Suspension Science: How Do Bridge Designs Compare?
Have you ever ridden in a car driving across a suspension bridge? Suspension bridges, with their tall towers, long spans and gracefully curving cables, are beautiful examples of the work of civil engineers. How do the cables and towers carry the load that is on the bridge, which includes you and the car you’re in when you cross the bridge? Can a suspension bridge carry a greater load than a simple beam bridge? You can try to answer these questions in this science activity!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
In a suspension bridge, the bridge deck (the part of the bridge that supports the load, such as cars and their passengers) hangs from, or is suspended by, massive cables. These cables stretch between the bridge’s 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 (in Washington in 1940) is an infamous example of this.
In this activity, you’ll build and test two types of bridges: a simple suspension bridge and a beam bridge. A beam bridge is the simplest type of bridge, and is typically supported by a raised part on either end. For example, a beam bridge could be as simple as a wood plank put down to cross a stream. Which type of bridge do you think can support a heavier load?
Extra: Test each bridge design a few more times, using a new bridge deck straw each time. Are your results always the same?
Extra: Try eliminating 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 now? Why?
Extra: Try this activity a few more times and focus on what part (or parts) of the bridge fails first. 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?
Observations and Results
Did the suspension bridge hold a greater number of coins compared to the beam bridge?
In this activity you should have seen that the suspension bridge was able to hold more coins than the beam bridge by around 150%, such as about 310 pennies (or 140 quarters) compared to about 200 pennies (or 90 quarters). When the beam bridge failed, this was likely because the bridge deck straw bent downward as more coins were added until it bent so much that it slipped down between the two towers. As coins were added to the suspension bridge, the cable (i.e., thread) was under tension and reinforced the bridge deck straw, pulling it upwards (while compressing the towers) and allowing the bridge to hold more coins. When the suspension bridge eventually failed, the bridge deck straw likely similarly bent into a V-shape, but because it was attached by the thread, the straw couldn’t fall and instead the cup may have slipped off of the straw.
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Teisha Rowland, PhD, Science Buddies
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