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Science

Galloping Gertie, Engineering's Most Misunderstood Failure (vice.com) 168

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.
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Galloping Gertie, Engineering's Most Misunderstood Failure

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  • 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.

    • by PsychoSlashDot ( 207849 ) on Tuesday December 15, 2015 @07:44PM (#51126063)

      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."

      • Re: (Score:2, Insightful)

        by Anonymous Coward

        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 w

      • by Roger W Moore ( 538166 ) on Tuesday December 15, 2015 @08:26PM (#51126307) Journal
        Actually it really is not like resonance but more like and anti-damping force. Resonance is when a periodic force is applied to the system and, when the frequency of that force matches the natural vibration frequency of the system, the steady-state response gives a large amplitude response. The key difference is that with resonance the system is in a steady state with a constant amplitude. With "anti-damping" (called aero-elastic flutter in this case) the amplitude of the system increases with each oscillation since you effectively have a negative damping ratio.

        Hence there is a clear difference in the motion between resonance and anti-damping which you can determine by studying the motion which the paper seems to have done. It is NOT just a fancy name for a resonance effect: the behaviour is transitory and not steady-state. However this has been known for over a decade now and I'd be surprised if it were still being taught as resonance in introductory physics courses. Certainly for the one I teach I describe it in terms of damping and point out the fallacy of the resonance explanation.
      • by phantomfive ( 622387 ) on Tuesday December 15, 2015 @08:41PM (#51126403) Journal
        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.
        • by msauve ( 701917 )
          Resonance of the bridge, not the wind.

          The authors seem the type who try to get out of a speeding fine by arguing the difference between speed and instantaneous velocity. GLWT. For the general public, "resonance" is a perfectly reasonable explanation.
          • The resonance of the bridge wasn't involved at all. For resonance, the wind would have to match the resonant frequency of the bridge.

            I'm actually fascinated to find how many people don't actually know what a resonant frequency is.
            • The resonance of the bridge wasn't involved at all. For resonance, the wind would have to match the resonant frequency of the bridge.

              From the bridge's perspective, this was most certainly resonance! The air pressure field at the bridge's surface exerted force on the structure with a fundamental frequency very close to the bridge's resonant frequency.

              Of course, flutter is a more complete explanation which considers the system of wind+bridge, rather than just the forces acting on the bridge. But isn't that obvious?

              • From the bridge's perspective, this was most certainly resonance! The air pressure field at the bridge's surface exerted force on the structure with a fundamental frequency very close to the bridge's resonant frequency.

                No, because if the wind had blown faster, the bridge would have fallen faster. It had nothing to do with the frequency.

            • The resonance of the bridge wasn't involved at all. For resonance, the wind would have to match the resonant frequency of the bridge.

              No, the argument is that the bridge was self-resonant [microwaves101.com] . Self-resonance isn't just for electronics; when you strike a bell or tuning fork and it resonates at a specific frequency, you're experiencing self-resonance.

          • Resonance of the bridge, not the wind.

            You do realize that resonance is a phenomenon caused by two different things vibrating at similar frequencies, right? If you pluck a guitar string (introduce energy) and the string produces a standing wave at a certain frequency, it string is NOT participating in resonance, as is defined in physics. It's just vibrating at a natural frequency. Similarly, if the wind adds a bunch of energy to the bridge in a non-periodic fashion, and the bridge oscillates in a standing wave (natural frequency of vibration,

            • As electronic engineers we talk about resonant circuits. They have a natural frequency, and when excited correctly, they resonate. So yes, I guess, strictly, you're right, but evferyday usage trumps here, I think.
            • You do realize that resonance is a phenomenon caused by two different things vibrating at similar frequencies, right?

              Is a tuning fork one thing, or two things because it has two tines? The two tines make the one fork, but without two tines a tuning fork doesn't work. Is a bridge one thing, or two things because it has two ends? The two ends of the bridge make one bridge, but the two ends of the bridge can resonate against one another. Self-resonance is a thing, and it's not just a thing in electronic circuits. The ongoing input of wind energy may have fed the reaction, but another bridge with the same wind wouldn't have b

            • by msauve ( 701917 )
              You do realize that "resonance" doesn't mean what you think it means, right? There are lots of physics sites which disagree with you - "a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object... [gsu.edu]"
              • You do realize that "resonance" doesn't mean what you think it means, right? There are lots of physics sites which disagree with you - "a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object..."

                Obviously you don't realize that this quotation doesn't mean what you think it means.

                "Resonance" is NOT the same as a "resonant frequency." The "resonant frequency" is the natural frequency where a system can experience resonance, but the frequency or the vibration at that frequency is NOT resonance itself.

                From the same site you linked, have a look at this graphic [gsu.edu], which basically says:

                "Resonance involves the existence of natural frequencies which are easy to excite and which a vibrating system picks out from a complex excitation."

                Resonance is NOT simply vibrations occurring at a resonant frequency within a system. That's just basic oscillation o

                • by msauve ( 701917 )
                  "which a vibrating system picks out from a complex excitation"

                  Like a wind musical instrument or an organ pipe. Or like a wind which blows across a bridge.
        • by gdshaw ( 1015745 )

          There's a difference:

          Resonance: a force at a particular frequency that causes increased motion.
          This: a powerful force caused increased motion.

          By that definition you would have to say that Helmholz resonance is not a form of resonance.

          As others have said, it all depends on what you define as the forcing function. The wind was applying a periodic force to the bridge due to the varying profile that it presented to the wind. It would not have been periodic in the absence of the bridge, but that's irrelevant because then there would have been no force at all (no area to act on).

          • The wind was applying a periodic force to the bridge due to the varying profile that it presented to the wind

            The periodic force of the wind was not relevant to the reason the bridge fell (although there was a periodic effect that came from vortex shedding, it didn't cause the bridge to fall).

        • 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.

          Couldn't it be argued that the force of the wind imparted on the bridge happened at a specific frequency since it only happened when the bridge's movement was in a certain position, and that this force is determined by the natural frequency of the bridge as the swinging was dependent on the innertial characteristics?

          It still sounds like a form of resonance to me. The originating force may not be, but the effect on the system does.

          • Couldn't it be argued that the force of the wind imparted on the bridge happened at a specific frequency since it only happened when the bridge's movement was in a certain position, and that this force is determined by the natural frequency of the bridge as the swinging was dependent on the innertial characteristics?

            If that happened, it would have been resonance (so good job, you're the first person who's replied to me who actually seems to understand resonance). But it had nothing to do with frequency of the force, and was instead just a really strong wind.

        • by pz ( 113803 )

          When you actuate a bow across violin strings, the strings oscillate at a given frequency. When you bow faster (the wind blows harder in the case of a bridge) the amplitude gets larger. The wind speed is not the frequency of energy input.

          Would you say that the violin string is not in resonance? I think more-or-less anyone would, except some particularly pedantic folk. So why would you say that the bridge was not in resonance? Unlike a violin string which is designed to sustain resonant activity, the bri

          • It was oscillating, and you can't get oscillations without resonant behavior.

            No, that's not true at all lolol. Objects have natural resonance. You can often see this in a bathroom stall, singing notes up the scale, and one note will be amplified much more than the others. You can often hear it playing an organ, as you play notes down the scale, different items in the room will shake loudly in resonance.

            In the case of the bridge, it would be each "gust" of wind hit at just the moment when the bridge was ready to swing in the direction of the bridge (except when I say "gust," I mea

      • by Beck_Neard ( 3612467 ) on Tuesday December 15, 2015 @08:51PM (#51126449)

        Nope, this isn't resonance, it's aeroelastic flutter: https://en.wikipedia.org/wiki/... [wikipedia.org]

        The important distinction is that resonance requires some oscillating energy input whereas flutter doesn't. Resonance doesn't directly depend on wind speed whereas flutter does.

        To be fair, the article does a surprisingly bad job of explaining it, hence the confusion.

        • Aeroelastic flutter is only a problem when it happens at the resonant frequency of the underlying physical object.

          Like when the forces are a consequence of the original motion.

          The bridge was torquing at it's center span's natural frequency x 2 (IIRC).

          Saying this wasn't resonance because the forcing function was a product of the oscillation in the first place is pedantic. Yes it was flutter, but destructive flutter is a resonant phenomenon.

          • > The bridge was torquing at it's center span's natural frequency x 2 (IIRC).

            Link?

          • Saying this wasn't resonance because the forcing function was a product of the oscillation in the first place is pedantic.

            Actually it is not pedantic the two types of motion are quite dissimilar: aero-elastic flutter generates an exponentially growing amplitude whereas resonance generates a large, but constant, amplitude. Aero-elastic flutter is essentially the inverse of damping whereas resonance requires an external, oscillating force which drives the system.

            • aero-elastic flutter generates an exponentially growing amplitude whereas resonance generates a large, but constant, amplitude.

              The bridge disintegrated while attempting to reach a constant amplitude.

      • To me it appears even more simple. Looks like they have found the exact way in which resonance happens for this bridge structure.
      • by endoboy ( 560088 )

        Perhaps we should require an explanation using the "ten hundred" most common words......

        Consider this an enthusiastic plug for Randall Munroe's (of XKCD fame) most recent book. "Thing Explainer-- Complicated stuff in simple words"

      • by Sique ( 173459 )
        But one of the most important properties of resonance (that makes it different from similar phenomena) is the fact that it happens only in a small frequency band. Resonance happens if the cause has the same frequency than the effect, and both interfere, and the interference pattern increases the amplitude.

        What we had with Galloping Gertie was a positive feedback loop. It would have happened at any swing frequency.

    • 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.

      It does sound a lot like how most people would think of resonance. Although actually a bit more like a really bad relationship. Forces kept trying to make it work, but each time got a teensy bit worse and had even more baggage, until suddenly it turned into a youtube video.

    • by Matheus ( 586080 ) on Tuesday December 15, 2015 @07:57PM (#51126143) Homepage

      Perhaps... but the difference is as follows:

      1) Resonance: This is a natural tendency of a physical object to self-increase its oscillation when caused to oscillate at the objects natural resonant frequency.

      2) (What Really Happened): This would be described as a reinforced feedback loop. In this particular case the reinforcement was coming from gravity acting on the bridge in one direction while wind was acting on the bridge in the opposite direction.

      The key difference here is that the amplification of oscillation leading to bridge failure was caused by **external forces not any natural resonance of the structure.

      In terms of knowing why the bridge failed and how to not have a future one fail in the same manner, the difference between those two is quite important.

      • In terms of knowing why the bridge failed and how to not have a future one fail in the same manner, the difference between those two is quite important.

        I'm going to say: No

        First of all... If you design a bridge that collapses, nobody is going to care when you tell them, "Yeah, but it wasn't destroyed by wind-driven amplification of the torsional oscillation! We're professionals. We had that issue covered!"

        Second... The fix for both is EXACTLY the same. You need to stiffen the structure and dampen any mov

      • Yup, and this is what happened to the Angers Bridge [wikipedia.org] back in 1850 too, or at least something similar. And yet for more than a century French physics teachers used it as a spectacular illustration of resonance, even though it's not that simple.
      • The key difference here is that the amplification of oscillation leading to bridge failure was caused by **external forces not any natural resonance of the structure.

        That's actually not true. According to the article:

        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.

        So, the bridge's own elastic forces worked in tandem with the externally applied forces to increase the amplitude of oscillation. The elastic forces in the bridge give the bridge a natural resonance frequency and the wind acted on the bridge with the same frequency. That's the definition of resonance. It's just like when you push someone on a swing. You apply gentle pushes with the natural frequency of the swing and the resulting amplitude can become q

        • It wasn't resonant with the wind. It was resonant with the aero forces generated by the twisting bridge.

          It all depends on what you define as the 'forcing function'.

          None of this is news. Nobody ever said the wind was gusting at the same frequency as the bridge. It's always been understood what happened.

          I put this whole thing down to reddit morons splitting hairs.

          Complete aside. The state employee that was supposed to buy insurance on the bridge pocketed the premium, until it started to really gallop

          • It wasn't resonant with the wind. It was resonant with the aero forces generated by the twisting bridge.

            What the hell are you talking about? Where are "aero forces" coming from except from the, well, AIR -- I.e., the wind??

            It all depends on what you define as the 'forcing function'.

            I can't seem to figure out what you're talking about except maybe that the bridge reinforced its own standing wave vibrations at its natural frequency. But that isn't resonance, and that's not a "forcing function." That's just an object vibrating at its natural mode of vibration when energy is introduced into the system. If you randomly strum a guitar string in a non-periodic fashion,

    • It's different. A resonant system has a particular frequency at which it 'likes' to oscillate; this frequency will have the lowest rate of energy dissipation, and so even a small amount of energy input at this frequency will tend to get stored and amplified over time. But in this bridge (FTA):

      "When the bridge bounced up and down, as it did for months and earlier in the morning of November 7th, it's thought that the vortex street was causing forced harmonic motion on the bridge. But observations and calculat

    • Resonance comes when a force matches a natural frequence in an object. The bridge normally did resonate and it had a resonant frequency. It normally oscillated along its length though, rather than twisting. The higher winds when it collapsed were stronger but also very gusty. If it was just resonance then it should have resonated at its normal frequency, perhaps with greater amplitude, but still oscillating up and down along the length of the span. The twisting motion was an oscillation but not at a re

  • I dunno, I bet there are a LOT more people that didn't understand all of the failings of Windows Vista.

  • If by "most misunderstood" we mean misunderstood by the most people, then that title would have to belong to the collapse of the twin towers.
    So many people still can't understand how a building could fall straight down, instead of sideways like in the cartoons. Forget resonance or oscillation, how about getting gravity into the public conscious. I guess it is just a theory after all...
  • I suspect we've all heard/read the slander "If houses were built the way software is written, the first woodpecker that come along would destroy civilization.". This example, and the Tay bridge disaster, are demonstrations of how we learned (usually) NOT to build bridges. Software is often as new to this field as those bridges were to Civil Engineering, so there are lessons to be learned.

    The real distinction is that most software projects don't take a decade and cost billions (California's government exam

    • Software fails catastrophically because one light bulb burning out causes the whole house to disintegrate. Production clusters with redundant servers, network switches and power supplies are a better equivalent of a house built to codes. I hope self driving car control systems follow the later model.

  • Not a fallacy (Score:2, Insightful)

    In logic, a fallacy [wikipedia.org] 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.
  • So.... Who shot John F. Kennedy?
  • After 20 years since studying the subject someone explained it to me. It is a kind of resonance since the wind force was applied in phase, but no one ever bothered with the details.
  • If I may extrapolate what I know about open-reel tape decks, this sounds like flutter. But in the other axis.

  • by JazzHarper ( 745403 ) on Tuesday December 15, 2015 @09:01PM (#51126497) Journal

    Isn't this conclusion pretty much identical to the findings sixty years ago? It's no surprise that the explanation was oversimplified to "resonance" by the popular press, but to claim that this is an entirely new result misrepresents what engineers learned from the failure.

    • +1 this. When I did my engineering degree in the 90s this was the reason that was put forward for the collapse. Best part about my physics lecturer is he started by saying that this bridge is used by most of the different schools to argue different reasons. If you do aerospace it will be the wind, if you do civils it will be resonance etc etc. The bridge is used as a cool subject for assignments, the teacher chooses what they want the assignment to be about.

  • Everyone knows Nikola Tesla's Oscillator can't resonate steel beams.

  • The old Jamestown Bridge in Rhode Island. That thing was a terror to drive over. It was all steel grate. Guess they figured you wouldn't get resonance or stresses from that. But driving over it wasn't fun.

    Now it's a big concrete structure. Nothing is moving that.
  • Seems to be a heap of stuff in the summary that isn't in the article "Each time the deck of the bridge twisted now, it sought to return to its original position (inertial forces)" etc- all of which defies physics on Planet Earth.

    As others have pointed out, "Tacoma Narrows wasn't a resonance" has been a bit of a mantra for 20 years or more, obviously things take a while to get to Texas.

    • by gweihir ( 88907 )

      Well, actually it was resonance, just one fed by wind, not by traffic. My guess is this confusion is because structural engineers think "traffic" when hearing "resonance" and "bridge"? Other disciplines do not have that limitation.

  • Of course it was a resonance - the excitation of a normal mode of a physical structure. That the excitation was due to complicated non-linear aerodynamics doesn't change the obvious fact that a normal mode was being excited.

  • by MouseTheLuckyDog ( 2752443 ) on Wednesday December 16, 2015 @12:22AM (#51127439)

    When it comes down to it, the basic argument is that the phenomena is aeroelastic flutter not resonance, because the driving force is nearly constant.

    However, I suspect if you work it all out the change in amplitude of the rotation is proportional to the Fourier transform of the driving force at the natural frequency of the bridge.

    Why is that? The nearly constant driving force is not nearly constant.
    It is a composition of oscillating forces spanning a range of frequencies. Of all those forces, only the force oscillating at the natural frequency contributes--hence it is proportional to the Fourier transform.

  • I learned about this bridge 20 years ago, and it was taught as "oscillations induced by wind". Of course, these need to be at or close to a resonance point, or they just get dampened out.

    Seems to be a non-story.

  • So more like the reed of a saxophone, a powerful and steady blow across it will cause it to resonate...sorry, cause it to flutter at its natural frequency.

    Compared with a tuning fork. If you have a C tuning fork and hold it near a piano when you play any C, the fork will noticeably vibrate from the tiny force of audio vibrations reaching it. It resonates with the weak, but in tune, audio force.

  • 15 fucking years ago, how much did this twin university "study" cost the taxpayers?

  • I thought this was common knowledge amongst engineers. Hell, I'm an electrical that only took one mechanical class and I heard this in college. My wife (she's a structural engineer) and I even recently had a conversation about it with some other friends.\

    More importantly, to a layperson, it's the same thing. Technically not correct to call it "resonance" but they don't care about the differences. Most engineers don't even care.

  • ... they'd have called it forced, (under)damped harmonic motion, no? Given that this is slashdot, it may well be called that in the article I couldn't be troubled to read ;-)

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