The members usually form one or more triangles in a single plane and are so arranged that the external loads are applied at the joints and theoretically cause only axial tension or axial compression in the members. The members are assumed to be connected at their joints with frictionless hinges or pins, which allow the members freedom to rotate slightly. Trusses have been used in bridge building since the middle ages. Trusses are made up of a top and bottom chord , diagonals and vertical posts. These are the primary members of the bridge.
Vertical posts are in tension .
Counter bracing or ties are diagonals, present in certain types of trusses, sometimes in the center panel. Floor beams span transversely from truss panel points. Stringers span longitudinally, parallel to the trusses spanning the floorbeams. Top and bottom lateral bracing and portal bracing, sway bracing and struts are secondary members which help to stiffen the structure. The deck is the surface on which traffic or pedestrians ride or walk.