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THE TRUTH ABOUT THE TACOMA-NARROWS BRIDGE


Newsreel footage of Tacoma Narrows Bridge shortly before collapse. Note that replay speed has
been accelerated to save file space.

The Tacoma-Narrows Bridge in Washington State collapsed in 1940, approximately six months after it opened. The bridge
was known as “Galloping Gertie” due to its excessive motions in relatively normal wind conditions. The cause of the failure
was a mystery for many years. The motion pictures that were taken at the time clearly showed the bridge oscillating in what
appeared to be a classical example of torsional resonance. However, after many years of investigations and analyses, it was
determined the failure was caused by a phenomenon, known to the aircraft industry, as aeroelastic flutter.

Mechanical resonance consists of a structure oscillating at a natural frequency whereby the inherent damping is insufficient to
dissipate its vibrational energy. The motions that can occur may be excessive, possibly leading to structural failure. However, it
was determined that classical resonance due to vortex shedding was not the cause of the failure. The failure of the bridge was
found to be caused by a complex aerodynamic and elastic interaction between the wind, which blew at a relatively constant
speed of 42 miles per hour, and the deformation of the bridge itself. The deformation of the roadbed in torsion, due to its
slender design, resulted in it behaving similar to an airfoil in flight whereby the roadbed assumed an angle of attack, i.e., an
angle with the direction of the wind. This caused a complex series of vortices, due to flutter, being shed from the roadbed
ultimately leading to greater levels of twisting. The flutter vortex shedding frequency was 0.2 Hz. This frequency was coincident
with the torsional natural frequency of the bridge leading to failure. Furthermore it was found that the more classical vortex
shedding frequency of the bridges side plate girders was 1 Hz. Hence it was proven that classical vortex shedding was not the
cause of failure of the bridge.

 


View of the bridge from the roadway. Footage replay speed is accelerated to conserve file space.

 

The failure of the bridge arose from the introduction of an unexpected failure mode, i.e., flutter, through the use of
solid side plate girders, as well as streamlining, and the slender shape of the bridge. The bridge was subsequently
rebuilt with the following added features:

•Increased damping
•Increased torsional stiffness
•Side members are open utilizing trusses in lieu of solid plate girders, to allow air to flow through,
minimizing the effects of vortex shedding

To see the full historic video of the Tacoma Narrows Bridge collapse, visit: archive.org/details/SF121

 

 

 



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