A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. A balcony protruding from a building would be an example of a cantilever. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 460m (1,500 ft), and can be more easily constructed at difficult crossings by virtue of using little or no falsework (temporary supports). The Hassfurt Bridge over the Main river in Germany with a central span of 38m (124 ft) was completed in 1867 and is recognized as the first modern cantilever bridge.
A simple cantilever span is formed by two cantilever arms extending from opposite sides of an obstacle to be crossed, meeting at the centre. In a common variant, the suspended span, the cantilever arms do not meet in the centre; instead, they support a central truss bridge which rests on the ends of the cantilever arms. The suspended span may be built off-site and lifted into place, or constructed in place using special travelling supports.
A common way to construct steel truss and prestressed concrete cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a balanced cantilever; when they attach to a solid foundation, the counterbalancing arms are called anchor arms. The parallel bridges which form the West-Link in West Dublin were built in this manner. In a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the balanced cantilever’s supports, the bridge superstructure often takes the form of towers above the foundation piers. The Commodore Barry Bridge is an example of this type of cantilever bridge.
The most famous example of this type of bridge is the Forth Railway Bridge in Scotland, which was the longest span in the world from 1890 until 1919 when the Quebec Bridge in Canada was built.