Clark Bridge, Alton, IL
Cable-stayed bridges may look similar to suspension bridges—both have
roadways that hang from cables and both have towers. But the two bridges
support the load of the roadway in very different ways. The difference lies in
how the cables are connected to the towers. In suspension bridges, the cables
ride freely across the towers, transmitting the load to the anchorages at
either end. In cable-stayed bridges, the cables are attached to the towers,
which alone bear the load.
The cables can be attached to the roadway in a variety of ways. In a radial
pattern, cables extend from several points on the road to a single point at the
top of the tower. In a parallel pattern, cables are attached at different
heights along the tower, running parallel to one other.
Parallel attachment pattern
Radial attachment pattern
How do cable-stayeds work?
Stand up and hold your arms out horizontally at each side. Imagine that your
arms are a bridge, and your head is a tower in the middle. In this position,
your muscles are holding up your arms.
Try making cable-stayeds to support your arms. Take a piece of rope (about five
feet long), and have a partner tie each end of the rope to each of your elbows.
Then lay the middle of the rope on top of your head. The rope acts as a
cable-stayed and holds your elbows up.
Have your partner tie a second piece of rope (about 6 feet long) to each wrist.
Lay the second rope over your head. You now have two cable-stayeds. Where do
you feel a pushing force, or compression? Notice how the cable-stayeds transfer
the load of the bridge (your arms) to the tower (your head).
Even though cable-stayed bridges look futuristic, the idea for them goes back a
long way. The first known sketch of a cable-stayed bridge appears in a book
called Machinae Novae published in 1595, but it wasn't until this
century that engineers began to use them. In post-World War II Europe, where
steel was scarce, the design was perfect for rebuilding bombed out bridges that
still had standing foundations. Cable-stayed bridges began to be erected in
the United States only recently, but the response has been passionate.
For medium-length spans (those between 500 and 2,800 feet), cable-stayeds are
fast becoming the bridge of choice. Compared to suspension bridges,
cable-stayeds require less cable, can be constructed out of identical pre-cast
concrete sections, and are faster to build. The result is a cost-effective
bridge that is undeniably beautiful.
In 1988, the Sunshine Skyway bridge in Tampa, Florida won the prestigious
Presidential Design Award from the National Endowment for the Arts. Painted
yellow to contrast with its marine surroundings, the Sunshine Skyway is one of
the first cable-stayed bridges to attach cables to the center of its roadway as
opposed to the outer edges, allowing commuters an unobstructed view of the
magnificent bay. Recently, in Boston, Massachusetts, a cable-stayed design was
selected for a new bridge across the Charles River—even though cheaper
options were proposed. City officials simply liked the way it looked.
Sunshine Skyway bridge
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