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Suspension Bridge

Suspension bridge
Suspension bridge
Golden Gate Bridge, San Francisco, CA

Aesthetic, light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet—far longer than any other kind of bridge. They also tend to be the most expensive to build. True to its name, a suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. These cables rest on top of high towers and are secured at each end by anchorages.

Suspension bridge anchorage
Suspension bridge anchorage

The towers enable the main cables to be draped over long distances. Most of the weight of the bridge is carried by the cables to the anchorages, which are imbedded in either solid rock or massive concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.

Try It!
What are the anchorages for?

Illustration of two books with string between Tie two loops of string around the tops of two hard cover books of similar size. Tie a third piece of string to each loop so that it hangs loosely between the books. Press down on the center string. What happens?

Illustration of string between two books, anchored at ends Next, stand two books about 10 inches apart. Put a stack of heavy books on one end of string to secure it to the table. Then pass the string over each book (letting some string hang loosely between the books). Place a second stack of books on the other end of the string. Press again on the center of the string. What happens? Notice how the anchorages (stacks of books) help to stabilize the bridge.

Akashi cable assembly
Akashi bridge cable assembly

Some of the earliest suspension bridge cables were made from twisted grass. In the early nineteenth century, suspension bridges used iron chains for cables. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking.

The Humber bridge
The Humber bridge

Currently, the Humber bridge in England has world's longest center span—measuring 4,624 feet. But this record won't stand for long. In 1998, the Japanese will unveil the $7.6 billion Akashi Kaikyo Bridge, linking the islands of Honshu and Shikoku. The bridge's center section stretches a staggering 6,527 feet. To keep the structure stable, engineers have added pendulum-like devices on the towers to keep them from swaying and a stabilizing fin beneath the center deck to resist typhoon-strength winds. Because suspension bridges are light and flexible, wind is always a serious concern—as the residents of Tacoma, Washington can surely attest.

Tacoma 3

Tacoma Narrows
Bridge Oscillation
Tacoma 3

Tacoma Narrows
Bridge Collapse
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At the time it opened for traffic in 1940, the Tacoma Narrows Bridge was the third longest suspension bridge in the world. It was promptly nicknamed "Galloping Gertie," due to its behavior in wind. Not only did the deck sway sideways, but vertical undulations also appeared in quite moderate winds. Drivers of cars reported that vehicles ahead of them would completely disappear and reappear from view several times as they crossed the bridge. Attempts were made to stabilize the structure with cables and hydraulic buffers, but they were unsuccessful. On November 7, 1940, only four months after it opened, the Tacoma Narrows Bridge collapsed in a wind of 42 mph—even though the structure was designed to withstand winds of up to 120 mph.

New Tacoma Narrows Bridge
New Tacoma Narrows Bridge

The failure came as a severe shock to the engineering community. Why did a great span, more than half a mile in length and weighing tens of thousands of tons, spring to life in a relatively light wind? And how did slow, steady, and comparatively harmless motions suddenly become transformed into a catastrophic force? To answer these questions engineers began applying the science of aerodynamics to bridge designs. Technical experts still disagree on the exact cause of the bridge's destruction, but most agree the collapse had something to do with a complex phenomenon called resonance: the same force that can cause a soprano's voice to shatter a glass.

Today, wind tunnel testing of bridge designs is mandatory. As for the Tacoma Narrows bridge, reconstruction began in 1949. The new bridge is wider, has deep stiffening trusses under the roadway and even sports a slender gap down the middle—all to dampen the effect of the wind.

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(next bridge description: cable-stayed)

Photos: (1) © Mark E. Gibson/Visuals Unlimited; (2,3) Matsuo Bridge Co., Ltd.; (4) Richmond and Rigg Photography; (5,6) Ed Elliot/The Camera Shop; (7) Corbis-Bettmann.

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