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The Tacoma Narrows Bridge fallacy

Ah HA: the infamous 1940 Tacoma Narrows Bridge did not collapse as the result of natural resonance after all!

The 1940 Tacoma Narrows Bridge, aka "Galloping Gertie".

Photographer unknown (image source). Use of this image appears to satisfy "fair use" law.

For one year, I lived on one side of the Tacoma Narrows Bridge in Washington state (USA) and attended school on the other side. This meant I had to commute across that world-famous (or should I say "infamous"?) span twice each day. This suspension bridge's bouncing and vibrating deck gave me nightmares that lasted many years after I relocated and my commute ended. But my nightmares and experiences were nothing compared to what actually happened one autumn morning in 1940, when "Galloping Gertie", as the original bridge was (and still is) known by the locals, twisted and vibrated in the winds before collapsing into the treacherous Tacoma Narrows, 57 metres (188 feet) below. Luckily, no one died, but my physics teachers made this event into an object lesson about the dangers of natural resonance -- a lesson that was incorrect. What did cause Galloping Gertie to collapse? This interesting video addresses this question by giving us a quick lesson about another phenomenon, aeroelastic flutter:

Visit MinutePhysics's YouTube channel [video link].

But Galloping Gertie did not die in vain. For example, the Bronx-Whitestone Toll Bridge that spans the East River in NYC is based on the design of the 1940 Tacoma Narrows Bridge. The resemblance was truly striking when I first saw the Whitestone.

Alarmed by Galloping Gertie's collapse, engineers installed steel trusses on both sides of the Whitestone's deck in 1943 to weigh it down and stiffen the span to reduce oscillation. However, after it was learned that aeroelastic flutter was the true cause of Galloping Gertie's collapse, the trusses were replaced with aerodynamic fiberglass fairings on both sides of the Whitestone's road deck in 2003. Besides making that span more resilient, this also restored the bridge's graceful arch, making it much more aesthetically pleasing -- and probably less likely to collapse in a windstorm.

Minute Physics is on facebook and can also be found on twitter @minutephysics


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