A cantilever bridge is a type of bridge that uses projecting beams, called cantilevers, which are supported only at one end to form or underpin the main span. The cantilevers may be simple beams for small footbridges, but for large cantilever bridges designed to handle road or rail traffic, trusses built from structural steel or box girders built from prestressed concrete are used. Cantilever bridges are especially useful for spanning a waterway without dividing it with river piers. The main features of a cantilever bridge are:
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Cantilevers: These are structures that project horizontally into space and are supported on only one end. They form the main elements of a cantilever bridge.
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Form and mechanics: A cantilever bridge is generally made with three spans, of which the outer spans are cantilevered, anchored at opposite ends to the opposite banks of a declivity to be crossed, such as a river valley, over which the free ends of the spans extend toward each other. The central span rests on the cantilevered outer spans. A cantilever bridge may carry vertical loads like a simply supported beam bridge. As the beams of the main span bend under such loads, the span undergoes horizontal compression on the top and horizontal tension on the bottom. The main span carries these forces to the outer spans, which carry them by compression vertically to the outer spans’ foundations.
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Construction: Cantilever bridges generally carry heavy loads over water, so their construction begins with the sinking of caissons and the erection of towers and anchorages. Caissons are large boxes or cylinders made of wood, metal, or concrete that are sunk into the riverbed to the level of stable soil. The towers and anchorages are then built on top of the caissons. The cantilever arms are built out from the towers, and the central span is built out from the cantilever arms.
Cantilever bridges were first built in the 19th century and were a major engineering breakthrough at the time, as they can span distances of over 1,500 feet (460 m) and can be more easily constructed at difficult crossings by virtue of using little or no falsework.
