This is interesting. After all these decades, it turns out that the Tacoma Narrow Bridge failed due to flutter, not resonance. They’ll have to rewrite a lot of texts.
I find it amusing that they note it “only” cost $0.75 to cross. That was actually a lot of money in 1940. Comparable to modern bridge tolls of $5 in the Bay area.
I thought it was always flutter. Flutter is just when your vortex shed at the same frequency as your twisting mode.
Ah, not quite, but the article does not do a good job of explaining this issue. It’s a subtle difference, but the key point is not that the resonance freq of the structure matches the natural shedding freq of the vortices (as in the aeolian harp effect) but merely that an impulse is delivered once each cycle or half-cycle in such a way that energy is added the the structure’s motion. There’s no requirement that this frequency have anything to do with the frequency of shedding in e.g. a von Karman street.
Flutter in aircraft wings occurs when the bending and torsional modes of the wing coalesce, which happens due to interaction with the unsteady aerodynamics – which typically is NOT vortex shedding.
Not exactly a new observation, but it is surprising how long the misunderstanding persists in the physics world.
Long and interesting article, if a bit confusing about how flutter is powered. But the real reason the bridge failed is buried almost at the end of the article:
“A year after the collapse, David L. Glenn, the PWA’s field engineer, revealed that he had not signed off on the bridge when it was completed in July 1940. He had submitted a report warning of faults in design and refused to recommend acceptance of the structure. But the PWA accepted the bridge, as did the Washington State Toll Bridge Authority. (The PWA fired David Glenn two weeks after the story made headlines.)”
The culture in the Public Works Agency which could
1) hire and pay Mr. Glenn,
2) ignore his negative recommendation, and
3) fire him for telling a truth
is the true culprit here.
Amen, brother!!!
Many many moons ago I was tasked to analyse the built-in-test capability on a computer networking segment (for which I had done the microcode and system software) of a large military system. After 3 days I came up with the two distilled numbers requested. A few days later the Program Manager showed up and asked me to sign the report. I saw that the numbers on the report were better than mine, and asked why. He said the numbers had been changed to meet the program requirements. I told him he’d have to sign the report, then, not me. Sadly, I did not have the guts to escalate this – I can only plead youth and naivete – but it still rankles more than 30 years later.
Nature will not be fooled. That’s why we lost two shuttles.
Anything is possible if you lower your standards far enough. And if you have a fall guy identified when the excrement hits the fan.
Okay, I’m going to have to school all you physicists and engineers. The Tacoma Narrows bridge didn’t collapse due to resonance or flutter, it collapsed due to catastrophic anthropomorphic climate change. With increasing CO2 emissions all our bridges will fail, and those of us that survive will face swarms of locusts and tropical diseases like Ebola and zombie attacks.
You just have to turn off the reason and logic centers in your brain and realize that our cis-gendered worldview supports the male patriarchy but blinds us to threats we haven’t even imagined yet, such as the accumulation of male pedestrian (foot propelled) dryer lint. Clean fuzzy socks are a sterling example of white male ambulatory privilege that endangers us all, since dryer lint is almost as explosive as C-4 and cannot be disposed of safely.
What does that have to do with bridge design or aero-elasticity? To the modern progressive mind, everything, because spouting meaningless nonsense about a ridiculous threat merits more social status than crunching lots of numbers and determining that a giant structure is going to fly violently apart.
Actually, $0.75 in 1940 is equal to $12.74 in current dollars., by the official government inflation rate. 17:1 since then. Probably understated too. Yeah, that’s a steep bridge toll.
Heh. maybe they charged extra for the thrill-ride…