joints. This prin ciple does not hold true for a framework composed of spaces in the form of rectangles, of which the members of the framework form the sides, because it is possible for a rectangular framework to become distorted without any side being either lengthened or shortened, by the simple failure of some of the joints and the movement of the members around the joints. For this reason the first thing to con sider in designing a truss is the arrangement of the members and the position of the joints so all of the open spaces will be in the form of triangles.
In Fig. 237 are shown two different methods of placing the pur lins. As will be readily seen, some of them are set so that their longer dimension in cross section is vertical, while others are set so that their longer dimension is at right angles to the rafters. Both of these methods are commonly employed. The tension members C are merely for the support of the lower chord or tie-beam D.
Fig. 23S shows a truss of the same general form as the one shown in Fig. 237, but of larger span. This truss is of such a span and has its joints and purlins arranged in such a way that it is similar to the trusses shown in plan in Fig. 236 and there marked A. In this truss also the veTtical members are not iron rods, as in Fig. 237, hut are composed of timber. The stresses in these members are, however, still tension stresses just the same as in Fig. 237, and for this reason it is a common practice to fasten them to the chords of the truss by means of iron straps, as shown at the points marked A in Fig. 238. In other respects this truss is constructed in a manner similar to that in which the truss shown in Fig. 237 is built.