Mythical Myths #74 – Tacoma Narrows Resonance

According to a story by astrophysicist Ethan Siegel in this week’s Forbes magazine: Science Busts The Biggest Myth Ever About Why Bridges Collapse . Wow! That headline reads as if “science” has just made a new myth-busting discovery and the article tells us that the previous understanding of what happened was based on the kind of resonance by which a singer can break a wineglass and is all wrong. But the “myth” is pretty much itself a myth here as the Federal Works Admin report way back in 1940 stated that “It is very improbable that the resonance with alternating vortices plays an important role in the oscillations of suspension bridges. First, it was found that there is no sharp correlation between wind velocity and oscillation frequency such as is required in case of resonance with vortices whose frequency depends on the wind velocity.” And with regard to whether or not the actual nonlinear effect should be described as a resonance I agree with the comments by Gregg Collins two years ago and Jeff Hester (also an astrophysicist by the way) one year ago in response to munutephysics’ video on this from 2011. Both of these commenters point out that despite not being a case of simple response to a periodic applied force “it is far ~more~ incorrect to say it was not a resonance at all”. What really happened is that the bridge had an inadequately damped torsional vibration mode which was excited not by a periodic change in the environment but by a nonlinear interaction with steady wind which pushed harder on the twist when the twist itself was greater (much as a child on a swing pumps herself up by reacting to her position rather than moving at a predetermined frequency).