Fig. 237 shows what is known as a king-post truss. Its distinguishing feature is the member A called a kingpost, 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 principle 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 consider 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.

Fig. 237. Section Showing Design of King Post Truss

Fig. 237. Section Showing Design of King-Post Truss.

In Fig. 237 are shown two different methods of placing the purlins. 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. 238. Section Showing Design of King Post Truss for Wide Span

Fig. 238. Section Showing Design of King-Post Truss for Wide Span.

Fig. 239. Section Showing Design of Trussed Roof Using Iron Castings at Joints

Fig. 239. Section Showing Design of Trussed Roof Using Iron Castings at Joints.

Fig. 238 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 vertical members are not iron rods, as in Fig. 237, but 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.

Fig. 239 shows a truss with the diagonal members running in a direction opposite to that in which run the diagonal members in the two trusses previously shown. This figure also illustrates the practice of placing an iron casting at each joint of the truss to receive the members which come together at that joint. This arrangement is, however, an expensive one on account of the castings, and it is doubtful if the advantage gained by the use of them is sufficient to warrant the additional cost. Usually the castings can not be kept in stock and must be made to order for each truss. Queen=Post Truss. Fig. 240 shows a modification of the kingpost truss, which is called the queen-post truss. Here there are two queen-posts instead of the single king-post. The queen-post truss is somewhat more popular in building work than is the king-post truss, but both are frequently employed in halls, warehouses, and stables, where an ornamental truss is not required, and also in churches and audience rooms, where they are to be concealed by other finish. Fig. 240 also shows how a floor or ceiling may be supported on the lower chord or tie-beam of the truss. The joists C are hung from the chord by means of stirrup irons or patent hangers. This arrangement makes the tie-beam act as a beam as well as a tie and in this case it must be made sufficiently strong to carry the load from the joists without sagging.

Fig. 240. Section Showing Design of Queen Post Truss

Fig. 240. Section Showing Design of Queen-Post Truss.

The queen-post truss, as will be seen, is not entirely composed of triangles, the center panel being in the form of a rectangle. In most cases this is not a serious disadvantage, since, when the truss is uniformly loaded, as it would be if it were an ordinary roof truss, there is no tendency to distort the center panel. It is almost always better, however, to introduce an additional diagonal member into this panel so as to divide it into two triangles. This obviates any danger of distortion of this panel.