Cantilever bridge
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| Cantilever bridge | |
|---|---|
| |
| The Firth of Forth rail bridge with its three double cantilevers | |
| Ancestor | Truss bridge |
| Related | None |
| Descendant | Cantilever spar cable-stayed bridge |
| Carries | Pedestrians, automobiles, trucks, light rail, heavy rail |
| Span range | Short to medium |
| Material | Iron, steel |
| Movable | No |
| Design effort | Medium |
| Falsework required | No |
A simple cantilever span is formed by two cantilever arms extending from opposite sides of the 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 it may constructed in place using special traveling supports.
A common way to construct steel truss cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a double cantilever; when they attach to a solid foundation, the counterbalancing arms are called anchor arms. Thus, in a bridge built on two foundation piers, there are four cantilever arms: two which span the obstacle, and two anchor arms which extend away from the obstacle. Because of the need for more strength at the double cantilever's balance points, the bridge superstructure often takes the form of towers above the foundation piers. The Commodore Barry Bridge is an example of this type of cantilever bridge.
Steel truss cantilevers support loads by tension of the upper beams and compression of the lower ones. Commonly, the structure distributes the tension via the anchor arms to the outermost supports, while the compression is carried to the foundations beneath the central towers. Many cantilever bridges use pinned joints and are therefore statically determinate with no beams carrying mixed loads.
Use of cantilevers during construction of arch bridges
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.List by length
World's longest cantilever bridges (by longest span) [link]- Quebec Bridge (Quebec, Canada) 548.64 m (1,800 feet)
- Forth Bridge (Firth of Forth, Scotland) 2 x 521.21 m (1,710 feet)
- Minato Ohashi Bridge (Osaka, Japan) 509.93 m (1,673 feet)
- Commodore Barry Bridge (Chester, Pennsylvania, USA) 501.09 m (1,644 feet)
- Crescent City Connection (Twin Spans) (New Orleans, Louisiana, USA) 480.06 m (1,575 feet)
- Howrah Bridge (Calcutta, India) 457.20 m (1,500 feet)
- Veterans Memorial Bridge (Gramercy, Louisiana, USA) 445.01 m (1,460 feet)
- San Francisco-Oakland Bay Bridge (East Bay Span) (San Francisco, California, USA) 426.72 m (1,400 feet)
- Horace Wilkinson Bridge (Baton Rouge, Louisiana, USA) 376.43 m (1,235 feet)
- Tappan Zee Bridge (New York, New York, USA) 369.42 m (1,212 feet)
Examples
Following are some commonly seen forms of steel truss cantilever bridge.
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