Various types of movable (top) and fixed (bottom) bridges.
A bridge is any device that spans an obstacle and permits traffic of some kind (usually vehicular) across it. Bridges that carry canals are more generally termed aqueducts.
The most primitive form is the beam bridge (or girder bridge), consisting of a rigid beam resting at either end on piers. The span may be increased by use of intermediate piers, possibly bearing more than one beam. A development of this is the truss bridge, a truss being a metal framework specifically designed for greatest strength at those points where the load has greatest moment about the piers. Where piers are impracticable, cantilever bridges may be built; from each side extends a beam (cantilever), firmly anchored at its inshore end. The gap between the two outer ends may be closed by a third beam. Another form of bridge is the arch bridge, essentially an arch built across the gap: a succession of arches supported by intermediate piers may be used for wider gaps. A suspension bridge comprises two towers that carry one or more flexible cables that are firmly anchored at each end. From these is suspended the roadway by means of vertical cables.
Movable bridges take many forms, the most common being the swing bridge, pivoted on a central pier; the bascule (a descendant of the medieval drawbridge), whose cantilevers are pivoted inshore so that they may be swung upward; the vertical-lift bridge, comprising a pair of towers between which runs a beam that may be winched vertically upward; and the less common retractable bridge, whose cantilevers may be run inshore on wheels. The most common temporary bridges are the pontoon, or floating bridge, comprising a number of floating members that support a continuous roadway; and the Bailey bridge.