Cantilever bridge


Cantilever bridge

BridgeTypePix
type_name=Cantilever bridge



image_title=The Firth of Forth rail bridge with its three double cantilevers
sibling_names=None
descendent_names=None
ancestor_names=Beam bridge, Truss bridge
carries=Pedestrians, automobiles, trucks, light rail, heavy rail
span_range=Short to medium|material=Iron, structural steel, prestressed concrete
movable=No
design=Medium
falsework=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 convert|1500|ft|m, and can be more easily constructed at difficult crossings by virtue of using little or no falsework.

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.cite book | last = DuBois | first = Augustus Jay | title = The Mechanics of Engineering| url = http://books.google.com/books?id=b-YeAAAAMAAJ| accessdate = 2008-08-10 | year = 1902 | publisher = John Wiley & Sons | location = New York | pages = 57,190] Several nineteenth century engineers patented continuous bridges with hinge points mid-span.cite journal | first = C. | last = Bender | year = 1890 | title = Discussion on "Cantilever Bridges" by C.F. Findlay| journal = Transactions of the Canadian Society of Civil Engineers| volume = 3 | pages = 75,79 | url = http://books.google.com/books?id=3RIOAAAAYAAJ| accessdate = 2008-08-10 ] The use of a hinge in the multi-span system presented the advantages of a statically determinate systemcite web| url = http://www.icomos.org/studies/bridges.htm| title = Context for World Heritage Bridges| accessdate = 2008-08-10| last = DeLony | first = Eric| authorlink = Eric Delony| year = 1996 |accessdate= 2008-08-10|work = World Heritage Sites | publisher = International Council on Monuments and Sites] and of a bridge that could handle differential settlement of the foundations.page 190] 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.page 79] 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.second paragraph of section on cantilevers]

The Kentucky River Bridge 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. [http://www.icomos.org/studies/bridges.htm#15] paragraphs 3 to 5 of cantilever section] 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. [http://www.icomos.org/studies/bridges.htm#15] paragraph 3 of cantilever section]

is seen in the use of wooden poles while the tension of the upper chord is shown by the outstretched arms. The action of the outer foundations as anchors for the cantilever is visible in the placement of the counterweights. [http://www.icomos.org/studies/bridges.htm#15 paragraph 6 of cantilever section] ]

Function

Quotation|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 Engineeringcite book
last = Waddell
first = J.A.L.
authorlink = John Alexander Low Waddell
title = Bridge Engineering - Volume 2
url = http://books.google.com/books?id=bxAkAAAAMAAJ
accessdate = 2008-08-19
year = 1916
publisher = John Wiley & Sons, Inc.
location = New York
pages = 1917
]

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.

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.

List by length

World's longest cantilever bridges (by longest span):citation |url=http://www.aisc.org/Content/ContentGroups/Documents/NSBA5/20_NSBA_LongestSpans.PDF |title=National Steel Bridge Alliance: World's Longest Bridge Spans |last=Durkee |first=Jackson |publisher=American Institute of Steel Construction, Inc |date=1999-05-24 |accessdate=2007-11-03]
# Quebec Bridge (Quebec, Canada) convert|1800|ft|m|0
# Forth Bridge (Firth of Forth, Scotland) 2 x convert|1710|ft|m|0
# Minato Bridge (Osaka, Japan) convert|1673|ft|m|0
# Commodore Barry Bridge (Chester, Pennsylvania, USA) convert|1644|ft|m|0
# Crescent City Connection (dual spans) (New Orleans, Louisiana, USA) convert|1575|ft|m|0
# Howrah Bridge (Kolkata, West Bengal, India) convert|1500|ft|m|0
# Veterans Memorial Bridge (Gramercy, Louisiana, USA) convert|1460|ft|m|0
# San Francisco – Oakland Bay Bridge (East Bay Span) (San Francisco, California, USA) convert|1400|ft|m|0
# Horace Wilkinson Bridge (Baton Rouge, Louisiana, USA) convert|1235|ft|m|0
# Tappan Zee Bridge (South Nyack, New York & Tarrytown, New York, USA) convert|1212|ft|m|0

Examples

References

External links

* [http://demonstrations.wolfram.com/CantileverBridge/ "Cantilever Bridge"] by Sándor Kabai, The Wolfram Demonstrations Project, 2007.
* [http://www.brantacan.co.uk/bridges.htm Bridges – Their Structure and Function] , Brantacan


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