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Suspension Bridges
How to Build a
Suspension Bridge
By Kevin Beck
Few human-made constructs
can instantly inspire awe from a distance (or up close, for that matter) in the
way a suspension bridge can.
This category of very long,
elegantly symmetrical structures includes some of the most famous bridges in
the world, including the Brooklyn Bridge in New York City, the Golden Gate
Bridge in San Francisco and the Akashi Kaikyo Bridge in Japan.
The reason suspension
bridges can be built to spectacular lengths (close to 4 km, or 2.5 miles, as of
2019) is that their unique tower-and-cable support system allows some of the
forces the bridge must withstand to be displaced laterally via the cables'
anchor points on the land on either side of the bridge.
You can explore this
principle yourself in a do-it-yourself experiment.
Suspension Bridge Design
Suspension bridges have a
characteristic appearance, the most notable feature being the pair of towers anchored
into the ground (usually under river, lake or sea water).
These support the two main
parallel cables running from end to end of the bridge over the
tops of the towers, forming a U-shaped curve in the middle mathematically known
as a parabola.
An array of vertical cables
are suspended from the main cables and support the deck that
serves the bridge's primary function of allowing passage over a gap.
Typically, there is at least
one roadway here. The length of the deck between the towers is called the main
span.
The lengths of the bridge on
either side of this span often add up to about the length of the main span.
General Concept of a
Suspension Bridge
Picture yourself and an
identically sized companion standing facing each other at a distance of about
15 feet (about 3 meters).
Imagine that each of you is
holding both hands high overhead, and that each hand – four in all between you
– supports a long rope that hangs almost to the floor between you.
If people standing behind
each one of you were to slowly apply an equal horizontal tension to
the ends of both ropes, pulling away from the center of this impromptu physics
party, the ropes would slide backward through your and your friend's hands,
with the "U" of the ropes gradually approaching a horizontal line
between your and your companion's hands.
Now imagine a series of
smaller ropes draped over the two main ropes on each side and attached to a
20-foot-long, lightweight board sitting the floor between your and your
friend's legs.
If the tension were
sufficient, the board would be raised off the floor as the main ropes became
more taut.
If you and your friend were
to "clamp" the board into place between your knees, this
"deck" would be supported by both the towers and the cables.
Suspension Bridge
Advantages
Suspension bridges are light
and flexible, since so much of them consists of cables and they are built
to sway with wind and other stresses.
Excessive stiffness (and
this may be counter-intuitive) is a disadvantage in bridges and other
structures that experience shear stress and other unusual forces.
The deck is built from
materials that are both compressible and able to withstand tension (extension),
allowing it to bend upward and downward slightly without breaking.
Of course, excessive sway
would produce hazards in its own right.
Suspension Bridge
Disadvantages
The main disadvantage of
suspension bridges is their susceptibility to wind damage, a
consequence of these bridges' prodigious size and lightweight construction.
To enhance stability,
suspension bridges are sometimes fitted with a box truss under the main roadway
to serve as a lower deck.
This not only makes the
bridge stronger but also sometimes allows for more traffic to pass on the
bridge.
Cable Suspension Bridge Kit
You can make a simple
suspension bridge model from everyday household and classroom materials, such
as drinking straws for the deck, string for the cables, masking tape, paper
clips and so on.
You can find a sample
classroom-style activity for younger and middle-school children in the
Resources.
Kevin
Beck holds a bachelor's degree in
physics with minors in math and chemistry from the University of Vermont.
Formerly with ScienceBlogs.com and the editor of "Run Strong," he has
written for Runner's World, Men's Fitness, Competitor, and a variety of other
publications. More about Kevin and links to his professional work can be found
at www.kemibe.com.
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