May 21st, 2008

For The Birds

Where "self-excitation" is not as dirty as it sounds

For more than (most of) you'd ever want to know [pdf] about the physics behind the 1940 Tacoma Narrows bridge incident. My professor was wrong (and apparently so were a bunch of physics textbooks): the culprit was not wind-induced forced resonance, but in fact: flutter. Thanks Wikipedia!

From the abstract:
It is ... demonstrated that the ultimate failure of the bridge was in fact related to an aerodynamically induced condition of self-excitation or ``negative damping'' in a torsional degree of freedom. The aeroelastic phenomenon involved was an interactive one in which developed wind forces were strongly linked to structural motion. This paper emphasizes the fact that, physically as well as mathematically, forced resonance and self-excitation are fundamentally different phenomena.