7/15/2008

Bridges Types (2) : Beam & Cantilever

Beam bridge



Beam bridges are horizontal beams supported at each end by piers. The earliest beam bridges were simple logs that sat across streams and similar simple structures. In modern times, beam bridges are large box steel girder bridges. Weight on top of the beam pushes straight down on the piers at either end of the bridge.

A beam is a direct descendant of the log bridge, now more normally made from shallow steel 'I' beams, box girders, reinforced concrete, or post-tensioned concrete. It is frequently used in pedestrian bridges and for highway overpasses and flyovers. As is its ancestor, this bridge is in structural terms the simplest of the many bridge types.

Like most bridges that are characterized by how they are supported, beam bridges consist of one horizontal beam with 2 supports usually on either end.

Decorative beam bridges, commonly built from cedar, pine, and redwood, can span a koi pond or country creek. These free standing bridges are usually built as functional walkways or just for decoration.


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 1,500 feet (460 m), 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.

Source : wikipedia

2 comments:

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