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Encyclopedia > Cable stayed bridge
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Sunshine Skyway, Tampa Bay, Florida

A cable_stayed bridge is a bridge that consists of one or more pillars, with cables supporting the roadbed. There are two major classes of cable-stayed bridges, differentiated by how the cables are connected to the pillars. In a parallel attachment design, the cables are made nearly parallel by having the height of attachment on the pillar be similar to the distance from the pillar along the roadway. In a radial attachment design, the cables all connect to or pass over the top of the pillar.


The cable-stay design occupies a sweet spot of length between cantilever bridges and suspension bridges. Within this sweet spot a suspension bridge would require more cable, while a full cantilever bridge would require considerably more material.

Contents

Comparison with suspension bridge

A multiple_pillar cable_stayed bridge may appear similar to a suspension bridge, but in fact is very different in principle and in the method of construction. In the suspension bridge, a large cable is made up by "spinning" small diameter wires between two pillars, and at each end to anchorages into the ground or to a massive structure. These cables form the primary load_bearing structure for the bridge deck. Before the deck is installed, the cables are under tension from only their own weight. Smaller cables or rods are then suspended on the main cable, and used to support the load of the roadbed, which is lifted in sections and attached to the suspender cables. As this is done the tension in the cables increases, as they are also increased with the "live load" of vehicles or persons crossing the bridge. The tension on the cables must be transferred to the earth by the anchorages, which are sometimes difficult to construct due to poor soil conditions.


In the cable_stayed bridge, the pillars form the primary load_bearing structure. A cantilever approach is often used for support of the roadbed near the pillars, but areas further from them are supported by cables running directly to the pillars. This has the disadvantage, compared to the suspension bridge, that the cables pull to the sides as opposed to directly up, requiring the roadbed to be stronger to resist these loads; but has the advantage of not requiring firm anchorages to resist a horizontal pull as in the suspension bridge. All static horizontal forces are balanced so that the supporting pillar does not tend to tilt or slide, needing only to resist such forces from the "live" loads.


A further advantage of the cable_stayed bridge is that any number of pylons may be used. While it is possible to build a suspension bridge with one central tower, they are usually built with a pair of towers. Note that the apparent four_tower western segment of the San Francisco-Oakland Bay Bridge is in fact a pair of two-tower suspension bridges with a massive central anchorage.


A related bridge type

Proposed San Francisco-Oakland Bay Bridge eastern span replacement

A self-supporting suspension span is similar in principle to the cable stayed type in that tension forces that prevent the deck from dropping are converted into compression forces vertically in the pylon and horizontally along the deck structure. Unlike a cable stayed bridge, this type must be supported by falsework during construction and so is more expensive to construct.


Examples of cable-stayed bridges

External links

  • Structurae: Cable_stayed Bridges (http://www.structurae.de/en/structures/stype/s1002.cfm)
  • Cable_Stayed Bridge (http://www.pbs.org/wgbh/nova/bridge/meetcable.html)
  • The Leonard P. Zakim Bunker Hill Bridge web site (http://www.leonardpzakimbunkerhillbridge.org)
  • Cable_stayed bridges on Bratacan (http://www.brantacan.co.uk/cable-stayed.htm)





 
 

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