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Demonstration of a Cantilever Bridge

A cantilever bridge is a bridge whose main elements are cantilevers—structures that are anchored at only one end while the other end floats horizontally without any support. A simple cantilever bridge consist of two cantilever arms extending from opposite sides of an obstacle to be crossed, such as a river. But the cantilever arms do not meet in the center; instead, they support another span—a central truss bridge which rests on the ends of the cantilever arms.

Back in the 19th century, when engineers first proposed the cantilever design to the public, it required more than a little convincing. After all, how can anybody trust a bridge that has no support underneath it?

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Demonstration of the Cantilever Principle of the Forth Bridge, circa 1887.

In the image above we see three gentlemen, engineers Sir John Fowler, Kaichi Watanabe and Sir Benjamin Baker, demonstrating the principle of the cantilever. Fowler and Baker are sitting on chairs with outstretched arms and wooden poles under them to support the tension in their arms. Between them sits the Japanese engineer Kaichi Watanabe, suspended and supported by the trusting arms of his two fellow engineers. Fowler and Baker represent the cantilevers, while Watanabe represents the load on the suspended central span. The bricks on either end represents the anchor for the cantilever beams.

The bridge whose design they are demonstrating is the Forth Bridge built between 1882 and 1890 in Scotland. It is one of the most famous early cantilever bridge in the world. For seventeen years, until the construction of the Quebec Bridge the bridge held the record for the longest span in the world of more than half a kilometer. It was also the first major structure in Britain to be constructed of steel; its French contemporary, the Eiffel Tower, was built of wrought iron.

The Forth Bridge was not the first cantilever bridge to be constructed, although it was the most important one. The Hassfurt Bridge over the Main river in Germany with a central span of 38 meters, and completed in 1867 is recognized as the first modern cantilever bridge. Other cantilever bridges such as the High Bridge of Kentucky (1877), the Niagara Cantilever Bridge (1883), and the Poughkeepsie Bridge (1889) were all important early uses of the cantilever design.

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A lesser known but possibly earlier version of the more famous photograph has a young boy in place of Kaichi Watanabe. Circa 1885. Photo credit: benjaminbaker.org.uk

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Forth Bridge. Photo credit: Mike McBey/Flickr

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Forth Bridge. Photo credit: fairlight13/Flickr

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Another early cantilever bridge—the Niagara Cantilever Bridge. It was replaced by the Michigan Central Railway Steel Arch Bridge in 1925. Photo credit: Library of Congress

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