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Cantilever bridges use horizontal structures supported on one end only, and can span over 1,500 feet with the right materials and engineering. The first cantilever bridge was built in 1867 by Heinrich Gerber. The Quebec Bridge in Canada holds the record for the longest single cantilever span, while the Forth Bridge in Scotland uses cantilevers balanced on river piers and on land. Cantilevers must be firmly anchored on one side to avoid shear stresses, and sometimes a beam or truss bridge is interposed between the arms of two cantilevers to create a suspension bridge. A balanced cantilever is another technique for covering long distances. The first step in building a cantilever bridge is to build and connect the arms to the shore.
A cantilever bridge is constructed using cantilevers, which are horizontal structures supported on one end only. With the right materials and engineering, a steel truss cantilever bridge can span over 1,500 feet (460m). This type of bridge has been used for pedestrians, trains and motor vehicles. Cantilevers are particularly useful for crossing a stream without dividing it with river piers.
Engineer Heinrich Gerber built the first cantilever bridge in 1867. He had invented the hinged girder just a year earlier. With this new invention, Gerber was able to lengthen the cantilevers and build a bridge long enough to cross the German Main River.
As of 2010, the Quebec Bridge in Canada holds the record for the longest single cantilever span. The simple cantilever bridge is 1,800 feet (549 m) long. In Scotland, the Forth Bridge is another famous cantilever bridge. Made of steel, the Forth Bridge uses cantilevers balanced on river piers and on land to achieve a combined span of 3,300 feet (1,006 m).
Cantilevers must be firmly anchored on one side to support the necessary weight on the free side and avoid shear stresses. A common example of a small cantilever is a trampoline. One side is firmly fixed to the ground so that the other side can support the weight of a person suspended over the water. Cantilevers must resist tension in the upper supports and compression in the lower ones.
Sometimes, a crevice or stream is too wide for a cantilever to pass through on either side. In this case, a beam or truss bridge is often interposed between the arms of the two cantilevers, connecting them into a single bridge. This type of bridge is known as a suspension bridge.
A balanced cantilever is another technique for covering long distances. In this situation, 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, meet with other cantilevers anchored in the ground or another pier to create an entire bridge.
The first step in building a cantilever bridge is to build and connect the arms to the shore. River docks for balanced cantilevers should not be constructed until the ground touching cantilevers have been completed. If a girder or truss bridge is needed, it cannot be added until the cantilevers are ready. The girder bridge is often built off site and lowered with a crane in place.
