Cable-stayed bridge

A cable-stayed bridge is a type of bridge that has one or more towers, from which cables support the bridge deck. A distinctive feature are the cables or stays, which run directly from the tower to the deck, normally forming a fan-like pattern or a series of parallel lines. This is in contrast to the modern suspension bridge, where the cables supporting the deck are suspended vertically from the main cables, which run between the towers and are anchored at both ends of the bridge. The cable-stayed bridge is optimal for spans longer than cantilever bridges and shorter than suspension bridges. This is the range within which cantilever bridges would rapidly grow heavier, and suspension bridge cabling would be more costly.
Cable-stayed bridges found wide use in the late 19th century. Early examples, including the Brooklyn Bridge, often combined features from both the cable-stayed and suspension designs. Cable-stayed designs fell from favor in the early 20th century as larger gaps were bridged using pure suspension designs, and shorter ones using various systems built of reinforced concrete. It returned to prominence in the later 20th century when the combination of new materials, larger construction machinery, and the need to replace older bridges all lowered the relative price of these designs.

History

Cable-stayed bridges date back to 1595, where designs were found in Machinae Novae, a book by Croatian-Venetian inventor Fausto Veranzio. Many early suspension bridges were partially cable-stayed in construction, including the 1817 footbridge Dryburgh Abbey Bridge, James Dredge's patented Victoria Bridge, Bath, and the later Albert Bridge and Brooklyn Bridge. Their designers found that the combination of technologies created a stiffer bridge. 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 wrought iron Bluff Dale Suspension bridge with wooden stringers and decking in Bluff Dale, Texas, or his weeks earlier but ruined Barton Creek Bridge between Huckabay, Texas and Gordon, Texas. In the twentieth century, early examples of cable-stayed bridges included A. Gisclard's unusual Cassagnes bridge, in which 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 Lézardrieux in Brittany. Eduardo Torroja designed a cable-stayed aqueduct at Tempul in 1926. Albert Caquot's 1952 concrete-decked cable-stayed bridge over the Donzère-Mondragon canal at Pierrelatte is one of the first of the modern type, but had little influence on later development. The steel-decked Strömsund Bridge designed by Franz Dischinger is, therefore, more often cited as the first modern cable-stayed bridge.
File:Abdoun Bridge.jpg|thumb|Abdoun Bridge, Amman, Jordan, example of an extradosed 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 Bridge. However, this involves substantial erection costs, and more modern structures tend to use many more cables to ensure greater economy.

Comparison with suspension bridge

Cable-stayed bridges may appear to be similar to suspension bridges, but they are quite different in principle and construction. In suspension bridges, large main cables hang between the towers and are anchored at each end to the ground. This can be difficult to implement when ground conditions are poor. The main cables, which are free to move on bearings in the towers, bear the load of the bridge deck. Before the deck is installed, the cables are under tension from their own weight. Along the main cables smaller cables or rods connect to the bridge deck, which is lifted in sections. As this is done, the tension in the cables increases, as it does with the live load of traffic crossing the bridge. The tension on the main cables is transferred to the ground at the anchorages and by downwards compression on the towers.
In cable-stayed bridges, the towers are the primary load-bearing structures that transmit the bridge loads to the ground. A cantilever approach is often used to support the bridge deck near the towers, but lengths further from them are supported by cables running directly to the towers. That has the disadvantage, unlike for the suspension bridge, that the cables pull to the sides as opposed to directly up, which requires the bridge deck to be stronger to resist the resulting horizontal compression loads, but it has the advantage of not requiring firm anchorages to resist the horizontal pull of the main cables of a suspension bridge. By design, all static horizontal forces of the cable-stayed bridge are balanced so that the supporting towers do not tend to tilt or slide and so must only resist horizontal forces from the live loads.
The following are key advantages of the cable-stayed form:

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, the horizontal forces balance and large ground anchorages are not required
Designs

There are four major classes of rigging on cable-stayed bridges: mono, harp, fan, and star.

The mono design uses a single cable from its towers and is one of the lesser-used examples of the class.
In the harp or parallel design, the cables are parallel, or nearly so, so that the height of their attachment to the tower is proportional to the distance from the tower to their mounting on the deck.
In the fan design, the cables all connect to or pass over the top of the towers. The fan design is structurally superior with a minimum moment applied to the towers, but, for practical reasons, the modified fan is preferred, especially where many cables are necessary. In the modified fan arrangement, the cables terminate near the top of the tower but are spaced from each other sufficiently to allow better termination, improved environmental protection, and good access to individual cables for maintenance.
In the star design, another relatively rare design, the cables are spaced apart on the tower, like the harp design, but connect to one point or a number of closely spaced points on the deck.

There are also seven main arrangements for support columns: single, double, portal, A-shaped, H-shaped, inverted Y, and M-shaped. The last three are hybrid arrangements that combine two arrangements into one.

The single arrangement uses a single column for cable support, normally projecting through the center of the deck, but in some cases located on one side or the other. Examples: Millau Viaduct in France and Sunshine Skyway Bridge in Florida.
The double arrangement places pairs of columns on both sides of the deck. Examples: Øresund Bridge between Denmark and Sweden, and Zolotoy Bridge in Russia.
The portal is similar to the double arrangement but has a third member connecting the tops of the two columns to form a door-like shape or portal. This offers additional strength, especially against transverse loads. Examples: Hale Boggs Bridge in Louisiana and Kirumi Bridge in Tanzania.
The A-shaped design is similar in concept to the portal but achieves the same goal by angling the two columns towards each other to meet at the top, eliminating the need for the third member. Examples: Arthur Ravenel Jr. Bridge in South Carolina, Helgeland Bridge in Norway, and Christopher S. Bond Bridge in Missouri.
The H-shaped design combines the portal on the bottom with the double on top. Examples: Grenland Bridge in Norway, Vasco da Gama Bridge in Portugal, Greenville Bridge in Arkansas, and John James Audubon Bridge in Louisiana.
The inverted Y design combines the A-shaped on the bottom with the single on top. Examples: Pont de Normandie in France and Incheon Bridge in South Korea.
The M-shaped design combines two A-shaped arrangements, side by side, to form an M. This arrangement is rare, and is mostly used in wide bridges where a single A-shaped arrangement would be too weak. Examples: Fred Hartman Bridge in Texas, and its planned sister bridge Ship Channel Bridge, also in Texas.

Depending on the design, the columns may be vertical, angled relative to vertical, or curved.

Variations

Side-spar cable-stayed bridge

A side-spar cable-stayed bridge uses a central tower supported only on one side. This design allows the construction of a curved bridge.

Cantilever spar cable-stayed bridge

Far more radical in its structure, the Puente del Alamillo uses a single cantilever spar on one side of a single span, with cables on one side only to support the bridge deck. Unlike other cable-stayed types, this bridge exerts considerable overturning force upon its foundation, and the spar must resist 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 de la Mujer, Sundial Bridge, Chords Bridge, and Assut de l'Or Bridge.

Multiple-span cable-stayed bridge

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