How to Build a Suspension Bridge

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 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.

References

About the Author

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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