Weird Wednesdays on Urban Planet takes a look at obscure, absurd, and curious things about cities around the world.
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!
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4 comments
Good Lord, those are some crazy torsional waves. I’m glad that computer simulations have reduced the incidence of such failures.
This is probably the most coherent (albeit w/an ambient music backdrop added) Youtube footage of the disaster out there — http://www.youtube.com/watch?v=j-zczJXSxnw&feature=related
(I’ve also found that said footage mashes up real well with Sophie B. Hawkins’ “Damn, I Wish I Was Your Lover”)
This post is a little off. The bridge fell not due to “cost cutting” but due to a simple failure in structural engineering theory to account for wind dynamics.
The previous worst collapse, the Quebec Bridge (the reason why Canadian engineers wear iron rings), was the fault of too much dead load. The bridge steelwork was heavy, which required more steelwork to support it, which made it heavier, etc.
Over the next few decades, engineers worked to make bridges lighter not strictly for cost-cutting (though that was a nice benefit) nor to make structures prettier (another nice side effect) but simply because they thought that if they made the structure lighter, it would have less dead load, and could therefore span a longer distance under less stress. It seemed to be a virtuous cycle.
This also coincided nicely with other new thinking at the time, such as assuming only a proportion of traffic being heavy trucks, and not having to deal with heavy railroad traffic (as all previous major crossings had).
Many bridges were therefore built in the 1930s with extraordinary thin decks and long spans — the original George Washington Bridge (before getting a second deck in the 1960s), the original Whitestone Bridge, etc. It was assumed that the dead load on these thin-deck bridges, while lessened, would still be enough to deal with the wind. As Tacoma showed, this was not true under certain wind conditions and deck shapes and changes were made to all future suspension bridge designs.
I remember that video too and now it’s bugging me!