If you have ever taken a physics course, whether in high school or college, there is a very good chance that you heard about the collapse of the Tacoma Narrow Bridge. In fact, if you are an engineering student, you probably have learned more about this bridge than the average student. Why? Because not only did this bridge collapse due to flaws in the design and construction, it collapsed in an amazingly dramatic fashion and was captured on film. This film, commonly shown as part of many physics or engineering curricula, leaves a lasting impression in the student’s memory.
Construction of the Tacoma Narrows Bridge began in September of 1938. It was a suspension bridge built over Puget Sound between the city of Tacoma and the Kitsap Peninsula in Washington State. At the time of construction, it was the third-longest suspension bridge in the world, by main span length. However, as soon as the deck of the bridge was built, the workers realized that something was not right. The deck of the bridge would move vertically during windy conditions – so much so, it was given the nickname “Galloping Gertie.” Some of the workman were even forced to chew on lemons to lessen the feeling of seasickness they experienced while working on the bridge.
Although the state’s engineers assured the local papers that the ‘bounce’ was normal, they began taking steps to eliminate the movement. They contracted for a wind tunnel study of the bridge to be performed to find the cause and a permanent solution. In addition, four hydraulic jacks were installed at the towers to act as shock absorbers. Although this modification was not very effective, the bridge was opened to traffic on July 1, 1940.
In October of 1940, as a preliminary suggestion from the wind tunnel study, tie down cables (1-9/16” diameter anchored restraining wires) were installed along the bridge’s side and mid-spans. This seemed to reduce the ‘bounce’ of the side spans, but was not effective at reducing the movement of the center span. The increased winds of autumn were now taking their toll on Gertie.
During the morning of November 7, 1940, a strong, icy wind was lashing at the bridge, causing two- to five-foot-high movements of the mid-span. This was followed later by a “lateral twisting motion” which was tilting the roadside up 28 feet one side, then the other, at an angle of up to 45 degrees. Approximately one hour after this twisting motion began, the Tacoma Narrows Bridge failed, and the center span of Galloping Gertie fell into Puget Sound.
The Tacoma Narrow Bridge was a suspension bridge and, as such, was designed to move. But why did this bridge move so much and, therefore, ultimately fail? As described in many physics and engineering books, the cause of the collapse is attributed to forced resonance: aeroelastic flutter produced by the wind that matched the natural frequency of the bridge. Several factors contributed to this. The wind tunnel study, completed just days before the collapse, determined that the solid stiffening girders, used underneath the roadbed, were partly to blame. Typically, bridges employed an open lattice beam truss, allowing the wind to pass through. However, this solid girder design caused the wind to be diverted above and below the structure, therefore, attributing to the flutter. Other factors identified throughout the investigation were the narrow deck design (only two lanes) and the overall lightness of the bridge partly due to the cost saving measures of limiting the amount of steel used during construction.
Although a tragedy financially, there was no loss of life from the collapse (except for a cocker spaniel, Tubby). And, as a direct result, research into aerodynamics-aeroelastics expanded, which helped influence future bridge designs preventing this from occurring again.
However, Galloping Gertie still lives on in physics and engineering classrooms and students’ memories.
References:
http://www.wsdot.wa.gov/tnbhistory/connections/connections3.htm
https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
Video showing the collapse:
https://commons.wikimedia.org/w/index.php?title=File%3ATacoma_Narrows_Bridge_destruction.ogv
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