A cantilever bridge can be used for pedestrians, trains, and motor vehicles.
A cantilever bridge is built using cantilevers, which are horizontal structures supported only at one end. With the right materials and engineering, a steel truss cantilever bridge can span more than 1,500 feet (460 m). This type of bridge has been used for pedestrians, trains and motor vehicles. Cantilevers are especially useful for crossing a watercourse without sharing it with river docks.
The Forth Bridge in Scotland is a famous cantilever bridge.
Engineer Heinrich Gerber built the first cantilever bridge in 1867. He had invented the hinged beam just a year earlier. With this new invention, Gerber was able to lengthen the cantilevers and build a bridge long enough to cross Germany’s main river.
In 2010, the Quebec Bridge in Canada holds the record for the longest single-span cantilever. The single cantilever bridge is 549m long. In Scotland, Forth Bridge is another famous cantilever bridge. Made of steel, the Forth Bridge uses balanced cantilevers on land and river piers to form a combined span of 3,300 feet (1,006 m).
Cantilevers must be securely anchored to one side to support the required weight on the free side and prevent shear stresses. A common example of a small overhang is a trampoline. One side is firmly attached to the ground so that the other side can support the weight of a person suspended above the water. The cantilevers must resist tension at the upper supports and compression at the lower ones.
Sometimes a gap or stream is too wide to be measured by an overhang on each side. In this case, a girder bridge or truss bridge is often placed between the arms of the two cantilevers, connecting them into a bridge. This type of bridge is known as a suspension bridge.
A balanced cantilever is another technique for traversing long distances. In this situation, the engineers build a dock in the middle of the river. They then anchor two cantilevers to the dock, each facing away from the other. These cantilevers, or anchor arms, join with other cantilevers anchored to the ground or another pillar to create a complete bridge.
The first step in building a cantilever bridge is to build and connect the arms to the shore. River docks for balanced swings should not be built until the swings touch the ground. If a beam or truss bridge is required, it cannot be added until the cantilevers are ready. The girder bridge is often built off-site and lowered by crane into position.