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Cantilever Bridge Facts: Lesson for Kids

Instructor: Elizabeth Foster

Elizabeth has been involved with tutoring since high school and has a B.A. in Classics.

Learn what a cantilever is and how people can use them to build bridges. Cantilever bridges also include double cantilevers, counterweights, and suspended spans. Cantilever bridges are found around the world.

Cantilevers

A cantilever is something that sticks out sideways from a support, and is only supported at one end. For example, if you hold out your arm sideways, then your arm is a cantilever.

Now imagine you're a giant. You and your giant friend want to make a bridge across the Grand Canyon so normal-sized people can get across. Because you're giants, you can just lie down on either side of the Grand Canyon and hold hands over the top. Then all the people can walk across your arms.

If you made a bridge that way, you'd be making a cantilever bridge. Your arm would be one cantilever, and your friend's arm would be another cantilever. By stretching the cantilevers across the Grand Canyon and touching hands in the middle, you'd make a cantilever bridge.

Cantilever Bridges

Stretching out your hands across the Grand Canyon would make a very simple cantilever bridge. But when engineers build real cantilever bridges, they use more complicated designs.

A human model of a cantilever bridge. Counterweights are circled in red. The suspended span is circled in green.
Human cantilevers

Counterweights

When you hold out your arm, your muscles hold it up. But if you build a bridge out of metal or concrete, there aren't any muscles. Instead, to hold the cantilever in place engineers use a counterweight. A counterweight is a very heavy weight that holds the cantilever steady.

To see a counterweight in action, take a ruler and put it on the top of a table. Slide the ruler so that part of it sticks out from the table. If you slide it too far, it overbalances and falls right off. But now try it again, and use something heavy to hold the ruler down on the table. This way, you can get the ruler to stick out a lot further before it falls, because you have a counterweight.

When engineers build bridges, they use heavy counterweights to hold the cantilevers in place.

Suspended Spans

In some cantilever bridges, the cantilever arms don't meet in the middle. Instead, there's a different section in the very middle of the bridge. This is called a suspended span.

Imagine that you're trying to hold hands with a friend to make a cantilever bridge, but you can't quite reach. So you grab one end of a stick and your friend grabs the other end. Now you have a bridge made of two cantilever arms with the stick held up in the middle. That's what a suspended span bridge is like.

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