Beam bridges are the simplest and most common type of bridge. A beam bridge is a beam supported at each end. The beam's size and height determine the length or span; the longer the beam, the weaker the bridge. Beam bridges do not typically span more than 250 feet. The forces exerted on the bridge are compressive on top, which causes tension on the bottom. Trusses are sometimes added to increase the rigidity of the bridge.
Arches are the only natural form of a bridge. The earliest arch bridges were built by the ancient Romans, and some still stand. Shape and structure give an arch bridge its strength; no additional support is needed. Because such as bridge is semicircular with abutments on each end, the weight load on the bridge is naturally carried outward along the two curving paths. This compressive force is pushed toward the abutments. Little to no tension exists on the underside because of this. The larger the arch, though, the more the tension affects the arch's underside.
A suspension bridge has cables strung across the expanse from which the bridge's base is suspended. The two basic types of suspension bridges are the M type and the A type. The M type has two towers and four anchorages. The towers support the majority of the weight. In the A type, or cable-stayed bridge, the cables are run from the base up to a single tower and secured. The tower in a cable-stayed bridge absorbs the compression forces. In both cases, the cables are under tension. In these bridges, the compressive force pushes down on the bridges' deck; because such bridges are suspended, the cables transfer the compression to the towers, where it is dissipated to the earth to which the towers are firmly secured. Almost all suspension bridges also have a supporting truss system beneath the bridge deck.
Cantilever bridges are cantilevers supported on only one end, and they combine concepts of arch and beam bridges. They use a series of balanced cantilevers for support, as well as foundation piers that are similar to those of a beam bridge. The central bridge is supported by the ends of the cantilever arms. The simplest cantilever bridge is made using two cantilever arms at opposite sides meeting at the center. The cantilever is similar to a bracket, protruding into space with a wide base so that no reasonable load can tip the structure.