A cantilever bridge is a variation of the simple beam bridge. A cantilever is a long arm that is anchored at one end and is free to move at the opposite end. A diving board is an example of a cantilever. When anchored firmly, a cantilever is a very strong structure. Imagine a 200 lb (91 kg) man standing on the end of diving board. The board bends only slightly, showing that it can hold a relatively large weight (the man).
A cantilever bridge consists of three parts: the outer beams, the cantilevers, and the central beam. Each of the outer beams of the bridge is somewhat similar to a short beam bridge. The on-shore edge of the bridge is attached to the ground itself or to a pier that is sunk into the ground. The opposite edge of the outer beam is attached to a second pier, sunk into the ground at some distance from the shore.
Bridge piers are vertical columns, usually made of reinforced concrete or some other strong material. In many cases, they are sunk into massive supporting structures known as abutments. Abutments are constructed in large holes in the ground, in contact with bedrock if possible, to withstand the forces created by the dead and live loads created by bridges and the traffic they carry.
Also attached to the off-shore pier is one end of a cantilever. The free end of the cantilever extends outward into the middle of the gap between the shores. An incomplete cantilever bridge consists, therefore, of two halves, one anchored to each side of the gap to be bridged and consisting of a cantilever facing toward the middle of the gap. The space between the two cantilevers, finally, is bridged by another beam, similar to that of a short beam bridge joining the two cantilevers to each other.
The distribution of forces in a cantilever bridge is fairly straightforward. The dead load and live load of the bridge is born by the two sets of piers that hold up the bridge, the outermost piers that hold up the outer edges of the bridge, and the inner piers that anchor the fixed end of the cantilever.
The two longest cantilever bridges in the world are the Forth Railway Bridge in Scotland, completed in 1890, and the Quebec Bridge in Canada, built in 1917. The former is 1,700 ft (520 m) in length and the latter, 1,800 ft (550 m) long.