The Bridge That Resonance Did Not Break

On 7 November 1940 the Tacoma Narrows Bridge in Washington twisted itself apart in a wind of about forty miles an hour, and film of the collapse became a staple of physics classrooms. Many textbooks explained it as resonance, with the wind pushing the deck at its natural frequency until the swings grew too large to survive.

In 1991 the engineers Billah and Scanlan surveyed those textbooks and objected. A steady wind has no rhythm to match. What twisted the deck was flutter, in which the motion of the deck altered the airflow around it, and the altered airflow fed the motion in turn. The error persisted for half a century, perhaps because a simple story that looks right is rarely checked by those who retell it. Why it was so rarely checked remains unclear.