Bridge Failures - Failure Case Studies in Civil Engineering - page 59

up to 1.3 m (50 in) in amplitude. For the most part, these oscillations were not con-
sidered to be dangerous. Studies of the bridge vibration problem were undertaken,
including wind tunnel testing of models at the University of Washington at Seattle,
and modifications were made, such as the installation of cable ties attached to
concrete anchors. These cables broke three or four weeks before the collapse.
Deflector vanes to change the aerodynamic characteristics had also been developed,
but their installation was under negotiation at the time of the collapse. After the
failure, a board of engineers was appointed by the Administrator of the Federal
Works Agency to determine the causes of failure.
Lessons Learned
The Board of Engineers concluded that the bridge was well designed and built to
resist safely all static forces. Its failure resulted from excessive oscillations made
possible by the extraordinary degree of flexibility of the structure. The Board
determined with reasonable certainty that the first failure was the slipping of the cable
band on the north side of the bridge to which the center ties were connected. This
slipping may have initiated the torsional oscillations. The Board recommended more
studies to understand the aerodynamic forces acting on suspension bridges.
Thus, incompetence or neglect was not the cause. The failure was due to the
torsional oscillations made possible by the narrow width and small vertical rigidity of
the structure. Those actions and forces were previously ignored or deemed to be
unimportant in suspension bridge design. This failure emphasized the need to
consider aerodynamic effects in the design of a suspension bridge. Modern bridge
decks are designed to eliminate the aero-elastic instability that pushed Galloping
Gertie to failure.
References
Ammann, Othmar H., von Karman, Theodore and Woodruff, Glenn B. (1941). The
Failure of the Tacoma Narrows Bridge, A Report to the Administrator ,
Federal Works Agency, Washington D.C., March 28.
Andrew, Charles E. (1943). "Observations of a Bridge Cable Unspinner," ENR ,
August 26, 89-91.
Andrew, Charles E. (1943), "Unspinning Tacoma Narrows Bridge Cables," ENR ,
January 14, 103-105.
Andrew, Charles E. (1945). "Redesign of Tacoma Narrows Bridge," ENR , Nov. 29,
64-67, 69.
Bowers, N. A. (1940). "Tacoma Narrows Bridge Wrecked by Wind," ENR ,
November 14, 1, 10, 11-12.
Bowers, N. A. (1940). "Model Tests Showed Aerodynamic Instability of Tacoma
Narrows Bridge," ENR , November 21, 40-47.
Delatte, Norbert J. (2009). Beyond Failure: Forensic Case Studies for Civil
Engineers , ASCE Press, 26-37.
ENR. (1940). “Tacoma Narrows Bridge Wrecked by Wind,” ENR , Nov. 14, 647.
ENR. (1940). “Model Tests Showed Aerodynamic Instability of Tacoma Narrows
Bridge,” ENR , 124, 674.