What really destroyed the Tacoma Narrows Bridge: resonance or something stranger?

Ask most physics students and they will tell you: the Tacoma Narrows Bridge shook itself apart because the wind hit its natural frequency, the way a singer can shatter a glass. It makes a great story, and in many physics textbooks the event is presented as an example of elementary forced mechanical resonance. But the engineers who studied it say it was more complicated. The bridge collapsed because moderate winds produced aeroelastic flutter that was self-exciting and unbounded.

The bridge opened on 1 July 1940 and fell into Puget Sound on 7 November. Even while it was being built, the deck moved up and down in the wind, so workers nicknamed it Galloping Gertie. It was slender: a long, narrow roadway only 39 feet wide, stiffened by shallow plate girders instead of the deep trusses the state had first proposed. Solid plate girders meant the wind could not pass through the structure: it was diverted above and below it.

On the final morning a steady wind of around 40 mph set the deck twisting, with one side going up while the other went down. The twisting kept growing instead of settling, until cables began to fail one after another and the deck tore apart. Flutter is exactly that: an unstable oscillation driven by the wind that does not limit itself but grows without bound.

So why does the textbook story persist? Part of the debate is about words, since resonance has no commonly accepted precise definition. But the measurements do not fit the simple version: the destructive twisting ran at about 0.2 Hz, neither a natural mode of the bare structure nor the roughly 1 Hz at which the deck would shed vortices in that wind. The difference is who sets the rhythm. In forced resonance an outside push happens to match the structure's beat. In flutter the wind supplies the power and the deck's own motion taps it, so the twisting feeds itself and, unlike resonance, grows steadily as the wind speeds up. The collapse changed bridge design for good: bridges built since are made to be rigid and to damp oscillations, and aerodynamic research became part of designing them.