Torsional Mode of the
Tacoma Narrows Bridge
original Tacoma Narrows Bridge was opened to traffic on July 1, 1940.
It was located in Washington State, near Puget Sound.
Strong winds caused the
bridge to collapse on November 7, 1940. Initially, 35 mile per hour
winds excited the bridge's transverse vibration mode, with an amplitude
of 1.5 feet. This motion lasted 3 hours.
The wind then increased
to 42 miles per hour. In addition, a support cable at mid-span snapped,
resulting in an unbalanced loading condition. The bridge response thus
changed to a 0.2 Hz torsional vibration mode, with an amplitude up to
28 feet. The torsional mode is shown in Figure 1.
information, please download: Tacoma
Narrows Bridge Failure PDF.