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.
Perhaps amend the definition of resonance (Score:5, Interesting)
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.
Re:Perhaps amend the definition of resonance (Score:5, Insightful)
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."
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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 w
Not resonance, anti-damping (Score:5, Informative)
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.
Re:Perhaps amend the definition of resonance (Score:5, Insightful)
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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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.
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I'm actually fascinated to find how many people don't actually know what a resonant frequency is.
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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?
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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.
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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.
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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,
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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
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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
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Like a wind musical instrument or an organ pipe. Or like a wind which blows across a bridge.
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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).
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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).
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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.
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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.
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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
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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
Re:Perhaps amend the definition of resonance (Score:5, Informative)
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.
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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.
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> The bridge was torquing at it's center span's natural frequency x 2 (IIRC).
Link?
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Link?
Zelda?!
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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.
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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.
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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"
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What we had with Galloping Gertie was a positive feedback loop. It would have happened at any swing frequency.
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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.
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.
Re:Perhaps amend the definition of resonance (Score:4, Informative)
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.
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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
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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
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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
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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,
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Yes, you're wrong. Pushing someone on a swing *is* how resonance works; you're adding energy to the system at the right time, i.e. resonant with the natural pendulum oscillation.
The analogy doesn't work for the bridge because the wind kept a steady speed; it's as if you're continuously pushing the swing, which, obviously, does not create an oscillation.
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The analogy doesn't work for the bridge because the wind kept a steady speed; it's as if you're continuously pushing the swing, which, obviously, does not create an oscillation.
Even a wind at a steady speed can cause a resonance condition. Perhaps a better analogy would be drawing a bow across a violin string, or blowing across the mouthpiece of a flute. Both are examples of resonance even those the energy input contains a multitude of frequencies.
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The analogy doesn't work for the bridge because the wind kept a steady speed; it's as if you're continuously pushing the swing, which, obviously, does not create an oscillation.
The wind kept a steady speed, but the force of the wind felt by the bridge did indeed oscillate as the bridge twisted (the surface area exposed to the wind changed with time). Just like you were standing behind the swing the entire time, but the force felt by the swing changed with time.
The moving wind was the source of the energy imparted on the oscillating bridge system, but the coupling of the wind to the bridge was not constant. Attributing this entirely to anti-dampening would imply that the force felt
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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
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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
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It was twisting in the at n=2 on the center span. Did you watch the video?
Most Misunderstood Failure? (Score:2)
I dunno, I bet there are a LOT more people that didn't understand all of the failings of Windows Vista.
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The Lacey V. Murrow sinking wasn't because of pressure washing without understanding displacement, they knew exactly how much they could flood the pontoons with contaminated water from hydraulic demolition of the bridge sidewalks during an expansion project. They failed to plan for a storm that dumped rain and lake water into the pontoons because they didn't close the watertight doors between demolition shifts.
It was an engineering failure, but not due to lack of understanding about displacement, just an a
Not the most misunderstood (Score:2)
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...
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Resonance vs flutter (Score:2)
refutes "woodpecker" slander, though (Score:2)
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
Re: refutes "woodpecker" slander, though (Score:2)
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)
WHo did it? (Score:2)
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So.... Who shot John F. Kennedy?
Why, John W. Booth did, in the library, with the wrench....
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It was the butterfly flapping it's wings.
Finally (Score:2)
Basically (Score:2)
If I may extrapolate what I know about open-reel tape decks, this sounds like flutter. But in the other axis.
Rediscovered? (Score:3)
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.
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+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.
Galloping Gertie was an inside job (Score:2)
Everyone knows Nikola Tesla's Oscillator can't resonate steel beams.
I recall (Score:2)
Now it's a big concrete structure. Nothing is moving that.
Terrible summary (Score:2)
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.
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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. (Score:2)
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.
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Rather obvious. Whether it is a bridge or a tuning-fork is pretty immaterial, it is a filter.
As to you climate-change denier morons: The only good thing about you people is that future generations will remember you by your extreme stupidity.
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As to you climate-change denier morons: The only good thing about you people is that future generations will remember you by your extreme stupidity.
I didn't catch that before. Of course, the modeling of climate science is actually mathematically very advanced; the system is never in steady state and even resonant type behavior like the El Nino-Southern Oscillation is not very linear. What the "deniers" don't seem to get is that the science was actually settled almost 30 years ago. The rest of the world has moved on, but we are saddled with a stupidity resonance being excited by a constant stream of money from petroleum interests.
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Neat fact: the non-linear aerodynamics will always tend to excite a structural mode around its natural frequency.
The fact that it's technically not steady state behavior and thus isn't rightly referred to in the language of linear steady-state analysis ("resonance") just goes to show you how it's the transient phenomena that real engineering is about. Any idiot can come to any comforting conclusion about steady state and average trends, but if you don't rigorously account for the nonlinearities and the transients, you don't have shit. I'm looking at you climate "scientists".
All physical systems (e.g., mechanical structures) are in fact non-linear. The simple fact is, though, that it is useful to analyze the response of a wide variety of systems to outside forcing in terms of the excitation of normal modes, either as an entirely linear system, or one with weak non-linearities (say, a limited amount of mode-mode cross-talk). If modes are excited, it is still appropriate to use the mathematics of resonant excitation, regardless of whether it is steady-state or not. This can be a
Has no one heard of Fourier transforms? (Score:3)
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.
News to me (Score:2)
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 (Score:2)
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.
learned this on history channel (Score:2)
15 fucking years ago, how much did this twin university "study" cost the taxpayers?
Common engineering knowledge (Score:2)
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.
In my antedeluvian physics courses ... (Score:2)
Aeroelastic flutter (Score:5, Informative)
Hey Texas dumb-shits, "wind-driven amplification of the torsional oscillation..." Sure as hell sounds like resonance to me. Unless they have some other definition.
The proper term for it is aeroelastic flutter [wikipedia.org]. It's a well understood phenomena most famous in jet airplanes but it occurs other places too including apparently this bridge.
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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."
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Not all oscillation is resonance. This was a positive feedback loop that would have happened regardless of the bridge's resonant frequency. Resonance is the result of the incoming energy having a frequency similar to the natural vibration frequency of the object. This would have happened at high constant windspeed, or as it did from non-periodic (but frequent) gusts.
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Can a bridge vibrate unstably at any frequency other than its resonance frequency?
There's no doubt here there was positive feedback, but I would argue the incoming energy did have a similar frequency to the natural frequency on the object as the force on the bridge depended on the current position of the bridge. I.e. just because the wind was constant doesn't mean the force on the bridge was.
Re:Aeroelastic flutter (Score:4, Informative)
Wow. The fact that this AC has been modded up to "+5 Insightful" makes me truly worry about the "nerd" factor at Slashdot these days. Not only do mods believe an AC spouting nonsense, but they aren't even capable of checking that nonsense or knowing enough basic physics to contradict it.
Let's clear this up. It's really quite simple.
Check your own wikipedia references.
Aeroelastic flutter is a type of "Simple harmonic motion"
. "Simple harmonic motion" is a type of "resonance"
You're skipping over a few steps here. Simple harmonic motion is NOT a "type of resonance." Let's explore further. As you say:
Simple harmonic motion
"...The motion is sinusoidal in time and demonstrates a single resonant frequency."
There's a difference between a "resonant frequency" and "resonance." SHM occurs at a specific frequency which is a natural mode of vibration of the system. That specific frequency could be USED to create resonance, but SHM isn't resonance itself.
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."
Exactly. Notice the external force part. The external force needs to DRIVE the motion at a specific frequency. THAT is resonance.
Or, let me try to put it in even simpler terms an AC might be able to understand:
Resonance: A system has a natural vibrational frequency of X. An external force also has a periodicity of X. Even a small external force with the same periodicity could drive the system to vibrate significantly. Example: place a tuning fork with pitch "middle C" on a piano string tuned to "middle C." The vibrations of one can drive the other to vibrate, because they both tend to vibrate at THE SAME frequency (both internal frequency and frequency of driving force).
Aeroelastic flutter: A system still has a natural vibrational frequency of X. But the external force is simply LARGE and roughly CONSTANT. The external force does NOT necessarily have a periodicity, and if it does, it isn't equal to X. So why does the "flutter" occur? Basically, there's too much energy flowing into the system and it can't dissipate it naturally. Random perturbations get it moving. Due to the natural characteristics of the system, it will tend to preferentially vibrate at one of its natural frequencies. Think of a flag fluttering in the wind -- you can see certain wavelike patterns happening even if the wind is relatively constant. The bridge is a little more complicated: it's more like a flag that's tethered on both sides. Again, in a strong wind you might see flutter "waves" happening -- that's not due to the periodicity of the wind, but to the natural reinforcement of standing waves in the flag itself when there's too much energy being pushed into it that it can't dissipate.
TL;DR -- Resonance requires a driving force with the same frequency as the system that's vibrating. Even a tiny external force could be potentially catastrophic if it reinforces the natural frequency of the system. Flutter just requires a large external force pushing energy into the system. The remedy in the case of the bridge is completely different -- for resonance, you'd have to worry about a particular windspeed for a particular length of bridge or something like that. Even a gentle wind might be able to set off nasty vibrations when resonance (matching frequencies) occurs. For flutter, you just need more damping material in general.
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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."
Exactly. Notice the external force part. The external force needs to DRIVE the motion at a specific frequency. THAT is resonance.
The original sentence might be slightly ambiguous but it doesn't explicitly state that the external force needs to drive the motion at the specific frequency. It just says there needs to be an external for
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The original sentence might be slightly ambiguous but it doesn't explicitly state that the external force needs to drive the motion at the specific frequency. It just says there needs to be an external force, and that it drives another system to oscillate at a specific frequency.
The sentence comes from the Wikipedia article on resonance. Read it in context. It's clear that it's referencing an oscillating driving force. The force's period doesn't have to line up exactly, but the closer it is, the more resonance. That's the point of what a "resonant frequency" is -- it's the frequency you can drive a system with an external oscillating force to produce maximum amplitude response.
This is really fundamental to the definition of resonance. If the driving force has a period very d
Wikipedia not right (Score:2)
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I never realised I needed to blow on and off 256 times a second to get a middle C. Maybe that's why I can't play the flute.
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Here's an older paper:
http://www.ketchum.org/billah/... [ketchum.org]
The distinction is drawn at the end of part III. Seems to me to be pure semantics. If the bridge were driven to flutter at a self-resonant frequency then yes, it was a resonant phenomenon. Does causing a wine glass to emit a tone by running your finger around the rim constitute a resonant phenomenon? The variation of the driving force being at the resonant frequency is caused by the wine glass vibrating at that frequency already, as was the variati
Can have flutter when system cannot resonate (Score:2)
The distinction is drawn at the end of part III. Seems to me to be pure semantics. If the bridge were driven to flutter at a self-resonant frequency then yes, it was a resonant phenomenon.
Resonance is a steady state phenomenon so if the amplitude is exponentially growing this is evidence of anti-damping (which is what aero-elastic flutter is) and not resonance. Both mathematically and observably the two phenomenon are different. Furthermore you can have aero-elastic flutter in a system which has enough damping that it cannot resonate (which is the case when the damping ratio is >1/sqrt(2) ).
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Resonance requires a driving force with the same frequency as the system that's vibrating
Close. If some part of the driving force is at that same frequency, you still get resonance. Feed enough noise into a hi-Q bandpass filter and you get resonant tone at the centre frequency. Apply positive feedback and the tone gets louder. With loop gain greater than unity it grows until limited either by the available power (saturation and clipping) or by physical damage that changes the system (either shifting its centre frequency, e.g. due to a broken window in the room changing its echo delay, or else
Definitions wrong (Score:3)
Simple harmonic motion is just the sinusoidal motion in time. To exhibit resonance you actually need a DAMPED harmonic oscillator (otherwise you have infinite amplitude at resonance) and not all damped harmonic oscill
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Which is why there you so seldomly run across Professor Anonymous Coward in any physics department.
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Agreed: clearly this is resonance. The bridge was observed oscillating at several different frequencies since it construction. However, it was only when the wind speed reached 42 miles an hour did the bridge oscillate at 1 cycle per 5 seconds. This particular frequency caused the bridge oscillation to gain energy, and the bridge tore itself apart.
"resonance is a phenomenon that occurs when a vibrating system or external force drives another system to oscillate with greater amplitude at a specific prefe
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I live near the bridge, and have driven across it a few times.
RIIIIGHT...
Let me guess, you want to sell me a bridge too...
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Just the salvage rights.
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I live near the bridge, and have driven across it a few times.
RIIIIGHT...
Let me guess, you want to sell me a bridge too...
I walked across the new one at Thanksgiving. My son lives less than a mile from it (there are now two). They rebuilt Gertie, using the original foundations, and there's an air vent down the middle. The new bridge is much wider and has a sidewalk/bike path.
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It is.
But the forcing function was a product of the motion and didn't have it's own freqency. Which makes it flutter.
Now...wait for it...flutter is an example of resonance.
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What I learned, back in the day, was that resonance was a coupling between the period of the input energy and the natural period of the system. If the input energy is steady or aperiodic, oscillation in the system doesn't automatically make it resonance. Is this not the way most engineers use the term?
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So, hearing a musical note when you pluck a guitar string doesnt involve resonance, because a single pluck doesn't match any natural period of the string?
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Indeed, but the sound you hear from the other strings, the reason your fingering on the other strings matters, that's resonance, as is the natural amplification of lower notes from the sound box in an acoustic guitar. The string itself: that's simple harmonic oscillation.
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a single pluck doesn't match any natural period of the string?
What is the Fourier Transform of a step function?
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Mod this parent up. A nice summary.