Suspension bridges suspend the roadway from cables attached to a main cable that runs over the length of the bridge. They are strong, light, and beautiful. The basic design has been around for centuries, and engineers continue to push the limits of the spans suspension bridges can span. A suspension bridge begins with the construction of the towers and the main cables are strung across them. The simple design of a suspension bridge allows for high headroom under the bridge and is less prone to collapse than other bridge types.
A suspension bridge is a type of bridge that is constructed by suspending the roadway from cables attached to a main cable that runs over the length of the bridge. In addition to being strong and light, these types of bridges are also beautiful, and some of the most famous bridges in the world are also built this way, including San Francisco’s iconic Golden Gate Bridge and New York’s Brooklyn Bridge. Designing a suspension bridge is simple and straightforward, and uses several techniques to distribute the weight of the bridge safely and evenly.
The basic design of a suspension bridge has been around for centuries: Thousands of years ago, people crossed streams and chasms by dangling their hands on suspended cables. Later, walkways were hung from the cables to facilitate the process, and the original screws and ropes began to be replaced with chains. Major bridges were still built using a truss design until 1808, when an American inventor named James Finley filed a patent on an early version of this type of bridge. Finley’s design involved stretching two strong chains across the tops of several towers and anchoring them on either side of the bridge. He hung minor chains from the two main chains and used them to suspend a rigid bridge, and the modern incarnation of this bridge was born.
In the 1830s, French engineers realized that twisted cables were safer than chains and began using them in the construction of suspension bridges. Bridges built today all use this wired design, but the basic shape has remained the same and engineers continue to push the limits of the spans suspension bridges can span. The longest in the world in 2007 was the Akashi Kaiyo Bridge in Japan, with an unsupported center span of 1.2 miles (1,991 meters) and a total length of 2.4 miles (3,911 meters).
A suspension bridge begins with the construction of the towers. If possible, the towers are located on land and anchored to the bedrock. If the towers are to be placed in water, structures called caissons are used to force the water out of the work area so construction can be done in a dry area. Once the towers have been built and anchored, the main cables are strung across them and anchored to points on land, although some bridges are self-suspending, meaning that the main cables are actually anchored to the bridge deck itself. Next, the suspension cables are threaded from the main cables and then the bridge is connected to them.
The simple design of a suspension bridge allows for high headroom under the bridge, which is useful when the bridge is being built over a major waterway or a very deep chasm. Such a bridge also has streamlined, clean lines that are aesthetically appealing to many engineers and is less prone to collapse than other bridge types such as those built on support trusses. It is also flexible, making it more likely to survive earthquakes and extreme weather conditions, although the flexibility also makes it only appropriate for vehicles and light rail: the concentrated forward weight of a heavy freight locomotive has too much energy for a suspension bridge to bear safely.
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