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posted by Fnord666 on Thursday May 25 2017, @09:34PM   Printer-friendly
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.


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  • (Score: 2, Interesting) by anubi on Friday May 26 2017, @06:42AM

    by anubi (2828) on Friday May 26 2017, @06:42AM (#515863) Journal

    Why in the hell isn't this blatantly obvious?

    Of course it was resonance, energy building up and driven as the bridge "sailed" in the breeze, whipping stronger and stronger as the energy built up. Classical control system theory will give you all the math you need to model it.

    Its a regenerative oscillator, works just like a whistle or woodwind. Limited by structural failure.

    Sure, I could see this "feature" of the bridge going un-noticed until they built it. There are many surprises that one discovers AFTER they build the thing. Remember the concave shaped building that melted cars? [cnn.com]

    I have made many things and failed to consider something else many times. Another word for this is "experience". I can't blame anyone for doing it - as I am also guilty as charged for doing the same, however to KEEP doing it ( like placing executables in documents ) is idiocy. ( I learned my lesson on an "ANSI-bomb" ).

    I was once asked to analyze an "investment opportunity" where one government-funded inventor was claiming over-unity power generation a few years ago. He was driving something like a bodybuilder's barbell torsionally at resonance through magnetically coupled drive, and even had NIST over to measure the torque on the rod connecting the two weights at each end. He was showing how he was only driving it at "low" torque and amassing a hundred times that torque in the resonant shaft. However he never used the words "resonant", and used "business techniques" to allow NIST to only document the torque, as what the shamster really wanted was to name-drop the trusted name of NIST in his presentations to give other governmental money-givers plausible deniability for funding this charade.

    However, I did not say the right thing. I did the "Joseph thing" and told what I believed to be the truth. No-one would listen to me. They found a PhD that had the business sense to say what they wanted him to say. He walked away well paid. I did not get a dime.

    Whether or not this stuff worked seemed to be the least of the suit-men's concern, rather the main concern was maintaining relationships with those who have the authority to compel the public to fund it.

    Gullibility of people in positions of authority to spend other people's money make me sick. Its like trying to tell governmental types the perils of embedding executables in a document. They hire the services of some team-player PhD to give them plausible deniability for doing incredibly stupid things, and even if they knew better, they will never fess up to it... doing this stupid stuff diverts incredible tax-payer monies to their cronies right under everyone's nose.

    --
    "Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]
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