Cantilever Bridges

(note : people are bit confused with  the cantilever bridges in dental with cantilever in engineering. so pls check it out.)

CANTILEVER BRIDGES

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 waddel

A cantilever bridge is constructed using cantilevers, which are horizontal structures supported only on one end.cantilevers may be simple beams for small bridges and for large bridges cantilevers are trusses which are made from structural steel.

A balanced cantilever is another technique for spanning long distances. In this situation, engineers construct a pier in the middle of the river. They then anchor two cantilevers on the pier, each facing the opposite direction from the other. These cantilevers, or anchor arms, meet with other cantilevers anchored in the ground or another pier to create one whole bridge.

function of  cantilver bridges :

The first step in building a cantilever bridge is to build and connect the arms to the shore. River piers for balanced cantilevers should not be built until the cantilevers that touch the land are completed. If a beam bridge or truss bridge is necessary, it cannot be added until the cantilevers are ready. The beam bridge is often built off-site and lowered with a crane into position.

A simple cantilever span is formed by two cantilever arms extending from opposite sides of the obstacle to be crossed, meeting at the center. With a 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.

with as balanced cantilever, the span is balanced by cantilevers going in the opposite direction. If these are anchored into solid foundations, they are called anchor arms. 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 through the anchor arms to the outermost supports, while the compression is carried to the foundations beneath the central towers.

Prestressed concrete balanced cantilever bridges are often built using segmental construction.

List of famous cantilver bridges in the world:

1.Quebec Bridge (Quebec, Canada) 1,800 feet (549 m)

2.Forth Bridge (Firth of Forth, Scotland) 2 x 1,710 feet (521 m)

3.Minato Bridge (Osaka, Japan) 1,673 feet (510 m)

4.Commodore Barry Bridge (Chester, Pennsylvania, USA) 1,644 feet (501 m)

5.Crescent City Connection (dual spans) (New Orleans, Louisiana, USA) 1,575 feet (480 m)

6.Howrah Bridge (Kolkata, West Bengal, India) 1,500 feet (457 m)

7.Veterans Memorial Bridge (Gramercy, Louisiana, USA) 1,460 feet (445 m)

8.San Francisco – Oakland Bay Bridge (East Bay Span) (San Francisco, California, USA) 1,400 feet (427 m)

9.Horace Wilkinson Bridge (Baton Rouge, Louisiana, USA) 1,235 feet (376 m)

10.Tappan Zee Bridge (South Nyack, New York & Tarrytown, New York, USA) 1,212 feet (369 m).

( A post on cable bridges will be published soon  :)