Abstract

Objective

The goal of this project is to investigate which type of bridge has the greatest weight bearing capacity.

Introduction

There are many different types of bridges, from simple plank bridges spanning a small stream to elegant suspension bridges supported by tall towers and soaring cables. Other bridges are supported with the structure of the arch, which was used by Roman engineers thousands of years ago. Yet another bridge type is the truss bridge, which gets its strength from a framework of triangles shaped from steel.

Deciding which bridge type to build for a particular location depends on many factors. What type of traffic will cross the bridge? What passes underneath the bridge? What locations are available for foundations for the bridge? How much money is available for building the bridge? What materials are to be used for building the bridge? See the "SuperBridge" website (WGBH, 1997) for an online exercise in picking the correct bridge for a particular site.

In this project, you will do background research to learn about the different types of bridges. In addition to your online and library research, you should also pay attention to the structure of bridges in your area. You will make models of at least three different bridge types, and then test each model to see how much weight it can support. Perhaps you will want to design one or more of your models to resemble a bridge near you.

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:

• strength-to-weight ratio,
• different bridge types, e.g.:
  • plank,
  • arch,
  • truss,
  • suspension.

More advanced students will also want to study the forces involved:

• compression,
• tension,
• torsion,
• shear.

Questions

• Can you identify the types of the various bridges you encounter in your everyday travels?
• Which bridge type do you think will be the strongest?
• Which bridge type will have the greatest strength-to-weight ratio?

Bibliography

• This website has tips for building model bridges from a former Science Olympiad participant, Garrett Boon:
  Boon, G., 2006. "Model Bridge Design," Garrettsbridges.com [accessed November 30, 2006] http://www.garrettsbridges.com.
• The PBS website, "Building Big" has a page on bridges that is a good place to start your background research:
  WGBH Educational Foundation, 2001. "Building Big: All About Bridges," PBS Online [accessed November 30, 2006] http://www.pbs.org/wgbh/buildingbig/bridge/index.html.
• 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 November 30, 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 November 30, 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 November 30, 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:

• stripwood
  • can use balsa or basswood (available at hobby stores), or
  • popsicle sitcks, or
  • coffee stirring sticks, or even
  • toothpicks;
• white glue,
• weights for testing bridge strength (can use metal weights, sand or water in a container).

The following equipment is optional:

• camera for before and after photos, and/or
• video camera for live movie of bridge testing,
• tripod or helper for video recording.

Experimental Procedure

1. Do your background research so that you are knowledgeable about different types of bridges: how they are constructed and the strengths and weaknesses of each type.
2. Select three or four different bridge types to model.
3. Choose the building materials to use for your model. Balsa or basswood can be obtained from local hobby shops or online. Both are lightweight and easy to work with. You can also use materials such as popsicle sticks, coffee stirrers, or toothpicks. To make fair comparisons, you should use the same scale and the same construction materials and methods for each of the bridges.
4. Construct your models. Pay careful attention to the glue work at joints. Allow the glue to dry fully (at least overnight) before testing your models.
5. Take photographs of each of your model bridges for your display board.
6. You might also want to weigh each bridge before testing.
7. Test how much weight each bridge can bear.
   • Place the bridge to be tested between two supports (e.g., tables, workbenches, or chairs). The bridge needs to be high enough off the ground to hang a container with weights underneath it. It is best if the container is close to the ground so it does not have far to fall when the bridge does collapse.
   • If you have a video camera, now is the time to start recording!
   • Hang a container from the bridge, and gradually add weight (e.g., metal, sand, or water) to the container.
   • Observe the bridge as you gradually add weight, and note any changes that you observe. Can you identify which part(s) of the bridge failed first? (Video playback can be helpful here, if you have a video camera.)
   • When the bridge fails, weigh the container. Record the weight that caused each bridge to collapse.
8. Take photographs of each of your collapsed bridges for your display board.
9. Which bridge type supported the most weight? Were the results what you expected? Why or why not?
10. If you weighed your bridges, which bridge had the greatest strength-to-weight ratio? You can calculate this by dividing the weight that just caused the bridge to fail by the weight of the bridge itself.

Variations

• How do material properties affect the strength of the bridge? Build the same type of bridge but with different building materials. Test the weight-bearing capacity of each bridge.
• Use the same materials and bridge type, but try building different span lengths. How does weight bearing capacity change as span length increases?

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

Credits

Andrew Olson, Ph.D., Science Buddies

Last edit date: 2012-01-31 11:00:00