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A cantilever, also called a fixed end beam, is a beam supported only at one end. The beam cannot rotate in any direction; thus it creates a solid support. The cantilever is considered the third of the three great structural methods, the other two being post-and-beam construction and arch construction. The cantilever thrusts down which is different from the thrust of an arch which is outward against its supports.

Cantilevers did not become popular in architecture until the invention of steel and its widespread adoption in construction, because the combined strength of steel and cement is needed to create an effective cantilever system. A building using cantilevers has an internal skeleton, from which the walls hang very much like curtains. Unlike more traditional building methods where the walls are used as support for the ceiling and walls, here they are dividers of space. This allows the interior of the building to be designed for purpose and creative architecture, rather than on where columns and other structural supports must be. The most famous architect to use the cantilever system was Frank Lloyd Wright. He first used it in the 1906 construction of the Robie House in Chicago. With the use of steel and concrete, Wright was able to extend the roof 20 ft (6 m) beyond its support. With the cantilever and Wright's belief in the use of the nature in which the building resided, an entire new school of architecture was created called the Prairie School.

Before cantilevers were used in buildings, they were used to create bridges. The first cantilever bridge was built in the late 1800s by Heinrich Gerber in Germany. He based his ideas on ancient Chinese bridges which, much earlier, used the concept of the cantilever. By using the cantilever, bridges would no longer need supports in their middles and, thus, could span deep ravines or rivers. In addition, bridges could be built across extremely wide bodies of water, or valleys, because fewer supports are needed, and the supports which are used can be further apart. Thus, the incorporation of steel, cement, and cantilevers changed the world of architecture and civil engineering.

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