Everyone has probably seen the footage of the original Tacoma Narrows Bridge and its crazy wobbling at one time or another. Most people would think about it as soon you say “earthquake” and “bridge”, but what caused the bridge to twist and bend and eventually collapse was actually the wind… and cost cutting.
There had been talk of building a bridge to connect Tacoma and Kitsap Peninsula in Washington since the 1880s. Almost 50 years later, engineer Clark Eldridge came up with the original blueprints. It was supposed to be a fairly standard suspension bridge costing $11 million. By the time the bridge was being constructed however, the design had undergone some tweaks by Leon Moisseiff, a New York engineer. Moisseiff already had a name for himself after working on both the Manhattan and Golden Gate bridges, and his redesign slimmed down the bridge at Tacoma Narrows, making it more “elegant” and also knocked about $4 million off the price tag.
Though it stretched almost 6,000 feet across the Narrows, the bridge was only built for two lanes of traffic and was considered quite narrow for a structure that length. What’s more, based on theoretical advances Moisseiff had co-published in 1933, his re-design got rid of the originally-proposed 25-foot-deep supports of lattice beam trusses (which allow air to pass through), and replaced them with a plated support system that was a third the depth.
Even before it was opened in July of 1940, the bridge was nicknamed “Galloping Gertie” by the construction workers who, as the stories go, suffered from motion sickness as winds would force the bridge to rise and fall in a transverse wave (see the up-and-down rippling during 0:06-0:11 in this video). Though the bridge was considered strong enough to remain sound, some attempts were made during construction to dampen the movement but none were effective (two of the three proposed solutions were either destroyed or broken before construction even finished). A study by the University of Washington proposed installing curved outriggers along the sides to make the desk more aerodynamic.
However, only five days after the study concluded, strong winds had caused excessively-violent swaying along a never-before-seen torsional wave (the side-to-side “twisting” motion shown the video above) that eventually brought the bridge down.
Interestingly, though the footage of the collapse is usually scene in black and white, it was actually captured on Kodachrome by a local camera shop owner. In 1998 the footage was selected for preservation in the National Film Registry by the Library of Congress.
Last month, the fantastic podcast series 99% Invisible did a feature on the bridge’s collapse (which includes a story about the attempts to rescue the collapse’s only victim, a cocker spaniel named Tubby) and the quirky Minute Physics explains how aeroelastic flutter caused the destruction.
Extra credit: Leave a comment if you remember what ’90s-era music video featured a clip of it — it’s been bugging me all day!