Abstract

Objective

The goal of this project is to compare the strength of two simple bridge designs: a beam bridge vs. a suspension bridge.

Introduction

The Akashi-Kaikyo Bridge (pictured at right) is the longest suspension bridge in the world, at the time of this writing (June, 2006). The bridge is 3911 m long overall, with a central span of 1991 m. It connects Maiko in Kobe and Iwaya on Awaji Island as part of the Honshu-Shikoku Highway (Wikipedia contributors, 2006). In addition to the sheer length of the bridge, the engineers who designed it also had to consider the environment: high winds, strong sea currents, salt air, and the potential for earthquakes in the area.

In a suspension bridge, the bridge deck is hung (suspended) from massive cables that stretch between the bridge towers, and are securely anchored at each end. The cables are thus under tension, while the bridge towers are under compression.

For long spans, the suspension bridge is usually the most economical choice, because the amount of material required per unit length is less than for other bridge types. However, since suspension bridges are relatively flexible structures, stress forces introduced by high winds can be a serious problem. The dramatic collapse of the Tacoma Narrows Bridge, captured on film, is a pointed example (see Ketchum, 2000).

In this project, you will use simple construction materials to build and test two types of bridges: a beam bridge and a simple suspension bridge. Which bridge supports more weight?

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• tension,
• compression,
• beam bridge,
• suspension bridge.

Questions

• Can you identify the forces acting on the loaded suspension bridge?
• Which parts of the bridge are in compression?
• Which parts are in tension?
• How does an engineer decide which type of bridge to use for a particular site?

Bibliography

• This project idea came from the PBS website, "Building Big." Their page on bridges is a good place to start your background research:
  WGBH Educational Foundation, 2001. "Building Big: All About Bridges," PBS Online [accessed June 13, 2006] http://www.pbs.org/wgbh/buildingbig/bridge/index.html.
• This fascinating website has information on different types of steel bridges and how they are made:
  Matsuo Bridge Co., Ltd, 1999. "Bridges," Matsuo Bridge Co., Ltd. [accessed June 13, 2006] http://www.matsuo-bridge.co.jp/english/bridges/basics.shtm.
• On this website, you can learn about different types of bridges (arch, beam, suspension, and cable-stayed), and then play "Build A Bridge." You'll be given a site description, and you have to decide which bridge type would work best there.
  WGBH, 1997. "Super Bridge," NOVA Online [accessed June 13, 2006] http://www.pbs.org/wgbh/nova/bridge/.
• Wikipedia has a good article on suspension bridges:
  Wikipedia contributors, 2006. "Suspension Bridge," Wikipedia, The Free Encyclopedia [accessed June 13, 2006] http://en.wikipedia.org/w/index.php?title=Suspension_bridge&oldid=58153872.
• What can happen when the design is not quite right:
  Ketchum, M., 2000. "Mark Ketchum's Bridge Collapse Page," [accessed June 13, 2006] http://www.ketchum.org/bridgecollapse.html.
• Check out this Science Buddies resource on building materials and forces:
  Pruitt, B., T. Bailey, and A. Tung, 2006. "Stress, Strain and Strength" Department of Mechanical Engineering, Stanford University (published by Science Buddies with permission) Stress, Strain and Strength.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• box of drinking straws,
• masking tape,
• dental floss or thread,
• scissors,
• 4 large paper clips,
• paper cup,
• pennies or metal washers,
• metric ruler.

Experimental Procedure

1. Cut two short pieces of straw, each 3 cm long. For each tower, tape two straws on either side of a short piece of straw, as shown below. Tape the long straws together at the top, too.

2. Tape one tower to the edge of a desk or chair. Tape the second tower to a second desk or chair of the same height. Position the towers 17 cm apart.
3. Place another straw between the towers so its ends rest on the short pieces. This straw is the bridge deck. Now you have a simple beam bridge.

4. Make a load tester by unbending a large paper clip into a V-shape. Poke the ends of the paper clip into opposite sides of a paper cup, near the rim. Use a second paper clip to hang the load tester over the bridge deck. Record how many pennies the paper cup can hold before the bridge fails.
5. Now change the beam bridge into a suspension bridge. Tie the center of a 100 cm cable around the middle of a new straw. Place the straw between the towers. Pass each end of the cable over a tower and down the other side.
6. To anchor the bridge, wrap each end of the cable around a paper clip. Slide the paper clips away from the tower until the cable pulls tight. Then tape the paper clips firmly to the desks. Test it again.

Variations

• If you eliminate the portion of the cable from the towers to the anchorage (leaving only the portion of the cable from the bridge deck to the towers) what happens when you test your bridge? Why?
• Failure analysis. When your suspension bridge failed, what part (or parts) failed first? For example, was the failure due to weakness of materials used or weakness at a joint? Can you think of ways to redesign your bridge to make the part (or parts) that failed stronger? Build your proposed solution and test it.
• Can you design and build a straw suspension bridge that spans a gap twice as wide and supports the same amount of weight? What parts of the bridge design need to change? Try it!

• For more science project ideas in this area of science, see Civil Engineering Project Ideas.

Credits

Edited by Andrew Olson, Ph.D., Science Buddies

Sources

This project idea, including the experimental procedure and construction images, is from:

• WGBH Staff, 2000. "Building Big Educator's Guide, Activity: Suspension Bridge Handout" Educational Print and Outreach Department, WGBH Educational Foundation [accessed June 12, 2006] http://www.pbs.org/wgbh/buildingbig/educator/act_suspension_ho.html.

Last edit date: 2006-06-26 10:45:00