Thursday Throughput: Death of a Titan Edition

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Michael Siegel

Michael Siegel is an astronomer living in Pennsylvania. He is on Twitter, blogs at his own site, and has written a novel.

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  1. Avatar Oscar Gordon
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    Did the get the instrumentation out of the beam steering housing?Report

  2. Avatar Oscar Gordon
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    ThTh2 & ThTh7: Two fluids stories! The Narrows bridge is an excellent study in fluid structure interactions. Also, I wonder why people assume buildings shouldn’t flex? Clearly this is someone who hasn’t lived in an area with earthquakes, or every been at the top of a tall building with a motion damping system.Report

    • Avatar DensityDuck in reply to Oscar Gordon
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      People often confuse “strength” and “stiffness”, even though those two terms have very different meanings in structural engineering. If you have a glass rod and a rubber rod, the glass rod is stiffer, in that you apply load and it doesn’t deform; but the rubber rod is stronger, in that you can apply more total load before the rod snaps.

      Now, strength and stiffness are usually coincident; you generally don’t want something to deform significantly when service loads are applied, because “stay in the same shape when typical service loads are applied” is generally part of the requirement set for useful artifacts. And indeed, as we see from the Narrows Bridge, while the bridge is able to handle the structural loads, it isn’t staying the same shape, and this makes it unsuitable for its purpose of being a bridge…Report

      • Avatar Oscar Gordon in reply to DensityDuck
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        I do hope you included that explainer for all the non-engineers on this site, right? And not for my benefit?Report

        • Avatar Kristin Devine in reply to Oscar Gordon
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          whoever it was for, it was helpful to me! I read it to my kids and they understood the principle. 🙂Report

          • Avatar Oscar Gordon in reply to Kristin Devine
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            Oh, good! Now let’s get into a discussion about plastic and elastic deformation, and yield, and fatigue, and how all of this gets way more interesting when we go from homogeneous materials to composites or heterogeneous materials!Report

            • Avatar Kristin Devine in reply to Oscar Gordon
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              yes, please do, using simple terminology, and I will devour every word of itReport

              • Avatar Oscar Gordon in reply to Kristin Devine
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                Oh geez, that’s a post in itself…Report

              • Avatar Oscar Gordon in reply to Oscar Gordon
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                says:

                FYI to DDs mini-post below:

                Metalic alloys are heterogeneous, but there is a new nanostructure material that is also called heterogeneous that is not technically a metallic alloy.

                https://www.tandfonline.com/doi/full/10.1080/21663831.2017.1343208Report

              • Avatar DensityDuck in reply to Kristin Devine
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                says:

                That was…apparently the comment box did NOT like my editing of the comment. Could someone maybe delete that and we’ll try it again?Report

              • Avatar Oscar Gordon in reply to DensityDuck
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                says:

                Now this paragraph is flexible, and this paragraph is stiff, and if you mix them together you get… well… that mess up there.

                Thus materials science and language structure are not analogous.Report

              • Avatar DensityDuck in reply to Kristin Devine
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                says:

                OK let’s try this again.

                Homogenous materials are like a bunch of gummy bears all mashed together. You can pull on the blob and it stretches, and eventually you pull hard enough that some of the gummy bears detach from the blob; that’s material failure.

                How hard you have to pull before the bears detach is a matter of the surface area of each bear. (This metaphor is gonna get a little weird.) The surface of the gummy bear has a particular stickiness, and if you have bigger bears then there’s more surface area, so each bear sticks more strongly to the other bears. (We’ll assume for now that ripping a bear’s head off is a lot harder than pulling it away from the other bears.)

                A heterogenous material — like an alloy — is as though you took a bunch of Jolly Ranchers, licked them, rolled them in gummy bears, and then mashed the resulting lumps together. The gummy bears stick to the licked Jolly Ranchers much more strongly than to each other, so each of the bear-Rancher lumps acts like a big gummy bear; and, as I said earlier, big bears have more surface area and stick more strongly because of it.

                You might also ask “why not just make the whole blob out of Jolly Ranchers”, but the problem there is that Jolly Ranchers are very rigid and brittle; the gummy blob can flex and stretch more, and so if you haven’t made your blob exactly perfect (and if you aren’t pulling exactly straight) the gummies will shift around to line up with the actual load, rather than just sitting there. If you apply the load off-center to a rigid structure, you start bending it in addition to pulling or pushing at it, and bending can concentrate or amplify the applied load. This is part of the strength-versus-stiffness thing; it might take a lot of pulling to separate a gummy from the blob but the blob will stretch waaaaay out before that happens, whereas a Jolly Rancher brick won’t stretch even a little bit but will snap almost right away. But a mix of the two will have both more strength than pure gummies (larger individual blobs) and will be stiffer (the Jolly Ranchers make up part of the mass, and they don’t stretch) without being brittle (the gummies shift around to minimize load concentrations.)

                Another option, if you have a bear of sufficient stiffness (stop that snickering!) is to use some advanced techniques to make a ginormous gummy bear. (Remember earlier when we said that ripping a bear’s head off is harder than separating bears?) When they talk about “single-crystal casting” things, picture someone pouring molten gummy into a mold and then slooooooowly cooling it so that it makes a single gummy bear thirty-five feet long and weighing approximately six hundred pounds…Report

            • Avatar veronica d in reply to Oscar Gordon
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              From a materials standpoint, it surprised me how much the Tacoma bridge was able to flex before failure. Naively, I would have expected it to break apart from even a third of that amount of deformation.

              The point: most of my materials intuition was formed by playing with legos! 🙂

              When I was (much) younger, one of my roommates was a materials engineer, mostly metallurgy — Cal Poly guy. Anyway, he introduced me to Tacoma Narrows and explained how it all worked out. It was my first introduction to both materials engineering and fluid dynamics. Cool stuff. I learned a bunch of words and a little bit of math.Report

        • Avatar DensityDuck in reply to Oscar Gordon
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          says:

          The thing is that people might look at something flexing and swaying and say “that does not seem very strong”, and what they mean is that it’s not very stiff, and it might actually be quite strongReport

      • Avatar DensityDuck in reply to DensityDuck
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        “as we see from the Narrows Bridge…” This was meant to refer to the Verrazzano Narrows bridge we see in the video, not the Tacoma Narrows bridge that collapsed. Obviously the Tacoma Narrows bridge was unsuitable for the purpose of being a bridge after it fell into the river!Report

  3. Avatar Kristin Devine
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    The apocryphal story of Gertie as I was told in school, is that the Professor (you can see him on the film trying to save the dog) just happened to be on the bridge when the shaking started. He convinced the people on the bridge to drive/walk down the middle of the road since that’s the place with the most stability and probably saved some human lives, even though he was not able to save the scared dog. I don’t know if that’s entirely true, but it did give me an understanding of how that kind of wave works.Report

  4. Avatar Kazzy
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    From what I recall, when bridges start moving like that, it is because some sort of harmonic mean is reached, where the movement spurs more movement. Or something? Is any of that right?

    I’ve walked across the GWB several times and feeling it sway and move is a trip. It is also a really fun way to freak someone out who has never experienced it before.Report

    • Avatar Oscar Gordon in reply to Kazzy
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      says:

      Possibly. A flexible structure like a bridge has a natural amount of damping of vibrations. Exceeding that damping can happen either through a resonance, or simply through the constant increase of input energy.

      So you could, say, just keeping increasing wind speed until the bridge fails because you’ve gone well beyond the physical limits of the structure.

      Or the wind speed can stay constant, but hits the structure resonance, in which case the wave amplitude grows despite damping.

      IIRC, modern bridges (etc.) have multiple damping modes, so you can’t hit a single resonance, or the design is such that the resonance frequency is so far beyond what could occur in nature.Report

      • Avatar Carl Schwent in reply to Oscar Gordon
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        My father’s masters thesis (1948) was on the aerodynamic characteristics of structural sections. Although it doesn’t actually mention bridges, he always told me that it was inspired by the Tacoma Narrows Bridge failure. If I remember his layman’s explanation correctly, as the bridge tilts, either towards or away from the wind, it catches the wind better, more tilt means even more force. And at certain speeds you could indeed get a resonance that would disastrously grow the amplitude.Report

  5. Avatar Michael Cain
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    No one, even the NSF (especially the NSF?) likes to pay for maintenance. What new things would have gone undone to keep Arecibo in good repair? Would it have attracted more money if more time had been spent looking for “rogue” asteroids — something I understand it was good at — and less on science? Can/will the USSF fund a replacement and let it be used for science some of the time?Report

  6. Avatar Oscar Gordon
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    says:

    Scientist have figured out what is killing salmon returning to urban waterways to spawn.

    Tires.

    More specifically, tire dust.

    Even more specifically, an anti-corrosion/anti-oxidation agent added to tire rubber to keep the tire from corroding.

    That’s gonna be a trick to fix.Report

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