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, au expensive one on account of the ings, 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 post 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 arc 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.
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.
Scissors Truss. The scissors truss is shown in Fig. 242. It has no beam and, therefore, it will exert considerable thrust on the walls of the building, which thrust must be taken care of by buttresses built on the outside of the walls. This is perhaps the most simple form
of truss which can be used when an open timber truss is required.
All the parts are of wood. If desired, an iron tie rod may be inserted between the two wall bearings of the truss, so as to elimi nate the thrust on the walls, and this rod need not detract seriously from the appearance of the open timber work.
Hammer Beam Truss. A very popular form of truss for use in churches is the hammer beam truss mentioned above. This is shown in Fig. 243. On the left is shown the framework for the truss, while on the right is shown the way in which it may be finished. Its characteristic feature is the hammer beam A. The sizes of the pieces can only be determined by calculation or experience, and depend entirely upon the span of the truss and the loads to be carried, which are different for different locations. It is common practice to insert a tie rod between the points B and C to take up the thrust which would otherwise come on the walls. All parts of the framework must be securely bolted or spiked together so as to give a strong, rigid foundation for the decoration, which should be regarded merely as decoration and should not be con sidered as strengthening the truss in any way.
Truss Details. There are several methods of supporting the purlins on wood trusses, but the method illustrated in Fig. 244 is one of the best as well as the most frequently employed. A block of wood A is set up against the lower side of the purlin, and prevents it from turn ing about the corner B, which it has a tendency to do. The block is set into the chord of the truss to a depth suffi cient to keep the purlin from sliding downward as it receives the weight from the rafters E. This figure also shows the most simple method of fram ing a strut into the chord of a truss.
The strut C is set into the chord D far enough to hold the strut in place. If it is perpendicular to the chord, it need not be so set into it, if the pieces are well nailed together, because in this case there is no tendency for the strut to slide along the chord. Care should be taken not to weaken the chord too much in cutting these mortises.
In Fig. 245 are shown the most common methods of forming the joint between the top chord and the tie-beam of a truss. The connection shown at A depends upon the bolts for its strength, while that shown at B depends upon the wrought-iron straps E, which are bent so as to engage notches cut in the tie-beam F. The piece C is very often added beneath the tie-beam, at the bearing, to strengthen it at this point, where the beam is subject to consider able bending stress. The block D is merely for filling and to pro tect the bolts where they pass between the chord and the tie-beam. It may be omitted in many cases. The plate G is placed between the nuts or bolt heads and the wood to prevent the crushing of the latter. Washers should be used with all bolts for this purpose.