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| The Tacoma-Narrows Bridge today, courtesy of Altavista.com. |
Washington State Department of Highways gave the project of designing the Tacoma Narrows Bridge to an engineer of their own, Clark Elderidge. After Elderidge had finished the plans, “the State retained Leon S. Moisseiff, the world-renowned suspension bridge builder to examine the plans concerning the superstructure.”(2) The author of Building Big states that “Moisseiff had designed the ultimate in slender bridges.”(1) Checking over the plans, Moisseiff made many changes. Most significant of these alterations was to replace the 25 ft. open girders with 8 ft. shallow plate girders. The major difference was that these shallow plate girders made the bridge much lighter.
Bridge Background
On July 1, 1940, the Tacoma Narrows Bridge in Tacoma, Washington was completed after two year of construction. It was the longest suspension bridge of its time spanning over 5939 feet with a center span of 2800 feet.(4) This massive bridge connected the Tacoma and Gig Harbor. Relying on the shear and bending stress in the main cables, the engineers designed the bridge with a new method of calculations. This method allowed them to design a less expensive, lighter bridge. However, this design wasn't adequate in any respect.
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| The Tacoma-Narrows in 1940, courtesy of SCRET. |
Failure and Aftermath
On November 7, 1940, only 4 months after completion, the Tacoma Narrows Bridge failed, only claiming the life of a dog trapped in a car on the bridge. The State closed the bridge at 10:00 AM because of enormous undulations. The shallow girder plates of the bridge caught the wind and caused the bridge to oscillate. In the construction process, a 4 mph wind caused the bridge to have minor oscillations. Tie down cables had been implemented, but had little effect. At the time of the collapse, the winds were exceeding 40 mph and “the the motion dramatically changed from a rhythmic rising and falling to a two-wave twisting motion.”(2) (This is shown in the animation below.) The amplitude of these waves ranged between 5 and 28 feet. After the collapse, the Federal Work Agency investigated the cause of the failure. The FWA found that the design live load was 1.5 times less than required. The lack of stiffness of the bridge also contributed to the failure. The extreme flexibilty of the bridge proved to be the greatest weakness of the Tacoma Narrows Bridge. However, the theories of the FWA were quite vague.(3) Engineers still debate on the actual cause of the failure.
1. Building Big. Accessed on April 28, 2001 at 10:30 PM. Building Big Web Site.
2. Galloping Gertie. Accessed on April 28, 2001 at 10:00 PM. Galloping Gertie.
3. Koughan, J. The Collapse of the Tacoma Narrows Bridge. The University of Texas at Austin. 1996. Accessed on April 29, 2001 at 9:30 PM. The Collapse of the Tacoma Narrows Bridge.
4. Mileham, G. The Tacoma Narrows Bridge. Accessed on April 27, 2001 at 8:00 PM. Greg Mileham's Web Page.
5. Tacoma Narrows Project. Accessed on April 28, 2001 at 10:00 PM. SCRET's Web Page.
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