On this date: The Tacoma Narrows Bridge collapse

TACOMA, Wash. (WHTM) — The construction workers called it "Galloping Gertie."

Construction of the Tacoma Narrows Bridge started in 1938, and it opened to traffic on July 1, 1940. It was the world's third-longest suspension bridge, with a central span of 2,800 feet and two side spans each measuring 1,100 feet. Only the Golden Gate and George Washington bridges were longer.

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It would survive less than five months.

Even while it was being built, the deck moved vertically in windy conditions — hence the bridge's nickname. The problem continued after it opened, despite efforts to dampen the vibrations. On Nov. 7, 1940, 40-mile-per-hour winds triggered significant oscillations in the deck, twisting it until the span collapsed and crashed into the river. There was just one fatality — a dog trapped in the car you see in movies of the disaster.

In fact, there was a lot of film shot of the collapse. At least four people took movies, much of it in color. (Kodak began selling Kodachrome movie film in 1935.) Most of the time, though, you see only black and white copies, made for newsreels for showing in theaters.

Midstate Markers: Rockville Bridge, Susquehanna Township

Some of the film was shot at 24 frames per second (the professional "Hollywood" standard), but other movies were shot at 16 frames per second (standard for home movies at the time). A lot of times the 16 fps footage gets run at 24 fps, giving the impression the bridge twisted a lot faster than it actually did. (To see the collapse in color,.)

All the movie films helped with figuring out what happened, but scientists and engineers have revised their explanations of the underlying causes over the years since the disaster. The initial conclusion was that the main cause was mechanical resonance, where an object is moved by a force (in this case, wind) that happens to be close to one of its built-in "natural" frequencies. Left unchecked, the force builds on itself until something happens to stop it, like a bridge deck collapsing.

But over the years, people have figured out there was a second process involved — aeroelastic flutter. This is defined by Wikipedia as "a self-feeding and potentially destructive vibration where aerodynamic forces on an object couple with a structure's natural mode of vibration to produce rapid periodic motion."

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Yes, it's complicated, and I haven't really found a simple way to explain it, but here's a NASA video showing it on an airplane. (The airplane test is in slow motion, and rest easy, the trial was stopped before anything ripped off.)

What was left of the bridge was sold as scrap metal. World War II stalled the construction of a replacement, but a new span opened in 1950 using the tower pedestals and cable anchors of the original and carefully engineered to eliminate the problems that caused the collapse. (A second bridge was added next to it as traffic increased.)

Part of the deck is still in the water, where it serves as an underwater reef.