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Bring Science Home

Feel the Forces of a Suspension Bridge

An easy engineering activity from CityScience
bring science home straw suspension bridge



George Retseck

Key Concepts
Forces
Compression
Tension

Introduction
Have you ever wondered how bridges stand tall in the face of gravity, weather and the heavy traffic crossing over them? Building them might seem complicated, but they are based on some surprisingly simple forces that you have probably experienced yourself. Have you ever stomped on an empty can? Or played tug-of-war? Then you might know more about engineering suspension bridges than you think!

"Compression" and "tension" are two forces that we can feel and see in the world around us. Where can you find examples of compression (hint: think of the empty can) and tension (hint: try a game of tug of war).

Using lots of math, civil engineers position the parts of a suspension bridge so compression and tension work together to hold up some of the world's longest spans. With a partner and some common household items, you be able to see what it feels like to be a suspension bridge—and compare the strength of a suspension bridge to a beam bridge.

Background
People have been building suspension bridges for more than 600 years to cross the widest rivers and canyons. Today, you can still find suspension bridges deep in the remote jungle as well as in most major cities. In the U.S. the two most famous suspension spans are probably San Francisco's Golden Gate and New York City's Brooklyn bridges.

The main parts of a suspension bridge are its towers, cables, decking and anchorages. As you might guess, the long cables are the unique part of a suspension bridge. Anchored on land at each end, the cables are strung over the top of tall towers and gently slope back down to support the bridge deck (the road and/or walkway) from above.

Materials
•    A partner (someone of similar size works best)
•    Three straws: one to use as towers and two to use as bridge decks—one for each of the two bridges to be built
•    Scissors
•    Piece of string, four feet long, to use as a suspension cable
•    Masking tape
•    Large paper clip
•    Load bucket (paper box takeout containers work well)
•    Heavy nuts, bolts or anything else small but heavy to serve as weights
•    Ruler

Preparation

•    Move two chairs of equal height seven inches apart.
•    Carefully cut one straw in half to make two towers.
•    Cut a small slit on the top end of each tower for the string to rest in.
•    Cut one four-inch piece of string.
•    Have the masking tape handy.

Procedure
•    To learn about the forces that keep suspension bridges standing, you can experience them yourself. To start, face your partner.
•    Join hands, and slowly, carefully lean backward. Don't let go! Do you feel a tug? What is the force associated with pulling? This is tension, which suspension bridge cables use.
•    Facing your partner again, place your palms against your partner's. Now slowly, carefully lean toward each other. What kind of pressure do you feel now? This is compression, which is found in bridge towers.
•    Now that we know what the forces of compression and tension feel like, we can build two types of bridges and compare their strength. We'll start by building a model suspension bridge. Tape one half of the cut (tower) straw to the top of one of the chairs so that the slit is on the top, pointing to the other chair. Now tape the other half of the straw to the other chair in the same manner. The two slits should be in a line, and the straws should be seven inches apart.
•    Wrap the middle of the four-foot string twice around a whole straw. This will serve as the deck—where a road or walkway would go.
•    Tape each end of this straw to a base of a tower straw so that it spans the distance between them.
•    Thread each end of the string through the slit in each straw down the other side. To anchor the bridge, tape each side of the string to the chair. You've just built a suspension bridge!
•    Loop a large paper clip around the deck straw and hang your empty load bucket from it. Begin to put your weights into the bucket, recording the number until the bridge breaks (by the straw bending or giving way).
•    Now we'll build a beam bridge. Remove the string and carefully cut the two side-tower straws in half.
•    Tape one straw "tower" to the edge of a chair. Tape the second tower to a second chair of the same height.
•    Position these towers seven inches apart.
•    Now tape each end of another straw to the bottoms of the two shorter side straws. This is your beam—where a road, path or railroad tracks would go. You've just built a beam bridge! Do you think this bridge will be able to hold more weight than the suspension bridge or less? Why?
•    Hang the load bucket from the deck and run the test again by adding weights one at a time.
•    Which bridge is stronger? Where did the bridges collapse? Where was each bridge weakest?
•    Extra: Design and build a longer span. See if you can make a bridge twice as long (14 inches) that supports the same amount of weight. What parts of the bridge will you need to change? (Tip: You might need to reinforce the tape situating the towers, wrap the string around the deck more times or tape the anchorages farther back.)


Observations and results
Which bridge was able to hold more weight? Why do you think that is?

When all the materials and circumstances are the same, suspension bridges can span longer distances than simple beam bridges. This is because the bridge deck, or roadway, is supported from above using tension in the cables and compression in the towers rather than just from bases. Suspension bridges are also less rigid, so they can better withstand outside forces, such as earthquakes. Have you crossed beam bridges or suspension bridges? See what kind of bridges you can find in your area.

Cleanup
•    Discard straws, string and tape
•    Return weights

More to explore
"100 Years Ago: Engineering a City—New York City's Bridges," from Scientific American
"Crowds Walking Can Make a Bridge Sway," from Scientific American
Brooklyn Bridge Timeline, from PBS's Ken Burns
Golden Gate Bridge, from PBS's American Experience

This activity brought to you in partnership with CityScience
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