[vlovich123]

> And of course, no story about natural resonant frequencies can pass without a reference to the collapse of the Tacoma Narrows Bridge in 1940.¹

Yes it can because it turns out it wasn't an issue with resonant frequencies & it's just promulgating an incorrect (but catchy) story.

> 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.

> You can calculate what the resonant frequency of the bridge would be, and there was nothing driving at that frequency. All you had was a sustained, strong wind. In fact, the bridge itself wasn't undulating at its resonant frequency at all!

I recommend reading the article but the long & short is it's something called "flutter" and they even have a video of the problem.

[1] https://www.forbes.com/sites/startswithabang/2017/05/24/scie...

[jdiff]

There's a footnote indicated by the superscript 1 in your quote:

> ¹ Follow-up 2: Yes, I know that the Tacoma Narrows Bridge collapse was not the result of resonance, but I felt I had to drop the reference to forestall the “You forgot to mention the Tacoma Narrows Bridge!” comments.

Damned if you do, damned if you don't, and damned if you do both, even.

[necovek]

Considering "follow-up" wording, it might have been added later, and might not really be true — but the important thing is that there is a correction now.

So it's good a commenter here posted about it, because this might have led to the clarification being put in in the first place.

[kaycebasques]

Follow-Up 2 has been there since at least Dec 2022: https://web.archive.org/web/20221231143310/https://devblogs....

[necovek]

Thanks for confirming that.

[jdiff]

The commenter quoted a section of text that included a reference (the mentioned superscript 1) to the footnote, it was already there.

[ptero]

It's closer to "damned if you knowingly make grossly incorrect statements even if you put a small footnote saying OK, I know it is not correct". And damned right that you are damned in this case :)

I would much prefer a reference to the event with the clarification, in the same paragraph, that it is due to a different phenomenon. My 2c.

[lelandfe]

Think I missed the grossly incorrect part

[colechristensen]

This is a pretty... dubious refutation.

The wind excited a vibrational mode of the bridge which caused it to kind of fail and when parts started breaking more modes were activated and it fell apart.

It's being sold as this gotcha! it's a myth! it wasn't _really_ resonant frequency!

And like... I studied aerospace structures... sure "flutter" is a bit of a better explanation, but saying "resonance" is a myth is a bit silly. Complex structures have lots of vibration modes. The first fundamental frequency can be picked usually and called _THE Resonance Frequency_ or whatever, but it's not like something anybody really places that much emphasis on being the boss in charge of all the vibration.

Myth != terminology nitpick in a layman's explanation

But you get a lot of layman going around correcting people and calling things myths.

It's like Internet people arguing about "just a theory", nobody who actually does science really cares at all about the precise meaning of the word "theory".

[WalterBright]

The movie of the galloping bridge looked like resonant frequency to me.

Back at Caltech, the dorm halls had poured concrete walls. Naturally, some students got a signal generator, a power amplifier, some speakers, and installed the speakers at the node of one of the halls (the halfway point).

Then turned it on, and tuned the frequency until it matched the resonant frequency of the hall. The energy in the halls quickly built up until the entire building was going whomp whomp whomp. Except that the frequency was too low to hear.

You just got a feeling that something was very very wrong. Residences would come out of their rooms wondering what the heck was going on.

A fine prank!

Of course, that was exciting the resonant response with a driving resonance frequency, which is not what happened at the Tacoma Narrows bridge, which had a different way of exciting it as already explained.

[WalterBright]

At Boeing, I worked on proving the elevators would not flutter. It wasn't about a driving force being at the resonant frequency. It was about the interaction of spring rate of the system and the force pushing on it.

The same thing happens if you stretch a rubber band in front of your lips and blow on it. Increasing the stretch will increase the frequency.

Musical wind instruments work the same way.

P.S. the elevators did not flutter in flight test or in service. Phew!

[nick3443]

Flutter is a resonance. That of a combined aeroelastic system not simply mechanical inertial-elastic.

[keybored]

When you’re about to quote a sentence with a footnote, you best read the footnote first.