A self-anchored suspension bridge is a suspension bridge type in which the main cables attach to the ends of the deck, rather than directly to the ground or via large anchorages. The design is well-suited for construction atop elevated piers, or in areas of unstable soils where anchorages would be difficult to construct. Image:Bridge-suspension-anchorages.svg, Suspension bridge with the main cables attached to the ground (black squares) Image:Bridge-self-anchored.svg, Self-anchored suspension bridge; the main cables are attached to the end of the road deck The load path of the self-anchored suspension (SAS) bridge converts vertical loads into tension forces in the main cables which are countered by compressive forces in the towers and deck. The system balances forces internally without external anchorage requirements making it suitable for sites where large horizontal forces are difficult to anchor. This is similar to the method used in a tied-arch bridge where arch member compression is balanced by tension in the deck.

History

The self-anchored suspension bridge form originated in the mid-19th century, with a published description by Austrian engineer Josef Langer in 1859 and U.S. Patent No. 71,955 by American engineer Charles Bender in 1867. The form was applied to a handful of Rhine River crossings in Germany during the first half of the twentieth century.John A. Ochsendorf and David P. Billington, "Self-Anchored Suspension Bridges," ''ASCE Journal of Bridge Engineering,'' vol. 4, No. 3 (August 1999): 151-156.

Examples

* The SAS portion of the

eastern span replacement of the San Francisco–Oakland Bay Bridge The eastern span replacement of the San Francisco–Oakland Bay Bridge was a construction project to replace a seismically unsound portion of the Bay Bridge with a new self-anchored suspension bridge (SAS) and a pair of viaducts. The bridge i ...

(2013) is a single-tower asymmetric bridge with a main span of . It is currently the largest SAS bridge in the world. * The Pingsheng Bridge (2006) in China is a single-tower bridge with a main span of . * The Konohana Bridge (1990) in Japan and the Yeoungjong Grand Bridge (2000) in

South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korean Peninsula and sharing a land border with North Korea. Its western border is formed by the Yellow Sea, while its eas ...

, both have two towers with a central span of . * The Three Sisters Bridges of

Pittsburgh Pittsburgh ( ) is a city in the Commonwealth of Pennsylvania, United States, and the county seat of Allegheny County. It is the most populous city in both Allegheny County and Western Pennsylvania, the second-most populous city in Pennsylva ...

are the earliest examples (1924–28) of this bridge type in the US. * The

Chelsea Bridge Chelsea Bridge is a bridge over the River Thames in west London, connecting Chelsea on the north bank to Battersea on the south bank, and split between the City of Westminster, the London Borough of Wandsworth and the Royal Borough of Kens ...

(1937) in

London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. * The in

Arkhangelsk Arkhangelsk (, ; rus, Арха́нгельск, p=ɐrˈxanɡʲɪlʲsk), also known in English as Archangel and Archangelsk, is a city and the administrative center of Arkhangelsk Oblast, Russia. It lies on both banks of the Northern Dvina near ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...

. Image:Haer PBG erection force Diagram part.png, Construction falsework on left; force schematic on right File:CalDOT-SASElevPlan4Q2005P42.svg, Elevation and plan view of single-tower SAS bridge portion of 2013 eastern span of San Francisco-Oakland bay bridge. Not shown: cable is continuous across the western end, under roadway.

Construction method

The nature of the self-anchored suspension bridge necessitates the temporary construction of

falsework Falsework consists of temporary structures used in construction to support a permanent structure until its construction is sufficiently advanced to support itself. For arches, this is specifically called centering. Falsework includes temporary s ...

, in the form of compression struts or an underdeck, before work begins on the permanent structure. This requirement is inherent in the structure's definition. In the absence of suspension via cableage, the deck of a suspension bridge is incapable of self-support. On a suspension bridge of the more usual ''earth-anchored'' type, both of the primary (''i.e.'', horizontal) cable's anchorages exist prior to construction in the form of solid terrain. In the ''self-anchored'' suspension bridge, however, the cable must be anchored to the bridge deck, which has yet to be built and will not bear its own weight; ''ergo'', falsework must be employed until the final method of suspension is possible. The cables apply strong axial forces onto the bridge girders, which may require strengthening. Therefore, self-anchored suspension bridge spans tend to be smaller than ''earth-anchored'' ones.

Cable anchors

As in a traditional suspension bridge, the primary cable type may be multiple parallel independent cables as in the image at right of the Hutsonville Bridge (no longer extant), or

eyebar In structural engineering and construction, an eyebar is a straight bar, usually of metal, with a hole ("eye") at each end for fixing to other components. Eyebars are used in structures such as bridges, in settings in which only tension, and ne ...

s, or a more conventional composite cable.

References

External links