(Tacoma Narrows Bridge – video below)
My three sons and I were talking about this the other day. I was explaining to them how a marching band can potentially set up a standing wave in a bridge setting up a feedback loop in the bridge’s construction eventually causing it to collapse. (Who says physics can’t be fun?)
There are historical examples where this has actually happened. In 1831 cavalry troops marched in step over a suspension bridge near Manchester, England causing the bridge to collapse. In 1850 the Angers Bridge in France collapsed when 478 soldiers marched across it in lockstep. Because of instances like these, groups marching in formation are now instructed to break step when crossing bridges.
So what is the science behind all this? Here is an answer from the physics department at the University of Wisconsin-Stout.
When troopers march in cadence across a bridge, the marching may match a natural resonance frequency of the bridge. Although only a small amount of energy is added with each step, because of the resonance effect this energy will be stored. As a result, the bridge will cumulatively absorb energy from the marching men, increasing the oscillation amplitude in the bridge (just as pushing someone on a swing, in cadence, increase the amplitude of the swing). Enough energy may be added this way to damage or destroy the bridge. (from: How to Cross a Bridge)
The Tacoma Narrows Bridge over Puget Sound in Washington is a famous example of a bridge that was destroyed by resonance vibrations. The original Tacoma Narrows Bridge, also known as “Galloping Gertie,” was a concrete and steel suspension bridge. It was the third longest span bridge in the world when it was built. Construction on the bridge began in 1939, and the bridge was put into service on July 1, 1940. Just four months later, on November 7, 1940, the bridge was destroyed by resonance vibrations set up by winds acting on the bridge. The bridge that had taken a year to build was destroyed in a matter of hours.
Here is actual video footage from 1940 of the bridge’s vibrations and eventual collapse. I remember seeing the footage of this concrete bridge bouncing up and down like a piece of rubber back in high school, and it was fun to find it online again. (Click below for video.)
(Video length: 1:03)
You can also visit the Internet Archive for additional video footage of the bridge’s construction, opening ceremony, and final collapse.