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posted by Fnord666 on Thursday May 25 2017, @09:34PM   Printer-friendly
from the tacoma-bridge-is-falling-down,-falling-down dept.

One of the most spectacular and famous bridge collapses of all was that of the the Tacoma Narrows Bridge on November 7, 1940. On that day, high, sustained winds sent the bridge into a twisting, rocking motion that led to its eventual collapse. It has been used as a classic example of the phenomenon of mechanical resonance, however, this is incorrect. Ethan Siegel has an article (behind an ad-blocker blocker) explaining how a much more intricate phenomenon known as aeroelastic flutter was responsible.

The collapse of the Tacoma Narrows Bridge on the morning of November 7, 1940, is the most iconic example of a spectacular bridge failure in modern times. As the third largest suspension bridge in the world, behind only the George Washington and Golden Gate bridges, it connected Tacoma to the entire Kitsap Peninsula in Puget Sound, and opened to the public on July 1st, 1940. 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.

[...] But as the wind passed over the bridge on November 7th, a stronger, more sustained wind than it had ever experienced before, causing vortices to form as the steady wind passed over the bridge. In small doses, this wouldn't pose much of a problem, [...] Over time, they cause a aerodynamic phenomenon known as "flutter," where the extremities in the direction of the wind get an extra rocking motion to them. This causes the outer portions to move perpendicular to the wind direction, but out-of-phase from the overall up-and-down motion of the bridge. This phenomenon of flutter has been known to be disastrous for aircraft, but it was never seen in a bridge before. At least, not to this extent.

When the flutter effect began, one of the steel suspension cables supporting the bridge snapped, removing the last major obstacle to this fluttering motion. That was when the additional undulations, where the two sides of the bridge rocked back-and-forth in harmony with one another, began in earnest. With the sustained, strong winds, the continued vortices, and no ability to dissipate those forces, the bridge's rocking continued unabated, and even intensified. The last humans on the bridge, the photographers, fled the scene.

But it wasn't resonance that brought the bridge down, but rather the self-induced rocking! Without an ability to dissipate its energy, it just kept twisting back-and-forth, and as the twisting continued, it continued to take damage, just as twisting a solid object back-and-forth will weaken it, eventually leading to it breaking. It didn't take any fancy resonance to bring the bridge down, just a lack of foresight of all the effects that would be at play, cheap construction techniques, and a failure to calculate all the relevant forces.


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  • (Score: 1) by khallow on Friday May 26 2017, @03:34AM (8 children)

    by khallow (3766) Subscriber Badge on Friday May 26 2017, @03:34AM (#515814) Journal

    Something seems wrong with the people who support dark matter.

    Don't judge a scientific opinion on the basis of a few bad apples. Dark matter, MOND, and similar theories are attempts to explain an ongoing discrepancy between observed mass and observed velocities. And we do already know that there is some degree of dark matter out there (neutrinos, massive compact halo objects, etc), we're just not sure if it's collectively enough to explain the anomalies we see. So dark matter theories do have a valid reason for existing.

    MOND has a similar claim to validity and it is ridiculous to see someone go crusader over this in such a pretentious manner. Currently, all these theories have the problem that they are ad hoc. They were developed to explain existing phenomena, and there is a great deal of latitude in the formulation of the theory that makes falsifying these theories collectively difficult. My take is MOND is operating at a significant disadvantage due to the need to introduce unknown physics and not working with general relativity very well (for example, a number of the MOND approximations use a tenuous Newtonian mechanics approach which is stretched thin at the level of galaxies).

    My view is that Ethan's behavior in this matter was unprofessional and anti-scientific. He wasn't helping to develop a better understanding of our universe by sabotaging rival viewpoints' ability to make their case. Underhanded tactics doesn't generate respect in me.

  • (Score: 0) by Anonymous Coward on Friday May 26 2017, @03:38AM (7 children)

    by Anonymous Coward on Friday May 26 2017, @03:38AM (#515815)

    a few bad apples

    Sure, but what about the second part of the post?

    • (Score: 2, Insightful) by khallow on Friday May 26 2017, @02:26PM (6 children)

      by khallow (3766) Subscriber Badge on Friday May 26 2017, @02:26PM (#515966) Journal
      Not seeing the problem with "conventionalist stratagems" (as discussed in the paper linked in your second part) at present. Dark matter and dark energy are modest alterations to current theory to attempt to explain things for which our earlier naive attempts failed. If the approach requires building a more and more complex series of alterations, then it will mean that the approach, like say the epicycle approach centuries ago, is probably hiding some substantial physics.

      MOND has the same problem. It's too easy to adjust to fit at present (which is what conventionalist stratagems are all about, modest altering of an existing model to fit observation) and we have no good explanation for why gravity should behave that way (alternately, why space-time should be shaped that way, if you're looking at it in a general relativity way).

      At this point, I think epicycle building is inevitable no matter which models we use. So rather than pick the one true model, which we already know will have problems, it strikes me as far better to nurse along many different approaches, including MOND, in order to have the greatest variety of ways of viewing the physics at hand. Even a model with problems and inaccuracies can describe a system well enough to illustrate particular aspects or characteristics of the system.
      • (Score: 0) by Anonymous Coward on Friday May 26 2017, @07:01PM (5 children)

        by Anonymous Coward on Friday May 26 2017, @07:01PM (#516085)

        You are talking about something completely different. My question is: What kind of scientists wouldn't be interested in that mass-discrepancy vs acceleration relationship in the figure to the point where it is not shown to students? That is a key piece of info about the dark matter issue, so ignoring it means there is something really wrong here.

        • (Score: 1) by khallow on Friday May 26 2017, @08:51PM (4 children)

          by khallow (3766) Subscriber Badge on Friday May 26 2017, @08:51PM (#516116) Journal

          My question is: What kind of scientists wouldn't be interested in that mass-discrepancy vs acceleration relationship in the figure to the point where it is not shown to students?

          Why do you think that is an issue? Googling around the "mass-discrepancy vs. acceleration relationship seems to come from a 2004 paper [arxiv.org]. It would not be surprising for cutting edge research to take time to appear in a book. Second, why is this particular observation so important to MOND? It appears, for example, there are several research papers claiming compatibility with dark matter models as well. If those papers are accurate, then this observation is at present more of a curiosity with little resolving power between the rival cosmological models.

          • (Score: 0) by Anonymous Coward on Friday May 26 2017, @11:11PM (3 children)

            by Anonymous Coward on Friday May 26 2017, @11:11PM (#516182)

            It isn't that it is important to MOND, MOND is irrelevant to it's importance.

            It is an apparently universal property of whatever is causing the "dark matter" problem. Universal properties of phenomena are important and need to be taught to people who go to grad school for that topic. It is really simple... The fact this info is missing says something is very wrong.

            PS Your responses are also bizarre to me. You seem literally incapable of grasping what I am saying when it is not complex at all.

            • (Score: 1) by khallow on Saturday May 27 2017, @03:33AM (2 children)

              by khallow (3766) Subscriber Badge on Saturday May 27 2017, @03:33AM (#516280) Journal

              PS Your responses are also bizarre to me. You seem literally incapable of grasping what I am saying when it is not complex at all.

              All I can say from my exposure to grad school in a related field (mathematical physics) is that it takes time for cutting edge research to find its way into textbooks. The most recent work in a textbook tends to be what the author was directly working on. And it is common for such textbooks to give a spotty treatment to subjects the author isn't interested or quite as knowledgeable in.

              So it is no surprise that work that is around 15 years old (with much of the research done in the last few years) isn't in textbooks, though maybe it should be getting some consideration by now. I guess my point is that while there could be some sort of weird social filtering, like ostracization or mental blocks (though I guess some sort of mental blockage as described in my previous paragraph are par for the course), it could also be a matter of the lag common to textbooks.

              I just don't see the point of interpreting the worst of such authors just because they've missed some research. Keep in mind that most of what a graduate student learns, outside of their own research, in their field comes straight from papers and the authors who wrote those papers. Not being in a textbook is not that significant.

              And why should my responses seem bizarre to you? What you are speaking of is complex, even the part about research being missing from textbooks.

              • (Score: 0) by Anonymous Coward on Sunday May 28 2017, @07:19AM (1 child)

                by Anonymous Coward on Sunday May 28 2017, @07:19AM (#516668)

                And it is common for such textbooks to give a spotty treatment to subjects the author isn't interested or quite as knowledgeable in.

                How in the world can you be interested in cosmology and not be interested in the "dark matter" problem? And if you are interested in the "dark matter" problem, how can you not be interested in that relationship? It makes no sense.

                • (Score: 1) by khallow on Sunday May 28 2017, @11:24AM

                  by khallow (3766) Subscriber Badge on Sunday May 28 2017, @11:24AM (#516711) Journal

                  How in the world can you be interested in cosmology and not be interested in the "dark matter" problem?

                  Because you can't be equally interested in every area of a huge subject.