e to which rigidity is given by staying and bracing, so that its figure shall be incapable of alteration by turning of the bars about their joints.
The simplest frames are of wood and of few parts.
More imposing structures are more complicated, the parts being employed in resisting extension or compression.
Composite trusses employ both wood and iron; in fact, few of any importance are destitute of bolts and tie-rods.
The principal simple forms are shown in roof, Fig. 4420-23, pages 1971-73.
Railway bridges of the present day exhibit many structures of this class See truss-bridge; also tubular bridge, and list under bridge.
Roof-trusses. d2, Fig. 6704, shows secondary rafters r′ r′, which are added to receive the sheathing, and connected to the principal rafters r r by purlins q q. The king-post is extended down to support the center of the tie-beam, into which the principal rafters only are mortised.
In Fig. 6704, e is a queen-post wooden roof-truss at the Greenwich Hospit