A cantilever bridge is a bridge built using structures that project horizontally into space, supported on only one end (called cantilevers). 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 , and can be more easily constructed at difficult crossings by virtue of using little or no falsework.

Origins

thumb|left|The original style of cantilever bridge

Engineers in the 19th century understood that a bridge 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. Several 19th-century engineers patented continuous bridges with hinge points mid-span. The use of a hinge in the multi-span system presented the advantages of a statically determinate system and of a bridge that could handle differential settlement of the foundations.

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 travelling supports.

500px|thumb|center|A diagram of the parts of the [[John P. Grace Memorial Bridge ]]

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 that 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.

Another way to build a cantilever bridge is to have a stiff material and make a long plate out of said material, and place it across a gap, and add weights on both sides. These are called counterweights. This is a true counterweight bridge. Also see cantilever bridge

500px|thumb|center|The old eastern span of the [[San Francisco–Oakland Bay Bridge, pictured in August 2014, is deconstructed in an order nearly reverse that of its construction. Similar temporary supports were used under each anchor arm during the bridge's construction.]]

List by length

World's longest cantilever bridges (by longest span):

{|class="wikitable sortable"

! No !! Bridge !! Location !! Country !! Date !! Length

|-

| 1 || Quebec Bridge || Quebec || Canada || 1919 ||

|-

| 2 || Forth Bridge || Firth of Forth || Scotland || 1890 || × 2

|-

| 3 || Minato Bridge || Osaka || Japan || 1973 ||

|-

| 4 || Commodore Barry Bridge || Chester, Pennsylvania || United States || 1974 ||

|-

| 5 || Crescent City Connection (dual spans) || New Orleans, Louisiana || United States || 1958 & 1988 ||

|-

| 6 || Howrah Bridge || Kolkata, West Bengal || India || 1943 ||

|-

| 7 || Gramercy Bridge || Gramercy, Louisiana || United States || 1995 ||

|-

| 8 || Tokyo Gate Bridge || Tokyo || Japan || 2012 ||

|-

| 9 || J. C. Van Horne Bridge || Campbellton, New Brunswick & Pointe-à-la-Croix, Quebec || Canada || 1961 ||

|-

| 10 || Horace Wilkinson Bridge || Baton Rouge, Louisiana || United States || 1968 ||

|-

| 11 || Tappan Zee Bridge || South Nyack, New York & Tarrytown, New York || United States || 1955 ||

|-

| 12 || Lewis and Clark Bridge || Longview, Washington & Rainier, Oregon || United States || 1930 ||

|}

Examples

<gallery mode="packed">

File:Quebec bridge pont de quebec.jpg|The Quebec Bridge is of the general structure described above.

File:VejleFjordBridge.jpg|The Vejle Fjord Bridge is a concrete bridge built using the balanced cantilever method.

File:SFOakBrEastPartVEast.jpg|Former eastern span of the San Francisco–Oakland Bay Bridge

File:Howrah Bridge, Kolkata.jpg|Howrah Bridge, Kolkata

File:ForthBridgeEdinburgh.jpg|The Forth Bridge with its three double cantilevers.

File:North Span Bluewater Bridge (Port Huron Mich).jpg|Original 1938 span of the Blue Water Bridge

File:Pulaski Skyway full view.jpg|Pulaski Skyway

File:Vrengenbrua.jpg|Vrengen Bridge, a concrete bridge.

</gallery>

References

  • "Cantilever Bridge" by Sándor Kabai, The Wolfram Demonstrations Project, 2007.
  • Biggest of Finished Girders Go Traveling: six giants of 70 tons gave engineers a hard nut to crack, Popular Science monthly, February 1919, page 79, Scanned by Google Books
  • Southwest Line