A self-anchored suspension bridge is a

suspension bridge A suspension bridge is a type of bridge in which the deck (bridge), deck is hung below suspension wire rope, cables on vertical suspenders. The first modern examples of this type of bridge were built in the early 1800s. Simple suspension bridg ...

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. File:Bridge-suspension-anchorages.svg, Suspension bridge with the main cables attached to the ground (black squares) File:Bridge-self-anchored.svg, Self-anchored suspension bridge; the main cables are attached to the end of the road deck (horizontal blue line) 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 A tied-arch bridge is an arch bridge in which the outward-directed horizontal forces of the arch(es) are borne as tension by a chord tying the arch ends rather than by the ground or the bridge foundations. This strengthened chord may be the deck ...

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 San Francisco-Oakland Bay Bridge, Bay Bridge with a new self-anchored suspension bridge (SAS) and ...

(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 China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

is a single-tower bridge with a main span of . * The Konohana Bridge (1990) in

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

and the Yeoungjong Grand Bridge (2000) in

South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...

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

Pittsburgh Pittsburgh ( ) is a city in Allegheny County, Pennsylvania, United States, and its county seat. It is the List of municipalities in Pennsylvania#Municipalities, second-most populous city in Pennsylvania (after Philadelphia) and the List of Un ...

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 Kensin ...

(1937) in

London London is the Capital city, capital and List of urban areas in the United Kingdom, largest city of both England and the United Kingdom, with a population of in . London metropolitan area, Its wider metropolitan area is the largest in Wester ...

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. * The in

Arkhangelsk Arkhangelsk (, ) is a types of inhabited localities in Russia, city and the administrative center of Arkhangelsk Oblast, Russia. It lies on both banks of the Northern Dvina near its mouth into the White Sea. The city spreads for over along the ...

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

. 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 ...

, 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 (physic ...

s, or a more conventional composite cable.

References

External links