n.
Structure that spans horizontally to allow pedestrians and vehicles to cross a void.
Bridge construction has always presented civil engineering with its greatest challenges. The simplest bridge is the beam (or girder) bridge, consisting of straight, rigid beams placed across a span (e.g., a tree trunk laid across a stream). Ancient Roman bridges are famous for their rounded arch form, which permitted spans much longer than those of stone beams and were more durable than wood. A modification of the arch bridge was the drawbridge, developed during medieval times. The lift bridge, another movable type, can change position to allow clearance for ships and boats. Suspension bridges (e.g., Brooklyn Bridge , Golden Gate Bridge ) are capable of spanning great distances; their main support members are cables composed of thousands of strands of wire supported by two towers and anchored at each end, and the roadway is supported by vertical cables hung from the main cables. Other bridges include the truss bridge, popular (e.g., for railroad bridges) because it uses a relatively small amount of material to carry large loads, and the cantilever bridge, typically made with three spans, with the outer spans anchored down at the shore and the central span resting on the cantilevered arms.