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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: 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 the tower(s).

Compared to other bridge types, the cable-stayed is optimal for spans longer than typically seen in cantilever bridges and shorter than those typically requiring a suspension bridge. This is the range in which cantilever spans would rapidly grow heavier if they were lengthened, and in which suspension cabling does not get more economical were the span to be shortened.

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 construction, including the 1817 footbridge at Dryburgh Abbey, and the later Albert Bridgemarker (1872) and Brooklyn Bridgemarker (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.

The earliest known surviving example of a true cable-stayed bridge in the United States is E.E. Runyon's largely-intact steel or iron bridgemarker with wooden stringers and decking in Bluff Dale, Texasmarker (1890), or his weeks-earlier but ruined Barton Creek Bridge between Huckabay, Texasmarker and Gordon, Texasmarker (1889 or 1890)., 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 in Brittany (1924). Eduardo Torroja designed a cable-stayed aqueduct at Tempul in 1926. Albert Caquot's 1952 concrete-decked cable-stayed bridge over the Donzére-Mondragon canal at Pierrelattemarker is one of the first of the modern type, but had little influence on later development. The steel-decked bridge designed at Strömsundmarker by Franz Dischinger (1955) is therefore more often cited as the first modern cable-stayed bridge.

Other key pioneers included Fabrizio de Miranda, Riccardo Morandi and Fritz Leonhardt. Early bridges from this period used very few stay cables, as in the Theodor Heuss Bridgemarker (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 or different 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.
Image:Bridge-suspension.svg|Suspension bridgeImage:Bridge-fan-cable-stayed.svg|Cable-stayed bridge, fan designImage:Bridge-harp-cable-stayed.svg|Cable-stayed bridge, harp design
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 of the cables, 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. span 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.

Variations

Side-spar cable-stayed bridge

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.

Cantilever-spar cable-stayed bridge

Far more radical in its structure, the Redding, Californiamarker Sundial Bridgemarker 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 architect Santiago Calatrava include the Puente del Alamillomarker (1992), Puente de la Mujermarker (2001), and Chords Bridgemarker (2008).

Multiple-span cable-stayed bridge

Cable-stayed bridges with more than three spans involve significantly more challenging designs than do 2-span or 3-span structures.

In a 2-span or 3-span cable-stay bridge, the loads from the main spans are normally anchored back near the end abutments by stays in the end spans. For more spans, this isn't the case, and the bridge structure is less stiff overall. This can create difficulties both in the design of the deck and the pylons.Examples of multiple span structures where this is the case include Ting Kau Bridgemarker, where additional 'cross-bracing' stays are used to stabilise the pylons; Millau Viaductmarker and Mezcala Bridge, where twin-legged towers are used; and General Rafael Urdaneta Bridgemarker, where very stiff multi-legged frame towers were adopted. A similar situation with a suspension bridge is found at both the Great Seto Bridgemarker and San Francisco – Oakland Bay Bridgemarker where additional anchorage piers are required after every set of three suspension spans - this solution can also be adapted for cable-stayed bridges.

Extradosed bridge

The extradosed bridge is a cable-stayed bridge but with a more substantial bridge deck that being stiffer and stronger allows the cables to be omitted close to the tower and for the towers to be lower in proportion to the span.

Cable-stayed cradle-system bridge

A cradle system carries the strands within the stays from bridge deck to bridge deck, as a continuous element, eliminating anchorages in the pylons. Each epoxy-coated steel strand is carried inside the cradle in a one-inch steel tube. Each strand acts independently, allowing for removal, inspection and replacement of individual strands. The first two such bridges are the Penobscot Narrows Bridgemarker, completed 2006, and the Veterans' Glass City Skywaymarker, completed 2007.

Related bridge types

Self anchored suspension bridge

A self-anchored suspension bridge has some similarity 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 tower and horizontally along the deck structure. It is also related to the suspension bridge in having arcuate main cables with suspender cables, although the self-anchored type lacks the heavy cable anchorages of the ordinary suspension bridge. Unlike either a cable stayed bridge or a suspension bridge, the self-anchored suspension bridge must be supported by falsework during construction and so it is more expensive to construct.

Notable cable-stayed bridges

See also: List of largest cable-stayed bridges and :Category:Cable-stayed bridges

Further reading

  • De Miranda F., et al., (1979), "Basic problems in long span cable stayed bridges", Rep. n. 25, Dipartimento di Strutture - Università di Calabria - Arcavacata (CS) Italy, (242 pagg.) September 1979.


References

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