Opened to traffic: July 1, 1940 Collapsed: November 7, 1940. Tacoma Narrows Bridge. Lewis Cluck - lxc130030 Advanced Requirements Engineering – SE 6361.Z01 Oct. 26, 2012 . Design of Collapsed Bridge.
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Modified Tacoma Narrows design used 8-foot deep girders and increased span to width ratio allowing for a slimmer and more elegant look. Also cheaper: $7 million
The structure experienced considerable vertical oscillations while it was still under construction, several strategies were used, in vain, to reduce the motion of the bridge. They included:
Professor Frederick Burt Farquharson from the at the University of Washington hired to perform wind-tunnel tests and recommend solutions in order to reduce the oscillations of the bridge.
Five days before the collapse, the test studies are concluded.
In other words, the oscillations increase in amplitude with each cycle because the wind pumps in more energy than the flexing of the structure can dissipate, and finally drives the bridge toward failure due to excessive deflection and stress.
The United States government requires that all bridges built with federal funds must first have their preliminary design subjected to wind tunnel analysis using a 3-dimensional model.