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  1. Mick West

    Mick West Administrator Staff Member

    Let's try to stick to the topic of the model please.

    I'm not modeling changes in size of the columns because they are (at this scale) vastly stronger than the actual column, and they don't really contribute to the collapse (as they largely fall away, with the floors slabs themselves doing the damage). As noted the floor slabs are about the same all the way up.
     
  2. Mick West

    Mick West Administrator Staff Member

    I think Bazant is irrelevant here. The columns are not being crushed in my model, and they were generally not crushed in the real collapses.
     
  3. Polly Math

    Polly Math Member

    Yes. Do you understand that the side in the model is vertically consistent where the towers were not ?

    No. The mechanical floors were below the top 11 floors and that was where the real weight of the mechanical equipment was, that's why the floors were twice as thick there. The sides on the model should get lighter as the height increases is what I am saying. I think that's relevant to what is being done.

    Well those are your thoughts.

    You could just as easily say that the middle of the sides were weaker if that sits more comfortably. They would in neither case present a consistent resistance along their length is the point that I am making. I think that could be replicated here.

    The core column structure is not there for the edges of the floor to tie into in the corners of the twin towers. This means that when you look at a floor of the tower from any side the floor truss system will be TRANSVERSE/CORE/TRANSVERSE on the inside.
     
  4. Polly Math

    Polly Math Member

    The floors in the twin towers took no dead load at all, just live load.
    And it's based on structural contemporary steel design volume 4. I don't have it to hand here.
     
  5. deirdre

    deirdre Moderator Staff Member

    that really wouldnt make a difference in this model though. it's the weight of the floors (the horizontals) that are causing the collapse. lighter walls would just make the connection breaks that much easier to "pull" the walls/columns in.

    and planing them would make an awful lot of sawdust to not show anything substantial.



    But the towers arent a cardboard box. Even *if* (and i'm not saying they are...i have no idea) the cattycornered corners were a mite bit stronger than the sides, the sides themselves are equally strong along the whole length. For one they werent one piece, and even if they were, the steel is the same in the middle as it is near the corners.

    and while, your theory of "stronger corners" sounds kinda good to me based on the catty-corner... the columns werent "crushed" like a cardboard box.

    because i have such a hard time at 110ish lbs crushing the box my Corona beer comes in... i think this ??? is what you are picturing for the towers? Try as i might i cant crush the corners at all. If i balance my weight on two corners the box will literally hold me up.

    More of a "mega giant godzilla" if he were to "step" in the towers? < not sure that's a good or accurate analogy... trying to figure out what youre picturing for the collapse. :)
     
  6. Polly Math

    Polly Math Member

    Precisely. It should be more difficult for the floors to do this lower down then, meaning that the resistance should indeed be increasing as the collapse progresses lower.

    Drilling them would be better. Keeps the outside diameter constant.

    So in the towers there is a 35ft x 60ft area at each corner where the floor system is transverse. The floors may well be equally strong in some sense along the side, but the floor truss has no core column connection to gain purchase on the outer columns and pull them in over these corner areas. The stress on the building structural system is not equal across the face. This is where them not being one piece as you said, would be crucial.

    I am thinking of something that is stronger at the corners in proportion with that.

    Yeah the godzilla hypothesis is right up there with the NIST report.
     
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  7. Keith Beachy

    Keith Beachy Active Member

    The floors are the same... it is the floors which have the same weight, and are not thinner, or smaller, etc. It is 12 floors that would make a lower floor fail. Steel failing and falling on floors is extra credit fail.

    108 and 109 were mechanical floors.
     
  8. Mick West

    Mick West Administrator Staff Member

    What resistance exactly. Can you draw a diagram of how you think my model should be adjusted?
     
  9. Polly Math

    Polly Math Member

    You don't need to adjust anything at all in your model. Your model is fine but to claim that is it "illustrating the progressive collapse of the wtc towers on 9-11" in any way shape or form is misleading. What you are illustrating is that if you hit something hard enough - it will break.
     
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  10. deirdre

    deirdre Moderator Staff Member

    but each step lower you go, the more weight is falling and knocking about. LOTs of weight.
     
  11. Keith Beachy

    Keith Beachy Active Member

    The connections of the floors are the same, the resistance of a floor failing does not get more resistance, the connections don't get stronger. The core and the shell get thicker, the accumulated mass, and the velocity of the collapse grow.
     
  12. Mick West

    Mick West Administrator Staff Member

    No, I'm illustrating that a tall stable structure that is made of column sections held together by floor slabs can collapse totally (and rapidly) if a fraction at the top starts to move downwards.

    Didn't you see the last video? When I hit the model it does not break, it just sways (like the towers did after the planes hit them). When the top part starts to fall, then the rest collapses.
     
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  13. Polly Math

    Polly Math Member

    The first corner connection to go in this scenario would break slightly later but that's more about the initiation. As the hypothetical collapse progresses the corners don't suddenly get weaker. I don't think it can be done with this model.
    Still respect to Mick for getting his hands dirty and putting it together. Oy has a long way to go, but it's an interesting journey. I think that the principle of these models is to build in accuracy wherever you can instead of guessing what difference the detail might make, and excluding it on that basis. The detail is then included to accommodate other developments in the model.
    I would drill out the side pieces to have less mass in the upper ones. But then again, it's easy saying that when it's Mick that has to do it.
     
  14. Polly Math

    Polly Math Member

    Your model is rocking yes. And not just in a hip and happening way !
    Look at the figure on the left and marvel at the balance of the twin towers. The up force of your model at the right hand side would be counteracted by the down on the left but also right through the system on ALL sides (including the right).

    For the sake of clarity - are you saying that your model in some way replicates this redistribution throughout the entire structural system ? Again, with no disrespect for a WIP, I would sincerely hope not.
    wtcaisi5_01.
     
  15. Mick West

    Mick West Administrator Staff Member

    I'm not saying that. It never occurred to me. But why wouldn't it redistribute loads as it sways?
     
  16. Mick West

    Mick West Administrator Staff Member

    But now I've read that page, yes it actually does replicate it in a way. In the original the Vierendeel trusses for opposing outer walls would be the connecting walls. In my model there are no connecting walls, however the lateral loads (like wind) are still essentially redistributed via the moment connection between by the floors and walls, which essentially form a truss.

    Note they specifically note that the floor system is not designed to carry any moments (rotating forces), however in my model it does (not really by design, that just what the magnets do). Hence my model is stable in simulated earthquakes and high wind loads.

    It's not particularly relevant to the collapse though, as this redistribution applies to lateral wind loads, not parallel dropping-block loads.
     
  17. drommelsboef

    drommelsboef Member

    We already know that a house of cards can collapse progressively because it the most instable structure possible and not designed for swaying in the wind or other small perturbations that will keep the structure stable. Certainly not designed to withstand a smaller plane. Mick's attemp to create a 2d structure with 3 colums can only be praised. Perhaps it would even collapse when you put it on fire. However Mick's model will probably collapse directly when an object with kinetic energy (and momentum) touches it and not create a hole into the structure that after impact is still very stable. In that case a gradual weakening mechanism should initiate the collapse. Mick's example already starts with huge kinetic energy, like Verinage demolition. In the Sauret video we also see that columns are shortened because first the core columns give way.

    If the model is 3d, there are more columns, trusses and braces and a gradual initiation leads to a similar collapse then we have in fact a scale model confirmation.
     
    Last edited: Apr 13, 2016
  18. Polly Math

    Polly Math Member

    Ok, We're done here. Good luck with your model.
     
  19. jaydeehess

    jaydeehess Senior Member

    You conflate collapse initiation with what Mick is actually modelling. He is modelling the progression of collapse POST-intiation..

    Fire is a concern for initiation
    Column strength is a concern for initiation
    Aircraft Impact weakening is a concern for initiation

    Mick's models depict what occurs AFTER the columns at the impact fire levels have given way already, and have nothing to do with how or why the initial column failures at that level happened.
     
  20. Oystein

    Oystein Active Member

    That's curious, coming from a person who persists in the false beliefs that the top 10% are much lighter than the average of the tower, and that the floors get more resistive the lower you go.

    You still have not really provided any reasons why Mick's model needs to make the corners stronger, and not made any actionable suggestion how to fix that "problem".

    I actually think you have a long way to go - step 1 would be "grasp what Mick is actually trying to model".

    Of course it is always nicer to model more details faithfully, but you have to stop adding details and accuracy at some point, unless you want to build an exact 1:1 replica. Whereever you stop, you need to think about why and how the things you include or exclude are relevant and significant, relative to your purposes.
     
  21. aka

    aka Member

    aka is denying what? That the towers collapsed? o_O
    Not a problem per se, but a very interesting peculiarity that I don't quite know how to address or amend. You point out yourself that...
    What this boils down to is the explanation given by some laypeople that the Twins were built with more lateral strength (against wind loads) than vertical strength (against gravity) - something which is not easy to achieve mechanically. With the magnets however, that is exactly what happens: it takes much less force to slide a magnet off a steel plate than to pull it right off. It seems to me that this plays a non-negligible role in how the model achieves its lateral strength, without losing its ability to allow for progression of collapse.
     
  22. Mick West

    Mick West Administrator Staff Member

    It doesn't though. I can throw things at the wall that are essentially hundreds of tons moving at hundreds of miles per hour, and the structure just sways. I can rock the structure back and forth in way that simulates a massive earthquake, and it similarly just sways.

    I did not set out to build something that would collapse like a house of cards. I set out to build a structure
    • that was stable (to scale),
    • where the floor slabs that could support the sudden application of six times their own weight
    • which had very strong vertical support from columns
    • and where the columns were held vertical by the floor slabs.
     
  23. aka

    aka Member

    TBPF, you did have to get the splices out of the way to ensure progression and prevent it from "jamming".
     
  24. jaydeehess

    jaydeehess Senior Member

    In the actual tower collapses do you expect that the truss supports were pulled off or sheared off?
     
  25. Mick West

    Mick West Administrator Staff Member

    Yes, because the jamming splice plates created an unrealistically strong seat connection. That would absorb vastly more force in the vertical direction than was realistic. So I really don't see the problem. The model was still stable. The splice plates still performed their function on the outer walls.
     
  26. Mick West

    Mick West Administrator Staff Member

    I've changed the title of this thread

    Towards A Replicable Physical Model Illustrating the Progressive Collapse of The WTC Towers on 9/11

    to:

    Towards A Replicable Physical Model Illustrating Aspects of the Collapse of The WTC Towers on 9/11

    As I think the original misled people into thinking I was trying to create a model that replicated the models structures, and all the factors involved in the collapse. Instead I am just illustrating certain aspects of that collapse using simple models. In particular I'm illustrating a stable structure that collapses as the floor slabs are stripped away from the walls.
     
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  27. Polly Math

    Polly Math Member

    It was actually a typo that was meant to say "IT" not "OY", but seeing as you were so good about it, have at it re any of these buildings. Feel free to start a thread.
     
  28. Polly Math

    Polly Math Member

    The towers had a roughly 6.1/2 : 1 Width : Height ratio, the model has about 3:1 or so, meaning the model is like 2 towers. So the drop per floor slat is about 9 storeys equivalent? Is that not a bit much ?
     
  29. Mick West

    Mick West Administrator Staff Member

    I don't think that ratio is not really relevant. For example I could simply use much longer floors, like 9 feet long (with more/bigger magnets) to get a better ratio. What difference would that make? The floor still falls the same distance, it's just heavier now.
     
  30. Polly Math

    Polly Math Member

    Making the floor spans longer? That would make it less accurate in terms of the ratio.
    Do you really think the slenderness of the structure is not important if you are trying to illustrate anything about the towers? Why would you NOT try to replicate these ratios and scales to some extent?
     
  31. Mick West

    Mick West Administrator Staff Member

    Which ratio exactly are you talking about?
     
  32. Polly Math

    Polly Math Member

    Height to Width. When you divide the height of the tower by the width you get about 6.5
    What does your model come out at, 3 maybe ?
     
  33. Mick West

    Mick West Administrator Staff Member

    I was not intending it to represent the full height of the structure. Imagine the same thing, but twice the height. Since the falling mass in my model is accelerating, it would obviously continue to take out anything beneath it. See the graphs here:
    https://www.metabunk.org/towards-a-...e-wtc-towers-on-9-11.t7396/page-6#post-179644

    There are a variety of scale problems. I can't really decrease the gap between the floors to 2" as then there would be magnets all the way down, giving an excessive retarding force that simply would not be there. The floors also are then very thick relative to their drop distance (like 6 feet thick with a 6 foot drop) but not correspondingly heavy (wood vs. steel and concrete).

    Hence the better suggestion is to make the floors wider. But it's just not practical to make something 20' wide and 130 feet tall.
     
  34. jaydeehess

    jaydeehess Senior Member

    How does that affect anything in the model? Height to width would be more relevant to lateral movement and slenderness concepts.
    Gravity does not scale.
     
    Last edited: Apr 14, 2016
  35. aka

    aka Member

    I was hesitant to say it, but now that you bring it up yourself... would it stand up?
    I read somewhere that back in the good old times when VFX did not mean CGI yet, models for movies were recorded at high FPS and the recording played back at a normal rate, which "slows down" their movement and makes them look big (children intuitively do something similar when they play with their cars - they make them fly in "slow motion").

    I tried a few speed settings and I think this is, overall, a relatively good fit:

     
    Last edited: Apr 15, 2016
  36. jaydeehess

    jaydeehess Senior Member

    Ok, what's your point? That you can time the actual collapse and adjust the model video to play through the same time period.
     
  37. Mick West

    Mick West Administrator Staff Member

    I'm sure I could get another two sections on it at least before it gets too squirrely - so taking it from 8 to 12 feet. The key thing is making everything square (i.e. corners all being exact right angles). A small error at the base can greatly magnify instability at the top. Like building a brick wall.

    With greater height the problem become the front/back motion, as it's only 3.5" thick. To make it more stable though I could simply change from using 3.5" wide lumber (1x4s) to 7.25" wide (1x8). I could also switch to a more homogenous material, like HDF, or modeling board for more precision with the angles. I'd also need to ensure it's on a level stable base.
     
  38. Mick West

    Mick West Administrator Staff Member

    A free-falling object (or object under constant acceleration) will describe a parabola when plotted against time.

    Parabolas, like all conics, remain parabolas when an affine transformation is applied (e.g. stretching or compressing one or more axis)

    Hence any two videos of a falling object can be made to match simply by compressing or stretching the parabola - i.e. by speeding up or slowing down the video, and stretching it to fit the distance.
     
  39. aka

    aka Member

    Yes, that was the point, thank you. You explained it better than I could have.

    I wonder whether the lack of lateral ejections highlighted by the direct comparison should be attributed to the coarser "granularity" or to the relative lack of resistance.
     
  40. Mick West

    Mick West Administrator Staff Member

    Both, but also the limited height of the model comes into effect. Less kinetic energy in the period covering initial floors of the collapse, and hence less energy for springing and bouncing.