Suspension
Bridge
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.
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.
What are the anchorages for?
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.
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.
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.
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.
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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? 
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.
