The failure of the Tacoma Bridge: A physical model

The cause of the collapse of the Tacoma Narrows Bridge has been a topic of much debate and confusion over the years. Many mischaracterizations of the observed phenomena have limited the understanding of the collapse even though there has always been an abundance of evidence in favor of a negative damping model. Negative damping, or positive feedback, is responsible for large amplitude oscillations in many systems, from musical instruments to the Tacoma Narrows Bridge failure. We discuss some of the more well known examples of positive feedback, and then show how the interaction of the wind with the oscillating bridge, especially the development of large scale vortices above and below the deck of the bridge, led to such a positive feedback instability. We support our model by computational, experimental, and historical data. (C) 2006 American Association of Physics Teachers.