The purpose of these lessons are to define what a suspension bridges, recognize that these types of bridges rely primarily on tension, and understand the process involved in building a cable-stayed bridge.
Lecture
The last type of bridge to define is the suspension bridge. This type of bridge can be easily be recognized by the large pylons (tall towers rising above the water) and the cables that gracefully hold up the road deck. Some examples of this type of bridge include the Brooklyn and George Washington Bridges in New York City. The cables are attached on top of the pylon and then pulled down in a parabolic or catenary shape down to two anchor points (one as you approach the bridge and the other as you leave the bridge) on land.
A disadvantage to a suspension bridge is its reaction to the wind. This was discovered during the collapse of the Tacoma Narrows Bridge in Washington State. This bridge began resonating with the wind and began to move like a wave in the ocean This bridge was not stiff enough. Although the Tacoma Narrows Bridge was designed to withstand 120 mph winds, it eventually collapsed due to the excessive movements of the bridge during a 40 mph wind which magnified through resonant frequencies.
The collapse of the Tacoma Narrows Bridge was a result of the differences in air pressure above and below the road deck. Similar to the wing of an airplane, the greater wind pressure below the deck and lower wind pressure above caused a lifting of the road. The varying wind speeds along different parts of the deck caused the road to lift and twist. You can experience this type of behavior when you walk across a suspended
foot-bridge. As you shift your weight back and forth or up and down you will notice the swaying motion. This is an excellent way to understand how cables of a suspension bridge remain in tension.
The Tacoma Narrows Bridge collapse forced engineers to redesign the road decks of this type of suspension bridge. Engineers realized that the roadway itself does not have to be rigid, but should flex when loads go across it. The flexes of the road are designed within tolerable and limits in large structures. The solution to keep the road stable and stiff was to add a larger truss below the deck so that wind approaching the bridge would be resisted more easily. The Verrazano Narrows Bridge that links Staten Island to Brooklyn is an another example of a suspension bridge.
Activity
Students will watch the video
NOVA - "Super Bridge"
. This movie tracks the work of engineers, construction crews, contractors, and surveyors. Students will be able to observe the construction of a bridge from the concept including engineering surveys to opening day of the newly constructed bridge.
During the movie students should be taking notes. They should be instructed to pay close attention to the following topics:
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• Constraints of the bridge design
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• The process of building a tower
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• Explain the dangers construction workers face including the definition of "the bends"
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• Explain problems encountered throughout the construction of the bridge and explain how they were resolved
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• Discuss the economics of building the bridge
Post Activity Discussion
The following are a list of questions that you can discuss with the class.
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• What are the steps taken in building a bridge from conception to final construction?
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• Describe the design criteria for this specific bridge.
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• What problems occurred before and during construction?
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• How can engineers avoid problems of this kind in the future?
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• What types of materials were used for the various parts of the bridge and why might this be?
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• Are there advantages to the cable-stayed bridge versus the suspension bridges of the
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past? Explain.
