The simplest type of suspension bridge is a simple rope bridge in which two ropes are attached to opposite sides of a span and a deck, usually made of wooden planks, is constructed and hung below the ropes. Sometimes known as "simple suspension bridges," these bridges date back to the 3rd century B.C. when they were first built in China. By the 1st century A.D. Chinese suspension bridges were being built with metal chains instead of rope and sometimes included vertical suspenders so that bridges could be made to cross spans too long for a bridge that was affixed only at its ends.
By adding one or more suspension towers to the middle of the span, along with the use of stronger materials, the bridge's length could be greatly increased, thus giving suspension bridges a key advantage over other traditional bridge designs.
While suspension bridges continued to be developed in China, early American civilizations constructed similar bridges independently. In South America, evidence of Inca simple suspension bridges dates back to the 7th century. Here the bridge cables were typically constructed from vines and used to make travel possible in rugged jungle terrain.
The evolution of suspension bridges was led by the introduction of new materials. The iron chains that replaced rope were much stronger but added weight. Steel cables and concrete towers made suspension bridges a more viable option for spanning long distances and supporting heavy decks that could carry vehicles laden with cargo.
Modern suspension bridges that function as part of a road system usually employ steel cables. Steel is an ideal material for this sort of construction since it retains its strength even under high tension. Small individual cables are wound together and bound to form the strong main cables that support the weight of the bridge's deck, along with the weight of people and vehicles crossing the bridge. Concrete or steel towers are used to extend the length a suspension bridge can span; the vertical suspension cables are constructed in a way similar to the main cables. The towers, also known as pillars, undergo compression from the weight of the deck. Pillars and cables are equally important in the design and construction of a suspension bridge, as the failure of either would be catastrophic.
The most famous suspension bridge is San Francisco's Golden Gate Bridge, opened in 1937. An architectural landmark, the Golden Gate Bridge was the longest suspension bridge in the world upon its completion. Today that distinction falls to the Akashi-Kaikyō Bridge in Kobe, Japan.
Other notable suspension bridges include New York's famous Brooklyn Bridge, which was the first to use steel wires in 1883. Washington State's Tacoma Narrows Bridge is the longest in the world to use a twin roadway design, with two separate decks side by side to allow traffic to move in both directions. (Other bridges use a single wide deck, or two decks stacked atop one another to allow for two-way traffic.)
Suspension bridges are suitable for certain applications, but not always the preferred choice. In cases of simple suspension bridges, the ease of construction is a major advantage. More advanced suspension bridges are capable of spanning long distances that would be impossible with other bridge designs. Suspension bridges also use fewer raw materials and are usually lighter in weight than other bridge types.
Among the key disadvantages of suspension bridges are the increased cost. While the materials may not be as expensive, the design process can be long, and a bridge design may require extensive testing. Computer simulations and wind tunnels make this testing easier and help to ensure long-term safety, but the prospect of a suspension bridge failure makes safety a concern of some importance.