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from the tacoma-bridge-is-falling-down,-falling-down dept.
One of the most spectacular and famous bridge collapses of all was that of the the Tacoma Narrows Bridge on November 7, 1940. On that day, high, sustained winds sent the bridge into a twisting, rocking motion that led to its eventual collapse. It has been used as a classic example of the phenomenon of mechanical resonance, however, this is incorrect. Ethan Siegel has an article (behind an ad-blocker blocker) explaining how a much more intricate phenomenon known as aeroelastic flutter was responsible.
The collapse of the Tacoma Narrows Bridge on the morning of November 7, 1940, is the most iconic example of a spectacular bridge failure in modern times. As the third largest suspension bridge in the world, behind only the George Washington and Golden Gate bridges, it connected Tacoma to the entire Kitsap Peninsula in Puget Sound, and opened to the public on July 1st, 1940. Just four months later, under the right wind conditions, the bridge was driven at its resonant frequency, causing it to oscillate and twist uncontrollably. After undulating for over an hour, the middle section collapsed, and the bridge was destroyed. It was a testimony to the power of resonance, and has been used as a classic example in physics and engineering classes across the country ever since. Unfortunately, the story is a complete myth.
[...] But as the wind passed over the bridge on November 7th, a stronger, more sustained wind than it had ever experienced before, causing vortices to form as the steady wind passed over the bridge. In small doses, this wouldn't pose much of a problem, [...] Over time, they cause a aerodynamic phenomenon known as "flutter," where the extremities in the direction of the wind get an extra rocking motion to them. This causes the outer portions to move perpendicular to the wind direction, but out-of-phase from the overall up-and-down motion of the bridge. This phenomenon of flutter has been known to be disastrous for aircraft, but it was never seen in a bridge before. At least, not to this extent.
When the flutter effect began, one of the steel suspension cables supporting the bridge snapped, removing the last major obstacle to this fluttering motion. That was when the additional undulations, where the two sides of the bridge rocked back-and-forth in harmony with one another, began in earnest. With the sustained, strong winds, the continued vortices, and no ability to dissipate those forces, the bridge's rocking continued unabated, and even intensified. The last humans on the bridge, the photographers, fled the scene.
But it wasn't resonance that brought the bridge down, but rather the self-induced rocking! Without an ability to dissipate its energy, it just kept twisting back-and-forth, and as the twisting continued, it continued to take damage, just as twisting a solid object back-and-forth will weaken it, eventually leading to it breaking. It didn't take any fancy resonance to bring the bridge down, just a lack of foresight of all the effects that would be at play, cheap construction techniques, and a failure to calculate all the relevant forces.
(Score: 3, Interesting) by Anonymous Coward on Friday May 26 2017, @04:32AM
This ^^
My father was a flight test engineer at Boeing. He (and the rest of that flight test crew) were up in an experimental B-17 when the Tacoma Narrows Bridge failed. They all witnessed the failure directly from the air. Note that it was named Galloping Gertie well before the failure, it flexed on windy days as soon as it was completed.
Famous aerodynamicist Theodore Von Karman and others investigated the failure, their 1941 report (pdf) can be downloaded from http://authors.library.caltech.edu/45680/ [caltech.edu]
Here are the first & last paragraphs in the summary at the start of the 287 page report (too many OCR errors in the pdf to paste the whole summary):