CANTILEVER BRIDGE

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 1500 feet, and can be more easily constructed at difficult crossings by virtue of using little or no falsework.
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 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.

Contents

Use of cantilevers during construction of arch bridges

List by length

Examples

See also

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, usually limiting this method to the spanning of narrow canyons.

List by length

World's longest cantilever bridges (by longest span) [1]
# Quebec Bridge (Quebec, Canada)
# Forth Bridge (Firth of Forth, Scotland) 2 x
# Minato Ohashi Bridge (Osaka, Japan)
# Commodore Barry Bridge (Chester, Pennsylvania, USA)
# Crescent City Connection (Twin Spans) (New Orleans, Louisiana, USA)
# Howrah Bridge (Calcutta, India)
# Veterans Memorial Bridge (Gramercy, Louisiana, USA)
# San Francisco-Oakland Bay Bridge (East Bay Span) (San Francisco, California, USA)
# Horace Wilkinson Bridge (Baton Rouge, Louisiana, USA)
# Tappan Zee Bridge (New York, New York, USA)
# Mystic River Bridge (Boston, Massachusetts, USA) 1,525 ft (465 m)

Examples

The distribution of forces is clear in this demonstration using chairs, poles, counterweights, and humans

Quebec Bridge is of the general structure demonstrated above

The Howrah Bridge in Kolkata, India was opened in February 1943

Following are some different types of cantilever bridge.

Jeremiah Morrow Bridge is a deck truss cantilever bridge, with the roadway running on top of the truss

Newark Bay Bridge has arched cantilevers

Vejle Fjord Bridge is a concrete bridge built using the balanced cantilever method

See also