13-12-2012, 03:16 PM
Cantilever bridge
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Origins
Engineers in the nineteenth century understood that a bridge which was continuous across multiple supports would
distribute the loads among them. This would result in lower stresses in the girder or truss and meant that longer spans
could be built.[1] Several nineteenth century engineers patented continuous bridges with hinge points mid-span.[2]
The use of a hinge in the multi-span system presented the advantages of a statically determinate system[3] and of a
bridge that could handle differential settlement of the foundations.[1] Engineers could more easily calculate the forces
and stresses with a hinge in the girder.
Heinrich Gerber was one of the engineers to obtain a patent for a hinged girder (1866) and is recognized as the first
to build one.[2] The Hassfurt Bridge over the Main river in Germany with a central span of 124 feet (38 meters) was
completed in 1867 and is recognized as the first modern cantilever bridge.[3]
The High Bridge of Kentucky by C. Shaler Smith (1877), the Niagara Cantilever Bridge by Charles Conrad
Schneider (1883) and the Poughkeepsie Bridge by John Francis O'Rourke and Pomeroy P. Dickinson (1889) were all
important early uses of the cantilever design.[3] The Kentucky River Bridge spanned a gorge that was 275 feet (84
meters) deep and took full advantage of the fact that falsework, or temporary support, is not needed for the main
span of a cantilever bridge.
Function
Cantilever Bridge.—A structure at least one portion of which acts as an anchorage for sustaining another
portion which extends beyond the supporting pier.
— John Alexander Low Waddell, Bridge Engineering[4]
A simple cantilever span is formed by two cantilever arms extending from opposite sides of an obstacle to be
crossed, meeting at the center. In a common variant, the suspended span, the cantilever arms do not meet in the
center; instead, they support a central truss bridge which rests on the ends of the cantilever arms. The suspended span
may be built off-site and lifted into place, or constructed in place using special traveling supports.
Construction methods
Some steel arch bridges (such as the Navajo Bridge) are built using pure cantilever spans from each side, with
neither falsework below nor temporary supporting towers and cables above. These are then joined with a pin, usually
after forcing the union point apart, and when jacks are removed and the bridge decking is added the bridge becomes
a truss arch bridge. Such unsupported construction is only possible where appropriate rock is available to support the
tension in the upper chord of the span during construction, usually limiting this method to the spanning of narrow
canyons.