Cantilever bridge
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| Cantilever bridge | |
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| The Firth of Forth rail bridge with its three double cantilevers | |
| Ancestor: | Beam bridge, Truss bridge |
| Related: | None |
| Descendant: | None |
| Carries: | Pedestrians, automobiles, trucks, light rail, heavy rail |
| Span range: | Short to medium |
| Material: | Iron, structural steel, prestressed concrete |
| Movable: | No |
| Design effort: | Medium |
| Falsework required: | No |
A cantilever bridge is a bridge built using cantilevers--structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue of using little or no falsework.
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[edit] Function
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 constructed in place using special traveling supports.
A common way to construct steel truss and prestressed concrete cantilever spans is to counterbalance each cantilever arm with another cantilever arm projecting the opposite direction, forming a balanced 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 balanced cantilever's supports, 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 members 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 truss cantilever bridges use pinned joints and are therefore statically determinate with no members carrying mixed loads.
Prestressed concrete balanced cantilever bridges are often built using segmental construction.
[edit] 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.
[edit] List by length
World's longest cantilever bridges (by longest span):[1]
- Quebec Bridge (Quebec, Canada) 1,800 feet (549 m)
- Forth Bridge (Firth of Forth, Scotland) 2 x 1,710 feet (521 m)
- Minato Bridge (Osaka, Japan) 1,673 feet (510 m)
- Commodore Barry Bridge (Chester, Pennsylvania, USA) 1,644 feet (501 m)
- Crescent City Connection (dual spans) (New Orleans, Louisiana, USA) 1,575 feet (480 m)
- Howrah Bridge (Kolkata, West Bengal, India) 1,500 feet (457 m)
- Veterans Memorial Bridge (Gramercy, Louisiana, USA) 1,460 feet (445 m)
- San Francisco-Oakland Bay Bridge (East Bay Span) (San Francisco, California, USA) 1,400 feet (427 m)
- Horace Wilkinson Bridge (Baton Rouge, Louisiana, USA) 1,235 feet (376 m)
- Tappan Zee Bridge (South Nyack, New York & Tarrytown, New York, USA) 1,212 feet (369 m)
[edit] Examples
 The distribution of forces is clear in this demonstration using chairs, poles, counterweights, and humans. |
 The Quebec Bridge is of the general structure demonstrated above. |
 The Howrah Bridge in Kolkata, India was opened in February 1943. |
 The Pierre Pflimlin bridge is a balanced cantilever made of concrete, shown here under construction. |
 The Vejle Fjord Bridge is a concrete bridge built using the balanced cantilever method. |
[edit] References
- ^ Durkee, Jackson (1999-05-24), National Steel Bridge Alliance: World's Longest Bridge Spans, American Institute of Steel Construction, Inc, <http://www.aisc.org/Content/ContentGroups/Documents/NSBA5/20_NSBA_LongestSpans.PDF>. Retrieved on 3 November 2007
[edit] External links
- "Cantilever Bridge" by Sándor Kabai, The Wolfram Demonstrations Project, 2007.
- Bridges – Their Structure and Function, Brantacan
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