Cable-stayed bridge

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 major classes of cable-stayed bridges, differentiated by how the cables are connected to the tower(s). In a harp design, the cables are made nearly parallel by attaching cables to various points on the tower so that the height of attachment of each 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 the tower. 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 a great deal more cable, while a full cantilever bridge would require considerably more material and be substantially heavier.

History of development
Cable-stayed bridges can be dated back to the 1784 design of a timber bridge by German carpenter C.T. Loescher. Many early suspension bridges were of hybrid suspension and cable-stayed bridge, including the 1817 footbridge at Dryburgh Abbey, and the later Albert Bridge (1872) and Brooklyn Bridge (1883). Their designers found that the combination of technologies created a stiffer bridge, and John A. Roebling took particular advantage of this to limit deformations due to railway loads in the Niagara Falls Suspension Bridge.
In the twentieth century, early examples of cable-stayed bridges included A. Gisclard's unusual Cassagnes bridge (1899), where the horizontal part of the cable forces is balanced by a separate horizontal tie cable, preventing significant compression in the deck, and G. Leinekugel le Coq's bridge at Lezardrieux (1925). However, the first widely recognised modern cable-stayed bridge was designed at Strömsund by Franz Dischinger (1955). Other key pioneers included Riccardo Morandi and Fritz Leonhardt. Early bridges from this period used very few stay cables, as in the Theodor-Heuss-Rhine River Bridge (1958). However, this involves substantial erection costs, and more modern structures tend to use many more cables to ensure greater economy.


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 is done the tension in the 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 the 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 bridge, that the cables pull to the sides as opposed to directly up, requiring the bridge deck to be stronger to resist the resulting horizontal compression 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 tower does not tend 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.


Variations
A side-spar cable-stayed bridge uses a central tower supported on only one side. The example shown in that article is not significantly different in structure from a conventional cable-stayed bridge, although this concept could allow the construction of a curved bridge. Far more radical in its structure, the Redding, California Sundial Bridge is a pedestrian bridge that uses a single cantilever spar on one side of the span, with cables on one side only to support the bridge deck. Unlike the other cable stayed types shown this bridge exerts considerable overturning force upon its foundation and the spar must resist the bending caused by the cables, as the cable forces are not balanced by opposing cables. The spar of this particular bridge forms the gnomon of a large garden sundial. Related bridges by the archictect Santiago Calatrava include the Puente del Alamillo and Puente de la Mujer.