The Tacoma Bridge Collapse was Not due to Resonant Frequency

[jedishrfu]

An interesting article on the Tacoma Narrows Bridge collapse of 1940 shown to students of physics throughout the years as an example of the power of resonant frequencies.

However, the explanation was much too simple to be true.

Here's the real story:

http://motherboard.vice.com/read/the-myth-of-galloping-gertie

 

[BobJ]

They cheaped out. Golden gate bridge is a of the same design. On Tacoma, they cut costs and the designers were not the same people who invented that style of bridge. The inventor was well aware of the resonance issues, but this knowledge was lost when others went to make a knock off.

 

[A.T.]

An interesting article on the Tacoma Narrows Bridge collapse of 1940 shown to students of physics throughout the years as an example of the power of resonant frequencies.

However, the explanation was much too simple to be true.

Seems more like arguing about the definition "resonance". Also, this sounds weird:

"Now, having been pushed into flutter by its new ability to twist, Gertie was no longer significantly affected by the aerodynamics, but largely under the influence of her own forces, and locked in a downward spiral. Twisting induced more twisting, then greater and greater twisting, and so on, in a runaway, exponential fashion, until eventually the bridge could no longer dissipate its energy fast enough."

So if the aerodynamic input was suddenly gone (wind stopped), the bridge would collapse anyway, through the "influence of her own forces" and "twisting inducing greater twisting"?

 

[BobJ]

I don't like how the article is written either... trying to make it super dramatic. I think the collapse speaks for itself. A great event to study in many regards. Like: "what happens when engineers are told to make the design cheaper?" ;)

 

[russ_watters]

Yeah, I think I was taught this properly in [engineering] school, and even if the type of failure hadn't been seen/understood prior to the incident, it would have to be immediately apparent what was happening: that the wind and twist were combining to provide a positive and sloped forcing function. It is more complex than the simplest case of resonance, but still pretty obvious. Also, though the video of the plane with fluttering wings looks like a 747, that issue was known at least as far back as the early 1950s and factored into the design of the 707.

This seems a overhyped, playing on popular literary devices such as lost or delayed knowledge myths. And I'm not sure I believe the "it's taught wrong in schools!" bit either, but that is a common one too.

 

[zoobyshoe]

Seems more like arguing about the definition "resonance".

Yeah, it seems to be mostly about separating the concept of "resonance" from the concept of "flutter."

Also, this sounds weird:

"Now, having been pushed into flutter by its new ability to twist, Gertie was no longer significantly affected by the aerodynamics, but largely under the influence of her own forces, and locked in a downward spiral. Twisting induced more twisting, then greater and greater twisting, and so on, in a runaway, exponential fashion, until eventually the bridge could no longer dissipate its energy fast enough."

So if the aerodynamic input was suddenly gone (wind stopped), the bridge would collapse anyway, through the "influence of her own forces" and "twisting inducing greater twisting"?

Yeah, that seems disoriented. Earlier he said, "Each time the bridge twisted, that is, it twisted a little bit more, not less, back in the other direction, in a steady buildup of twisting energy that was reinforced by the wind." Which makes more sense.

I think the point of the article is interesting but the author rambles around, not sure how to organize his case. It's probably 4 times longer than it needs to be, padded with a lot of near-repetitions or re-explanations. Makes it hard to read.

 

[A.T.]

Yeah, that seems disoriented. Earlier he said, "Each time the bridge twisted, that is, it twisted a little bit more, not less, back in the other direction, in a steady buildup of twisting energy that was reinforced by the wind." Which makes more sense.

Yeah, but then at another place he writes:

"...aerodynamic forces are not a driving factor for bridge flutter"

 

[zoobyshoe]

Yeah, but then at another place he writes:

"...aerodynamic forces are not a driving factor for bridge flutter"

By which he means, I think, flutter arises from the properties of the thing that flutters, and not from the properties of the forces that cause the flutter. Not that he couldn't have been much more clear. "Driving factor" is not a good choice of terms in this context. Too easy to think he means, "driving forces."

 

[DrClaude]

There recently was a long discussion on the subject, based on the same article:
https://www.physicsforums.com/threads/expunging-myths-from-the-classrooom.849853/

 

[HallsofIvy]

Yeah, I think I was taught this properly in [engineering] school, and even if the type of failure hadn't been seen/understood prior to the incident, it would have to be immediately apparent what was happening: that the wind and twist were combining to provide a positive and sloped forcing function. It is more complex than the simplest case of resonance, but still pretty obvious. Also, though the video of the plane with fluttering wings looks like a 747, that issue was known at least as far back as the early 1950s and factored into the design of the 707.

This seems a overhyped, playing on popular literary devices such as lost or delayed knowledge myths. And I'm not sure I believe the "it's taught wrong in schools!" bit either, but that is a common one too.

Every differential equations text book that I have seen in the last 30 years, if they mentioned the Tacoma Narrows bridge did so to say was NOT an example of resonance.