This truss, however, is not a true truss, as it depends partly upon the resistance of the joints for its stability, and whenever used probably exerts more or less thrust on the walls.
As ordinarily constructed the truss shown in Fig. 44 may be considered as two compound rafters merely fastened securely together at the top and supported at the bottom by the walls, as illustrated by Fig. 45. It is evident that the tendency of the loads in Fig. 45 is to bend the rafters, as shown in Fig. 46, and also to tear them apart at the joint. The action of the truss, Fig. 44, therefore, consists in resisting the tendency of the compound rafters to bend, and also the tendency to pull apart at joints a b c. Any bending of the rafters must produce an outward thrust on the walls, and any give in the joints would also have the same effect.
To return to Fig. 45, it will be seen that if the walls are sufficiently stable that they cannot be thrust outward, only a pin or butt joint will be required at the top, and only the lower purlins will tend to produce flexure.
It is also evident that the greater the depth of the compound rafters, and the more firmly they are fastened at their intersection, the greater will be the resistance to bending and tearing apart.
In practice it is probable that where trusses of this type are used, they derive their stability both from the resistance of the walls and buttresses and from the resistance of the framework.
The writer is doubtful if the exact resistance of the truss shown in Fig. 44 (which has actually been built) can be determined.
Whenever used the walls should be strengthened opposite the trusses by buttresses.
A truss of the form shown in Fig. 47 is a true truss and the strains in it can be accurately determined. When built in the proportion shown in the figure however, the tensile strain in the members t, t and r is very great. In practice the strains in these pieces are somewhat lessened by placing braces at d d and by the resistance of the joints a, b and c.