What Is a Suspension Bridge?
A suspension bridge is an engineered structure used to span a valley, waterway or other object. It allows vehicles or pedestrians to safely travel across an obstacle rather than finding a way around it. Suspension bridges are suspended from heavy wire cables, and are used when traditional beam or arch bridges are not a viable option.
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Features
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A suspension bridge is made up of a continuous steel or concrete deck that is supported from above by wire cables. The cables are connected to towers at either end of the bridge, and are balanced by anchors or counterweights. Generally the towers will be supported from below by concrete piers, which rest close to the ends of the bridge. The cables support the entire weight of the bridge itself as well as any live loads.
Benefits
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The primary benefit of this type of bridge construction is the relatively long distance it is capable of spanning. While other types of bridges require intermediate supports, a suspension bridge can cover long distances using only the strength of its cables. It also has a very high strength to weight ratio and is flexible enough to withstand earthquakes and other stressors.
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Drawbacks
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Suspension bridges are highly vulnerable to wind, and have been known to collapse or fail due to seemingly minor wind gusts. They must be carefully engineered and braced to minimize the risk of failure. This high level of technical engineering, combined with the difficulty of building over a long span, tends to make suspension bridge construction more costly than that of other bridge designs. These bridges are also not generally capable of supporting heavy loads such as those associated with railroads.
Construction
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The construction of a suspension bridge begins with the installation of footers and foundations to support the piers. These supports are built from concrete and steel and are placed deep underground, or even underwater. Next the vertical piers are built from concrete or steel and the cable support towers are constructed on top. Anchors or counterweights are built at either end of the span using concrete or masonry, and the cables are connected from these anchors to the towers. The bridge deck is made from pre-formed steel or concrete panels that are connected to the cables through the use of a crane. Finally vertical bands of cable are added to support the bridge between the towers.
History
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The earliest suspension bridges are dated back to the Mayan Empire during the seventh century and were made from twisted grass and vines. By the 19th century, builders were relying on cables made from iron links. Though effective, these bridges could fail if even a single link was damaged. In 1883, the Brooklyn Bridge was the first bridge in the world to use the thin wire cables that are still widely used today.
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References
- Photo Credit Greenwood Photography: Flickr.com
