What is a Cantilever Bridge?
Cantilever bridges are designed for road or rail traffic and typically built from structural steel or boxed girders made from prestressed concrete. The defining feature of a cantilever bridge is, well, a cantilever. Cantilevers are structures that project horizontally with support on only one end. Cantilevers may also be beams for smaller footbridges. The larger steel truss cantilever bridges can be as long as 1,500 feet. The Quebec Bridge currently has the longest main span in the world at 1,801 feet long, surpassing the most famous cantilever bridge called the Forth Bridge at 1,700 feet long for the main span.
The History
The cantilever bridge was put into practice in the early 19th century when engineers would add multiple hinge points to distribute the load throughout the bridge and calculate stresses on the girder easier than before. It would help in reducing the stress in the girder or truss and allow for a longer bridge to be built. The Hassfut Bridge was the first cantilever bridge, built in 1867 over the Main river in Germany, and was 124 feet long.
How it Works
A cantilever span is formed when two cantilever arms extend from opposites sides and cross in the middle. In a suspended span, the arms don’t cross in the middle, and instead, they support a truss bridge that rests on the cantilever arms. When the prestressed concrete cantilever spans are constructed, the arms are counterbalanced projecting from opposite directions to create a balanced cantilever. When they are attached to a solid foundation they are called anchor arms. As an example, a foundation pier will have two balanced cantilever arms, and two anchor arms to add more strength at the balanced cantilever supports. The steel truss cantilevers use the tension of the upper members and compression of the lower to support loads.
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