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Galloping Gertie, Engineering's Most Misunderstood Failure (vice.com)
tedlistens writes: Generations of physics teachers, textbooks, and articles have taught that the spectacular collapse of the Tacoma Narrows Bridge, 75 years ago, in November 1940, was caused by resonance. But this explanation is inaccurate, and despite the fact that the collapse is not a mystery—that the bridge, in a sense, twisted itself apart—the fallacy continues to spread. Not only that: according to a new study by Don Olson and colleagues at Texas State University and East Carolina University, parts of the famous footage that immortalized it are misleading too. According to the most complete recent research, he and his co-authors write, "the failure of the bridge was related to a wind-driven amplification of the torsional oscillation that, unlike a resonance, increases monotonically with increasing wind speed." Each time the deck of the bridge twisted now, it sought to return to its original position (inertial forces). And as it did so, twisting back with a matching speed and direction (elastic forces), the wind and the vortices caught it each time, pushing the deck just a little bit more in that direction (aerodynamic forces). With each twist and each twist back, the size of the twisting slightly increased.
Intuitively, this phenomena as described has the feel of what one thinks of given the word 'resonance'. Perhaps 'pseudo-resonance' would be a good term to apply.
Pretty much. I'm reasonably well-read, and the summary leaves me hearing "resonance was the cause but we engineers have a bunch of other words we'd prefer to use because they're technically more accurate but for anyone not in bridge-building the distinction is meaningless."
"Oh no... he found the
In logic, a fallacy is a form of faulty reasoning. This is not a fallacy, it's a mistaken explanation of the causes of the collapse. Not the same thing at all.
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TFS leaves me thinking some popular-press hack didn't understand the explanation. Pretty sure these
sought to return to its original position (inertial forces)
twisting back with a matching speed and direction (elastic forces)
Should be reversed. ie: the forces that make it return to its original position are elastic forces in the deformed bridge members, while the speed of its return are inertial forces.
The bit that makes it non-resonant is the monotonic increase with wind speed. ie, that it doesn't depend on 'just the right speed,' but that the failure would have happened faster at higher wind speed. And that the oscillation was presumably apparent even at lower wind speeds that people should have seen - probably even during construction.
Check your own wikipedia references.
Aeroelastic flutter is a type of "Simple harmonic motion".
"Simple harmonic motion" is a type of "resonance"
Simple harmonic motion
"...The motion is sinusoidal in time and demonstrates a single resonant frequency."
Resonance is :
"a phenomenon that occurs when a vibrating system or external force drives another system to oscillate with greater amplitude at a specific preferential frequency."
This is like saying "A mallard isn't a bird, it is a duck."
There's a difference:
Resonance: a force at a particular frequency that causes increased motion.
This: a powerful force caused increased motion.
In this case it was just a powerful wind. The frequency didn't matter. If the wind had been faster, the bridge would have fallen sooner, whereas if it were resonance, a higher frequency would have reduced the chances of breakage.
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