Astrophysicist and author Ethan Siegel, writing for Forbes, just helped to expose a longtime myth about good ole’ Galloping Gertie, a bridge that (in)famously collapsed just a few short months after opening to public traffic.
To help jog your memory, here is footage uploaded to YouTube of the bridge twisting and bouncing around:
The story we learned in Physics class back in school was that Galloping Gertie’s fatal flaw was related to “resonant frequency” — the same phenomena responsible for wine glasses shattering when exposed a tone of specific frequency. Siegel proposes an alternative explanation:
But it wasn’t resonance that brought the bridge down, but rather the self-induced rocking! Without an ability to dissipate its energy, it just kept twisting back-and-forth, and as the twisting continued, it continued to take damage, just as twisting a solid object back-and-forth will weaken it, eventually leading to it breaking. It didn’t take any fancy resonance to bring the bridge down, just a lack of foresight of all the effects that would be at play, cheap construction techniques, and a failure to calculate all the relevant forces.
This wasn’t a total failure, however. The engineers who investigated its collapsed began to understand the phenomenon quickly; within 10 years, they had a new sub-field of science to call their own: bridge aerodynamics-aeroelastics. The phenomenon of flutter is now well-understood, but it has to be remembered in order to be effective. The two bridges currently spanning the Tacoma Narrows’ previous path have shorn up those flaws, but London’s Millennium Bridge and Russia’s Volgograd Bridge have both had “flutter”-related flaws exposed in the 21st century.
Don’t blame resonance for the most famous bridge-collapse of all. The true cause is much scarier, and could affect hundreds of bridges across the world if we ever forget to account for, and mitigate, the fluttering effects that brought this one down.
Read all of Siegel’s piece for the details…
AEC Forensics 