What are the types of bridges, and how do they work?
There are more than half a million bridges in the United States. They transport people across valleys, streams, and railroads, and they are constructed as one of four basic types: a beam bridge, an arch bridge, a suspension bridge, and a cantilever bridge. The simplest and most common form of bridging, the beam bridge has straight slabs or girders carrying the roadbed. Its span is relatively short (often not more than 250 feet [76 meters]) and its load rests on its supports or piers. The arch bridge, usually made of steel or concrete, looks like an arch, and thrusts outward on its bearings at each end. They can span up to 800 feet (244 meters). In the suspension bridge, the roadway hangs on steel cables, with the bulk of the load carried on cables anchored to the banks. Suspension bridges can span great distances—2,000 to 7,000 feet (610 to 2134 meters)—without intermediate piers. A cantilever bridge is a bridge built using cantilevers—structures that are built horizontally into space, supported on only one end. The cantilevers may be simple beams; however, large cantilever bridges made to handle road or rail traffic use trusses, an interconnected framework of beams, built from structural steel.