Bridge

Bridge, structure that spans an obstacle and permits traffic across it. The beam (or girder) bridge consists of a rigid beam resting at either end on piers. A development of this is the truss bridge, with a metal framework designed for greatest strength at those points where the load has greatest moment around the piers. Where piers are impracticable, a cantilever bridge may be built on beams (cantilevers) extending from each side. An arch bridge consists of one or more arches. A suspension bridge comprises two towers that carry one or more flexible cables that are firmly anchored at each end; the roadway is suspended by means of vertical cables. Movable bridges include 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, a pair of towers between which runs a beam that may be winched vertically upward; and the retractable bridge, whose cantilevers may be run inshore on wheels. Temporary bridges include the pontoon or floating bridge, in which floating structures support a continuous roadway.