Cantilever

Cantilever

A cantilever is any structure supported at one point which carries a load at the other end. Examples are a dome crane, an overhanging roof, and an unsupported balcony.

The cantilever's support need not be at one end, but can be in the middle of its horizontal span. For example, a seesaw is a cantilever. Horizontal cantilevers often have a counterweight on the their other end, such as with a crane. An aircraft wing is an example of a horizontal cantilever, where the support is on the end of the wing, supported by the wind, and the plane is the weight supported by the beam (that is, the wing).

Cantilevers need not be horizontal. A flagpole stuck in the ground is a cantilever, where the force of the wind against both flag and pole is the load causing stress. (Notice that the force against a cantilever's beam need not be applied only to its end, but can be along its entire length.) The ground acts as the point of support. Other examples of vertical cantilevers are skyscrapers, chimneys, and trees, the largest vertical cantilevers found in nature.

Some of the most important applications of the cantilever are in bridges. A steel truss is constructed horizontally, supported by a foundational support set in the river or gorge to be crossed. The truss then "juts out" over and past the support, meeting a truss from the other side. Sometimes a central connecting section, or "spanning section," is needed to complete the bridge between the trusses.

The earliest cantilever bridges were made of timber, and primitive cantilever bridges that are still standing can be found across the world. The Forth Railway Bridge in Scotland was the first cantilever bridge made entirely of steel, completed in 1892. It consisted of two connected suspended spans, each 1,710 ft (521 m) long. Another early steel cantilever bridge was over the East River in New York City, completed in 1901, with two main spans of 1,182 ft (360 m) connected by a central anchor span of 630 ft (192 m) on Blackwell Island.

The Quebec Bridge across the St. Lawrence River provided tragic lessons in the engineering of steel cantilever bridges. Begun in 1904, it was a railroad bridge designed to have a 1,800 ft (550 m) total span, which would have made it the world's longest. There were reports of a piece of metal on a tower buckling, and not long after an entire 9,000 ton (80 million newtons) section of the bridge fell into the river with 86 men on it, killing 75. Analysis showed that the collapse was a result of errors in calculating the weights and stresses and in assembling the parts.

After recover and redesign, the spanning section was again assembled and attached to the ends of the cantilever arms, only to break when a lifting stirrup broke away, killing 13 men. The new span was finally erected a year later, and the bridge opened in 1918, and has been in use ever since--the longest cantilever span in the world. (Today's steel strength would allow, in theory, a span of 2,500 ft (760 m)). The Golden Gate bridge in San Francisco is part cantilever bridge and part suspension bridge, with a cantilever span of 1,400 ft (430 m).