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This is employed mostly to overcome construction difficulties where it is not practical to temporarily support the bridge from below during construction. Being relatively rigid it is suitable for heavy rail applications, suitable to support the concentrated load of a locomotive. While as a form of architectural structure a cantilever usually has but one extended side, in bridges the structure is usually composed of at least two balanced pairs of cantilevers. An advantage of the cantilever is that it can be constructed by working only from the support caissons - this is done by building each side of the cantilever pair in synchronization to ensure the balance of the structure. Most bridge cantilevers uses a pair balanced structures, with two equal cantilevers extending, one to each side, from a central tower. Typically, two such double cantilevers upon completion will be securely anchored to massive supports at their outer spans to resist the inward tipping of the cantilevers, with a substantial gap between the two cantilevers. Between each pair of cantilevers a prefabricated central span may be lifted into place using cables or the central span may be fabricated in place from pieces, once the extreme portions of the bridge are securely anchored. The eastern span of the San Francisco-Oakland Bay Bridge uses this basic structure but with built-up lattice beams and plate rod tension elements. This portion of the Bay Bridge is soon to be replaced by a more modern (and supposedly more earthquake resistant) span. Probably the most famous of all cantilever bridges is the Forth Rail Bridge over the Firth of Forth in Scotland, notable for its innovative use of tubular structural elements and its use of three balanced cantilevers.
The cantilever principle is used during the construction of compression arch bridges. In most modern long span construction, temporary vertical towers are used with cable stays to hold up the arch, which is built in sections, each attached to a new stay. This method is similar to that used to construct a cable stayed bridge. Substantial backstays are anchored in the ground to resist the inward tipping forces on the towers. Upon near completion a short jack bridge is placed across the final gap and the arches are pushed apart to receive the final arch segments. Upon completion the stays, backstays, jack bridge, and temporary towers are removed. The use of the temporary tower greatly reduces the amount of material required and simplifies the design. Compression arch bridges may be built using self supporting cantilevers during construction but as the loads on elements can vary in tension and compression between construction and final use this may lead to a less efficient use of materials.
A pontoon bridge consists of a deck supported by tank- or boat-like floats. To allow passage of boats or ships requires either a conventional bridge at some point or a means of pivoting a section, or the whole of the bridge, for passage. This type of bridge is typically used for temporary military use, but a number of permanent bridges of this type have also been built.
For small bridges these bridge movement may be enabled without the need for an engine. Some bridges are operated by the users, especially those with a boat, others by a bridge-person, sometimes remotely using video-cameras and loudspeakers. Generally the bridges are powered by electric motors, wether operating winches or hydraulic pistons.
There are often traffic lights for the road and water traffic, and moving barriers for the road traffic.
250px Tower Bridge (1885), London, with a central double bascule bridge and suspension elements To allow ships to pass which can not pass under it, a bridge may be constructed such that it (or part of it) can be turned up (drawbridge; either one part or two, also called a bascule bridge). When first constructed this bridge was operated by steam power.
A swing bridge is a cantilever bridge that pivots in a horizontal plane and so opening a passage on each side of the central support.
In this type an entire span is lifted vertically while staying horizontal. There will be towers at each end that have guiding rails, cables, and counterweights.
Alternatively, if road traffic is very light, a transporter bridge may be used. These are suspended from cables far above the water. These cables carry travelers that support suspension cables for a single deck section. This deck section traverses at water level, but does not touch the water. Its operation is similar to that of a ferryboat.
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