A Cable-Stayed Bridge

by ce on November 22, 2014

A cable-stayed bridge is a bridge that consists of one or more columns (normally referred to as towers or pylons), with cables supporting the bridge deck. There are two theo major classes of cable-stayed bridges: In a harp design, the cables are made nearly parallel by attaching cables to various points on the tower(s) so that the height of attachment of each cable on the tower is similar to the distance from the tower along the roadway to its lower attachment. In a fan design, the cables all connect to or pass over the top of top of the tower(s). The cable-stayed design is the optimum bridge for a span length between that of cantilever bridges and suspension bridges. Within  this range of span lengths a suspension bridge would require a grat deal more cable, while a full cantilever bridge would require considerably more materials and be substantially heavier. Of course, such assertions are not absolute for all cases.

 

Comparison with suspension bridge

A multiple-tower 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 towers, 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 from the main cable, and used to support the load of the bridge deck, which is lifted in sections and attached to the suspender cables. As this done the tension in cables increases, as it does with the live load of vehicles or persons crossing the bridge. The tension on the cables must be transferred to earth by the anchorages, which are sometimes difficult to construct due to poor soil conditions.

In the cable-stayed bridge, the towers form the primary load-bearing structure. A cantilever approach is often used for support of the bridge deck near the towers, but areas further from them are supported by cables running directly to the towers. This has the disadvantage, compared to the suspension, that the cables pull to the sides as opposed to directly up, requiring the advantage of not requiring firm anchorages to resist a horizontal pull of the cables, as in the suspension bridge. All static horizontal forces are balanced so that the supporting tower does not end to tilt or slide, needing only to resist such forces from the live loads.

Key advantages of the cable-stayed form are as follows:

  • much greater stiffness than the suspension bridge, so that deformations of the deck under live loads are reduced
  • can be constructed by cantilevering out from the tower – the cables act both as temporary and permanent supports to the bridge deck
  • for a symmetrical bridge (i.e. spans on either side of the tower are the same), the horizontal forces balance and large ground anchorages are not required

A further advantage of the cable-stayed bridge is that any number of towers may be used. This bridge form can be as easily built with a single tower, as with a pair of towers. However, a suspension bridge is usually built only with a pair of towers.

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