Suspension bridge types

A suspension bridge is any type of bridge that makes significant use of tension rather than or in addition to compression. A suspension bridge usually has main cables (else ropes or chains), anchored at each end of the bridge. Any load applied to the bridge is transformed into a tension in these main cables. The earliest suspension bridges had the cables anchored in the ground at either end of the bridge, but some modern suspension bridges anchor the cables to the ends of the bridge itself. The earliest suspension bridges had no towers or piers, but these are present in the majority of larger suspension bridges. Although the earlier types of suspension bridges are suitable only for relatively short spans, all of the 14 longest bridges in the world are suspension bridges (see List of longest suspension bridge spans). Ignoring the possibility of pre-Columbian trans-oceanic contact, there were two independent inventions of the suspension bridge, in Eurasia (probably in China) and in Central and South America.[1]

Types

Types of suspension bridge include the following:

Simple suspension bridge: the earliest known type of suspension bridge, and usually a footbridge. The deck is flexible and lies on the main cables, which are anchored to the earth.
Underspanned suspension bridge: an early 19th-century descendant of the simple suspension bridge. The deck is raised on posts above the main cables.
Stressed ribbon bridge: a modern descendant of the simple suspension bridge. The deck lies on the main cables, but is stiff, not flexible.
Suspension bridge: the most familiar type. Though technically all the types listed here are suspension bridges, when unqualified with adjectives the term commonly refers to a suspended-deck suspension bridge,. This type is suitable for use by heavy vehicles and light rail. The main cables are anchored to the earth. The deck is carried below the main cables by "suspenders" and usually is stiff.
Self-anchored suspension bridge: a modern descendant of the suspension bridge, combining elements of a cable-stayed bridge. The main cables are anchored to the ends of the decks.

A pure suspension bridge is one without additional stay cables and in which the main cables are anchored in the ground.[2] This includes most simple suspension bridges and suspended-deck suspension bridges, and excludes self-anchored suspension bridges.

Hybrid types

Some suspension bridges are of unusual hybrid types. Among these are suspension bridges that have an "intermediate deck".[3] These bridges have a portion of deck that resembles an underspanned suspension bridge. Some of the earliest suspended-deck suspension bridges were of this type, and they continue to be constructed. Examples constructed in the 20th century include a viaduct over the river Oberargen near Wangen, Germany. A 258-metre (846 ft) span of the viaduct has a cable support below the deck, with one end of the cable anchored at a pier and the other end tied into a conventional cable stay. The underspanned portion of the span is 172-metre (564 ft) long and has three vertical members.[3]

The Akashi-Kaikyō Bridge, one of the longest suspension bridges in the world, is a suspended-deck suspension bridge with a stiff truss girder deck.[4] Its main span is 1,991 meters long.[4]

Construction

Unlike many other types of bridge, suspension bridges often can be built without use of falsework. In many cases, the main cables are constructed first, then the deck is added. This often involves the use of a pilot cable. For details of their construction methods, see the articles about each type of bridge.

Provided the cables are of sufficiently high quality, suspension bridges are suitable for the longest spans. However, their construction costs are high, so that usually they are economical only for spans in excess of 1000 feet. Shorter spans often are constructed for aesthetic reasons. The economy of longer span suspension bridges is due to their relatively low weight, but because of the greater flexibility that comes with low weight these bridges are more suitable as road bridges than railroad bridges.[2]

See also

References

  1. ↑ Tom F. Peters (1987-01-01). Transitions in Engineering: Guillaume Henri Dufour and the Early 19th Century Cable Suspension Bridges. Birkhauser. p. 260. ISBN 3-7643-1929-1.
  2. 1 2 R. L. Brockenbrough, Frederick S. Merritt (2005). Structural Steel Designer's Handbook: AISC, AASHTO, AISI, ASTM, AREMA, and ASCE-07 Design Standards (4 ed.). McGraw-Hill Professional. p. 800. ISBN 0-07-143218-3. page 15.7
  3. 1 2 Leonardo Fernández Troyano (2003-11-30). Bridge Engineering: A Global Perspective. Thomas Telford. p. 775. ISBN 0-7277-3215-3. pages 517-520
  4. 1 2 James D. Cooper (July–August 1998). "World's Longest Suspension Bridge Opens in Japan". Public Roads. 62.
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