Tacoma bridge is a narrow bridge in Washington state which is popular for its destruction just after it had opened. The accident fortunately did not cause any damage to human life but a dog was left on the bridge. This case is now a representative case which notified the importance of aerodynamics when building bridges. However, the accident brought an unfortunate life to the architect, Leon Moisseiff. He was expelled from the architecture industry after the destruction and died of heart attack after living his rest of life poorly.

Video of Tacoma bridge oscillating due to aeroelastic flutter

When learning resonance or waves in physics class, there are many students who bring the destruction of the Tacoma bridge as an example of resonance. The argument that the cause of the destruction is resonance is refuted by the fact that the frequency of destructive mode was 0.2 Hz which is not equal to the natural mode of the bridge. To understand this rebuttal more clearly, it would be good to know the resonance.

Resonance is a natural phenomenon where the amplitude of oscillation is increased when an external force with frequency that is equal to the natural mode of the object is exerted to the oscillating object. When this phenomenon is maximised, the object is destroyed. Because this phenomenon occurs when the frequency of external force is equal to the natural mode of the object, the rebuttal can falsify the claim that the bridge was destroyed because of resonance.

Then, what was the true reason for the destruction of the Tacoma bridge?

The true cause of the destruction was because of the aeroelastic flutter which was caused by the wind. Aeroelastic flutter is defined as “an unstable, self-excited structural oscillation at a definite frequency where energy is extracted from the airstream by the motion of the structure”. This phenomenon is caused because of the positive feedback between the object’s deflection and the external force. When the object oscillates because of the external force exerted(which in this case, exerted by the wind), it reaches a flutter point where the object is undergoing a simple harmonic motion. Therefore there is zero net damping and so any further decrease in net damping will result in self-oscillation and failure. The torsion stress that the object gets during the self-oscillation results in fatigue fracture which is a phenomenon that occurs when the stress exerted to the object exceeds the endurance limit.

Eunsang(Leo) Jang

Member of NLCS JEJU
Member of NLCS JEJU Engineering society

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