1. Mick West

    Mick West Administrator Staff Member

    Source: https://www.youtube.com/watch?v=6obTq2UrCiU

    A large part of the reason that 9/11 conspiracy theories persist is simply that people not understanding the physics of the collapse. I was talking to a conspiracy theorist a while back, trying to explain why the towers collapsed. I said that the floor trusses were heated until they sagged, then they pulled away from the walls. Here's a visualization of this with the floor slabs removed so you can see the trusses.

    Source: https://youtu.be/XpfDkL-vFdk?t=48s

    He asked why would they pull away from the walls. They were not any heavier. They were still attached to the walls. It was still the same mass being supported by the ends of the truss, so why would it pull? It made no sense to him.

    I realized I'd not really given it much thought myself. I had to consider for a moment before realizing it was the loss of stiffness that essentially turned the floors from rigid rods to ropes. Catenary forces come into play. I tried to explain this.

    The video above is my attempt to demonstrate it. I do it two ways. Firstly with two identical chains. One has the links welded together so it's a stiff rod. The other is just loose. The loose chain pulls in the columns. The stiff chain does not.
    Metabunk 2018-03-06 14-08-35.

    Secondly I make a "floor" out of a steel box tube with some weights. I heat it until it sags in the middle. Then it pulls in the end "columns".
    Metabunk 2018-03-06 14-10-46.

    I think another part of the problem with a lot of 9/11 explanations is that they lack a practical demonstration, so people don't really get a real sense of what the explanations mean. Hopefully this will help a little.

    UPDATE: Here's another demonstration I did this morning. In this one the chain is encased in ice, and gradually transitions from rigid to sagging.

    Source: https://www.youtube.com/watch?v=zTygQK3vriI
    Last edited: Mar 8, 2018
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  2. Landru

    Landru Moderator Staff Member

    Myles Powers had a pretty good explanation on one of his 9/11 debunking videos where he discussed the unique construction of the twin towers and how the buckling of the floor joists pulled the walls inward. I'll see if I can find it.
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  3. Mick West

    Mick West Administrator Staff Member

    I was asked what the temperature was that the "beam" failed at. I'm estimating about 650°C (1200°F) on the upper end. I repeated heating the bar with a cheap thermocouple attached. It seems to top about over 700°C when heating the thermocouple directly. It was in the 600°C to 650°C range heating the bar itself. It cools down very rapidly.
    Metabunk 2018-03-06 15-28-12.

    The bar did glow orange in this temperature test. But I don't recall it glowing in the bend test.
    Last edited: Mar 6, 2018
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  4. Mick West

    Mick West Administrator Staff Member

    No thermal expansion was visible to the naked eye, HOWEVER there's some interesting effects. When I first heat the beam in the middle with the smaller blue propane torch, the two block rise up. I'm heating the bar on top, the thermal expansion is greater on top, so it makes the bar want to curve upwards. So the blocks rise very slightly and rotate outwards

    The beam then does seem to expand a bit (maybe 1mm) up until the sag starts to set in. Watch the top of the "column" on the right here.

    (Note the background shifts slightly too. I think this is due to the image stabilization in my Nikon P900)
    Last edited: Mar 6, 2018
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  5. James W Wing

    James W Wing New Member

    Did you consider that the softening of the material in the beam begins to cause the beam to lengthen (pushing the wall outwards at first as you said) but then as the beam begins to be pulled downward (it would not go up under load), the chord is now shorter than the beam's original length and the weight of the beam begins to be off-centered (no longer straight down so the forces are pushing the walls outward as well) which adds to the downward pull and the walls are then pulled in as the softening of the beam causes the chord to become still shorter as the beam fails?

    The pinning on to the walls also failed as well - I'm not familiar with how these are actually constructed, but your analysis seems to be right on. Once the pinning on either end is broken, the other end hasn't a chance to hold it and the softened beam pulls away, the weight of the bent beam with broken pinning falls as a chunk taking floor after floor as it falls. Isn't that the way the final report explained it, too? Even if the beam did bend upwards at first, that would stress the pinning and when the beam began to fail, it would become shorter as it sagged and the floor would fall as soon as it got off the pinning surface. It would be a fairly catastrophic sag - the ends would be not inches but feet inside the walls. The falling floors would pull the walls inward and the weight above would make the whole thing come down in the center. Just like it did.

    What is more surprising to me is that it doesn't appear that they use more off-sets to stiffen things between floors - it looks like the buildings are built on the outside structure with nothing inside after the first floor or two. I have some experience with single family dwellings and there they use offset walls to provide the support and keep the upper floors from flexing - I'm sure it is a completely different ballgame.
  6. JFDee

    JFDee Senior Member

    That's likely aided in parts by the slight rotation of the 'cross beam' between the nails. Its lower edge is pushing against the outer nails
    Last edited: Mar 7, 2018
  7. econ41

    econ41 Active Member

    A different perspective on this very issue which may interest or amuse. A few years back in a discussion with Tony Szamboti I threw in a casual comment "But catenary sag is a very effective force multiplier." The discussion was with engineers/physicists and I took the comment as being self evident...several pages of opposing discussion followed and in frustration I made an observation that the concept was simple enough that a 10 year old could understand it.

    Then my grandson - 6 yo at the time - came visiting and I set up an experiment. It was somewhat fun parody BUT the physics is spot on accurate - a couple of leading questions when I "interviewed" the 6 yo star performer to allow for his age and vocabulary. Here is the post:
  8. Mick West

    Mick West Administrator Staff Member

    I think you might need a diagram there :)
  9. Mick West

    Mick West Administrator Staff Member

    I think it's one of those things that's self-evident from a practical perspective - i.e. if you've ever tried to hang a hammock between two sturdy seeming sticks you pounded into the ground. Or if you've tried to make a taut 30 foot length of rope not sag in the middle. Or if you've done the math.

    But if you get into the physics for the first time it's rather confusing. Where are the forces going? Why is the force at the walls so massive.

    It can be helpful to think of the floor first as a rigid slab, and then as two rigid slabs joined by a hinge in the middle, then with progressively more hinges. Then imagine putting weight on each of these different floors (or imagine the floor sections themselves being made of heavy steel bars, which essentially they are).
    Metabunk 2018-03-07 07-10-39.
    In case A, the undamaged floor you can see that, like my rigid chain, the weight is simply supported at the ends.
    Metabunk 2018-03-07 07-13-10.

    But in case B, with a single hinge in the middle you get essentially massive torque (a turning moment, a bending force) at the supported ends. What happens then depend on how the ends are connected (and how well they are connected).

    If they are simply resting on the seats, them they will simply pivot up and off. If they can pivot, then they will rotate about that point, and pull inwards as the center of the floor moves down. If they are firmly fixed to the walls (moment resisting) then they will twist the wall, pulling in the wall above the connection, and pushing out the wall below it (although they will also be pulling in so the major motion will be inwards, depending on the length of the floor span).

    The reality of the situation is a more complicated system, with variable stiffness in the floor due to uneven heating. It's a struggle to explain the forces involved even to people with some basic physics knowledge, so I think the practical demonstration route is the way to go.
  10. Oystein

    Oystein Active Member

    With some JREF help, I did the hammock math seven years ago:
    Last edited by a moderator: Mar 7, 2018
  11. Mick West

    Mick West Administrator Staff Member

    I have to modify my temperature claims from around 650°C to (possibly) around 750°. Keep in mind I'm measuring this with a thermocouple that came free with the multimeter, but it seems to respond well, and is accurate at low temperatures.

    The question of how hot my torch can make things is an interesting one as it relates to heat (an amount of energy) vs. temperature (a measure of how energetic some piece of matter is).

    My torch is a Bernzomatic TS8000 MAP-Pro (Propene/Propylene). The specs say the flame temperate is 3,730°F, 2054°C. Does that mean it can melt steel (melting point 2750°F/1510°C)? No

    If I heat my thermocouple alone, it gets to 1150°C, glowing yellow hot even in sunlight. It gets to this temperature in 12 seconds.
    Metabunk 2018-03-07 14-31-56.

    If I heat the thermocouple clamped on top of a thin steel square tube it gets to 1050°C in 60 seconds, then no higher.
    Metabunk 2018-03-07 14-34-07.

    If I clamp the thermocouple underneath a 1/4" steel bar:
    Metabunk 2018-03-07 14-35-36.
    (That's 25.9°C there)

    I then heat it from the top, it gets to 764°. It took over five minutes for this to happen. At that temperature even minor movements of the torch would cause the temperature to drop.
    Metabunk 2018-03-07 14-37-45.
    The reason being is that the flame can only supply heat at a certain rate. The metal the flame contacts is swimming in 2000°C hot gas, but that simply creates a heat flow from the gas to the metal. Simultaneously there's a heat flow along the bar, and another heat flow out of the bar into the much cooler surrounding air, and yet another heat flow from the radiating heat - especially when it gets above red hot.

    The hotter you heat it (heat in), the faster the heat flows (heat out). At some point they match, and the steel will not get any hotter.

    A thermocouple by itself has restrictions on how much heat can flow away, because it's tiny and not touching anything. So it can get a lot hotter. Still not as hot as the 2000°C flame though. At around 1150°C it's yellow hot, and is simply radiating out temperature faster than it can convect it in from the flame.

    With the thermocouple attached to something, then even if it's heated directly there's now an additional escape route for the heat, through the steel. So the maximum temperature is lower, 1050°C

    And when it's on the other side of the steel, the heat has to go through that steel to get to it. The thermocouple is not a bit downstream of the actual heat source, but more accurate reflects what the temperature of the steel is (as opposed to what the temperature of a thermocouple in flame is). So now the hottest I got it was 750°C.

    These issues of heat flow and thermal equilibrium are important to understanding some aspects of the WTC collapses.
  12. Mick West

    Mick West Administrator Staff Member

    To expand on that slightly, a large fire burns hotter than a small fire. Not just because the flames are hotter (they are), but because the heat flow is limited by the surface area of the fire, whereas the heat production is limited by the volume of the fire. It's also limited by the oxygen supply, but that can be improved with air flow.

    Flames are hotter? Yes they are - because the temperature of the area is hotter, the gases that burn start out hotter. The chemical reaction of combustion adds the same amount of heat, so the end result is hotter flames.

    Both of these things play a significant role in kerosene pool fires. If a pool of kerosene is large then it will be heating a lot more of the metal above it, so there nowhere for the heat in the middle to flow out, as it's being heated all along. The general area heat also heats up the kerosene before it ignites, resulting in a hotter flame temperature, and more heat. This is all modulated in probably complicated ways by the availability of oxygen.
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  13. econ41

    econ41 Active Member

    That is where I presumed too much practical common sense understanding in my discussions. Recall that I had two truther engineers - "enik" and Tony Szamboti - and a few "sideline lurker" debunkers who were (still are) FEA fluent practising engineers. enik outright rejected the assertion "...catenary sag is a very effective force multiplier." Tony Sz took it in his stride. And all of them - opponents and lurkers - come from a background where they rely on FEA as foolproof and I had falsified several examples where they lost the plot. BUT the specific "catenary sag" issue is quite straightforward if you dont overcomplicate it.

    Maybe. I took a simpler approach and went directly to the central point.
    A rigid beam supported at the ends puts 1/2 the vertical load onto each support and has no horizontal inwards force exerted on the columns.

    The "half the vertical on each column" does not change if the rigid beam becomes flexible. BUT it does start to pull inwards as we increase the flexibility. So take the extreme of a fully flexible rope and the physics is still simple - the amount of sag determines the inwards pull. Straight forward application of vectors.

    - MickWestVectors.
    The "V-vector" is half the applied load - and the "H-vector" and "Tension" in the rope determined purely by the geometry. The more sag in the rope the LESS the inwards pull. Au contraire - reverse that - the less sag - the tighter we pull in the hammock - the more the inwards pull. In the example of the graphic the Horizontal pull in is visually estimated at a bit over 2 times the vertical - itself half the applied load.

    And that is for fully flexible i.e. a rope. If the beam is only partly flexible then there is a mix. BUT it is the catenary sag component which causes the inwards pull. Which was all I needed for that other discussion. And the catenary sag component can be isolated from all the considerations of where bending moment resistance comes into play for a less than fully flexible rope>>beam. IF you try to track what happens in the beam....but can be ignored if you can take the beam as a "black box" where the only interest is the external forces on the end points.
    Last edited: Mar 7, 2018
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  14. Tony Szamboti

    Tony Szamboti Active Member

    Catenary loads are only possible when the structural member can no longer take moments and will only transmit tensile loads like a cable.

    The reason the NIST model did not produce catenary forces from sagging floor trusses on the WTC Tower perimeter columns is that the trusses could still take moments and withstand compression to a degree. They behaved like Mick's rigid chain just applying force from their vertical load onto their seats.

    It seems whoever modeled WTC 7 for NIST understood this and took the moment capacity away from the beams and girders by cutting notches in them. See Figure 12-35 in the NIST WTC 7 report.
  15. Jeffrey Orling

    Jeffrey Orling Active Member

    Perhaps the end connections of the trusses to the spandrel beams on the exterior and the belt girders around the core were not strong enough to pull the columns together because the those connections failed?

    The connections were often the weak link in the floor destruction mechanism.
  16. Oystein

    Oystein Active Member

    I am fairly certain this is wrong.
  17. Oystein

    Oystein Active Member

    NIST found, a little against expectations, that this is exactly what did NOT happen in the twins: The floor-to-column connections did NOT fail in tension, which would have caused some first floor to drop, pancake-like, onto the floor below. This is explicitly the FEMA hypothesis which NIST refuted.
  18. Oystein

    Oystein Active Member


    this may expand the purpose of this thread a little farther than you intend:
    We need to be clear that, when many inches[1] of inward bowing were observed at the twins' fire floors prior to collapse initiation, this amount of inward bowing was NOT the result merely of sagging trusses pulling inward. Rather, the sagging trusses only provided an initial tug[2], enough inward bowing for the perimeter columns' capacity to drop below extant load. The load then would push the wall further down and further inwards, until enough load had been redistrubuted to other columns with yet sufficient capacity. Most of the visible inward bowing certainly was caused by the vertical load, not by the lateral pull.

    [1] I do not recall specific numbers, but certainly more than a foot, or the depth of a perimeter column
    [2] Again, I have no numbers, but suggest on the order of a single inch
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  19. Mick West

    Mick West Administrator Staff Member

    Metabunk 2018-03-08 07-40-39.

    Even a cable has some resistance to moments (bending). Try bending these cables.
    Metabunk 2018-03-08 07-47-29.
    if you cut the cables on the outside ends of the bridge, then would the towers not fall inwards, despite the presence of some moment resistance in the middle cables?

    Catenary loads are possible whenever a stiff structural member suffers a reduction in it's ability to resist moments. If it's actively sagging then it's no longer fully resisting the moment forces, and hence must be acting less like a rigid structure, and more like a chain.

    The removed of the sections of beams in the global LS-DYNA model, above, does not remove the moment resistance from the beams, it just greatly reduces it in line with the weakening of the steel from the increased temperatures. This is a simplification for economy in the the model.
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  20. Mick West

    Mick West Administrator Staff Member

    At the very least the pulling in helped initiate the column deformation, contributed to it's loss of vertical strength, and determined the direction of the bowing.

    I might try an experiment with loaded columns AND a loaded floor. It's increasingly difficult to heat larger structures though. I might need a bonfire.
  21. Jeffrey Orling

    Jeffrey Orling Active Member

    There were no floor to column connections per se. The facade side was connected to the spandrels... horizontal members of the 3 column 3 story high assembly.. ON the core side there was a continuous belt girder surrounding the core perimeter columns. The double floor trusses did not "align" with the columns so the cantilevered girder was a transfer of floor loads to the core perimeter columns.

    When the OSS flooring collapsed the weak link was the floor truss to spandrel and to belt girder connections. It's possible that the cantilever belt girders were ripped off by the floor collapse... they certainly were at the top because few horizontal beams survived the floor collapse.
  22. Mick West

    Mick West Administrator Staff Member

    Perhaps your objection here is a bit of a tautology, using "catenary loads" as the idealized loads from an idealized infinitely flexible rope dangling from two points? Perhaps we should adopt the more general term "pull-in forces", as used by NIST?

    NCSTAR 1-6C
    Discussing approximations made when modeling
    What actually happened:
    Metabunk 2018-03-08 08-26-44.

    (14 kip = 14 kilo-pounds = 14,000 pounds force)

    NCSTAR 1-6D describes how the trusses transitioned from stiff to less stiff, and how that resulted in catenary forces:

    Metabunk 2018-03-08 08-39-39.

    Note they explicitly refer to the "residual moment capacity" of the floor. It has not turned into a floppy rope, but it is unable to fully resist the moment (bending) forces, which is why it is sagging.
  23. Jeffrey Orling

    Jeffrey Orling Active Member

    That NIST explanation sounds like garbledy gook to me.

    The core columns WERE laterally restrained by the bracing. Look at the plan of the core NIST.

    The pull in of the facade was because the CORE side of the floor where you see pull in had no core side support... the floor at that point was essentially cantilevered and the connection pulled at the facade until the bolts sheared and the connection failed. Shear strength of a 5/8"Ø bolt is 1,500#. If the claimed pull in force was 15,000#... each connection would have had to have more than 7 bolts. I don't think the top chords of the double truss had 10 bolts... each one had 2. if the truss had buckled / sagged as they claim the 2-1"Ø did nothing.



    Attached Files:

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  24. Mick West

    Mick West Administrator Staff Member

    No, see NCSTAR 1-6D section 2.5.2: http://ws680.nist.gov/publication/get_pdf.cfm?pub_id=101366
    You seem to only be considering the seat bolts, and those only in perfect perpendicular shear.
    Metabunk 2018-03-08 09-25-37.
    The welded gusset plate can be seen in the diagram above. I suspect also that some of the force would apply to the bolt as axial tension as the bearing angles pivot on the seat angle.

    Here's the strap anchors and spandrel studs in plan view. The strap anchors are studded into the slab, but also appear to be connected the the columns.
    Metabunk 2018-03-08 09-31-44.

    As you cans see there is essentially a truss-to-column connection. The seat is on the spandrel, but it's directly behind a column., The adjacent columns are attached via the strap anchors:

    The complexity of these connections (especially with the involvement of the concrete slab) made them very hard to model in the global simulation. NIST seems to have used estimated pulls based on observed deformation rather than calculated pulls.
  25. Jeffrey Orling

    Jeffrey Orling Active Member

    Perhaps difficult to model because of weld performance... but not difficult to test full scale.... which they didn't bother to do. Connections will fail at the weakest point... and YES that may be hard to model...

    Why didn't NIST assemble a truss connection to a spandrel and apply tensile stress im destructive testing?

    They built trusses and subjected them to fire didn't they?


    Attached Files:

  26. Mick West

    Mick West Administrator Staff Member

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  27. mrfintoil

    mrfintoil Active Member

    Perhaps it's a good thing to include a proper explanation what a truss system is, why it's designed the way it is, and how the truss system's stress resistance decreases as the original shape of the system is deformed.

    That would likely help some to understand the chain analogy even better.
  28. Tony Szamboti

    Tony Szamboti Active Member

    Vertical loads on horizontal structural elements create bending loads which require tension resistance on the bottom and compression resistance on top. A cable has virtually no ability to generate internal reactions against compression besides friction between strands and wires. The large bridge cable you show would have a very limited amount of moment resistance and would only seem to be able to take moments when carrying a vertical load which was orders of magnitude lower than what it was intended for in tension. That does not make it a moment carrying member. If you cut the cables of a suspension bridge on the outside the reaction to their catenary force would be removed and the load that caused the catenary force would no longer be supported and would fall. That would be the deck, not the towers. The cables do not support the towers of a suspension bridge. The towers of a suspension bridge are self supporting, and are actually supporting the cables vertically and are loaded in compression from the cables on top of them on a saddle. The towers would be relieved of that load at that point and have only their own weight to support, which is far less than they need to be capable of handling without buckling. See https://science.howstuffworks.com/engineering/civil/bridge6.htm

    Sagging composite floor slabs and trusses in the WTC Towers would not have lost moment resisting ability as they could still take significant compressive loading at 700 degrees C, even with some diagonal buckling in the truss, as claimed by NIST. It is the top flange of a beam that goes into compression and resists moments due to a vertical load on it and the concrete slab would not have lost its ability to take compression. This is why the NIST FEA model did not produce the pull-in forces on the columns with the composite floor involved. Had the composite floors been unable to take compression the vertical load would produce a tensile force to support it and the FEA program would have shown a pull-in force. It didn't because the trusses and their composite slabs could still take more than enough compression to resist the moment due to their vertical load, even at elevated temperature.

    Catenary forces/loads are only possible with transverse loaded cables or normally stiff structural members that have suffered a near total reduction of their ability to take compressive loads and resist moments, such as when you heated the flat bars to 700 C in the middle of their span with a torch. It would be more realistic if you made up a small metal frame which was composite with concrete on top and tried the same thing while heating it to 700 C. It will not cause a catenary force in that configuration, just like the real configuration didn't for NIST.
    Last edited: Mar 9, 2018
  29. Tony Szamboti

    Tony Szamboti Active Member

    Your are right about the NIST "sagging trusses pulled the exterior columns inward and caused the collapse" explanation being wrong as the composite floor slab and trusses could not generate the catenary force in their model as I explained to Mick above.

    However, you are wrong about what caused the exterior column pull-in. It is clear that the antenna dropping before the exterior roofline in the North Tower collapse shows it was a core column failure that pulled the exterior columns inward and initiated the collapse.

    The 1,500 lb. value for the shear failure load on the 5/8" diameter bolts you give is not correct and far too low. You were taking a Safe value, which is conservative with a Factor of Safety included. The failure would also not be at the thread root, in the tower situation. It would be across the larger area of the shank. The shear failure load also depends on the strength of the bolt.

    The truss seat bolts were per ASTM A325, with a minimum tensile yield strength of 92,000 psi. Shear yield strength is 57.7% of that and would be about 53,000 psi. The bolt shank shear area is .307 sq. inches. Thus each bolt could take 53,000 psi x .307 sq. inches = 16,271 lbs. of shear force. For two bolts that would be 32,542 lbs. See https://en.wikipedia.org/wiki/ASTM_A325 for the strength of ASTM A325 bolts.

    ASTM A325 bolts are somewhat equivalent to the Grade 5 SAE bolts in the chart you showed. If you look at the bottom of the chart you showed you will see that the Grade 5 SAE 5/8" bolt tensile breaking load is 27,100 lbs. for a coarse thread. That would be about 15,600 lbs. in shear. For two of those bolts the shear failure load would be about 31,200 lbs.

    31,200 lbs. at each truss connection would have been more than enough to pull the exterior columns in when the core went down.

    However, it wasn't just two 5/8" diameter bolts which comprised the truss to core and exterior connections.

    In the thread above Mick shows the complete situation on the exterior; with the two 5/8" diameter truss bolts, two anchor straps which have four 2.5" tall shear studs each in the slab and are also bolted to the spandrels with 5/8" diameter bolts, a 6" long shear stud in the side of the slab between trusses, and the 3/8" thick x 4.5 to 6 inch wide x about 10 inch long gusset plate welded to the spandrel and tops of the trusses. Of course, the truss bolts themselves are enough, but the real situation would have allowed for a much higher load without connection failure than just the two 5/8" diameter truss bolts in shear. There were also damper connections at the exterior.

    The core connection was two 5/8" diameter bolts and four 7" long shear studs in the side of the slab at each truss. There is also evidence of the composite and high strength welded wire fabric reinforced slab being continuous into the core, with shear stud plates in the core. There was a lot of additional connection strength, besides the two 5/8" diameter bolts, there also.
    Last edited: Mar 9, 2018
  30. Jeffrey Orling

    Jeffrey Orling Active Member

    Regardless of the actual shear strength of the bolts... the failure of the connection is a complex calculation above my pay grade. But there are many reasons why catenary pull in makes no sense to me.

    We agree that the collapse of the twins from the plane strike zone down was a result of a core failure regardless of the core. The building movements tell us this.

    The planes obviously destroyed a number of columns and may have put a huge dent (damage) to several others. Of the 47 core columns only 24 were supporting the OOS flooring and in both building a number of them were destroyed by the plane. We also know that loss of this axial capacity at the strike level was not sufficient to immediately trigger the top to collapse. That can after more than an hour in both cases. What did happen after the planes hit was fires were started... sprinklers failed, power lost above the strike zone.

    2wtc's top tipped toward the side with the loss of columns (axial support) and the dropping top hinged and translated NW.. The scores of thousands of tons of debris was more than enough to drive through the building overwhelming all the slabs below. All the columns survived the floor collapse and fell from instability... facade peeled and pushed outward like a peeling banana skin...and the core columns teetered and fell either toppling or buckling at the end connections from Euler buckling (the tallest ones). The columns connected by bracing beams fell over like a ladder.

    1wtc's top was the victim of the collapsing antenna supported by core columns and the hat truss which spread its 360 concentrated axial load which was over only 3 columns. Those columns likely were severed by the plane parts plowing through the core. Intact columns above severed columns were left hanging from the truss. The column to column end connections were designed for compression not designed for tension... Clearly these connections failed and imparted an downward force on the hat truss as they did. Clearly the truss was mortally weakened in its center... and the antenna on its 25'x25' base plunged into the roof and down into the core... destroying columns in its path before toppling over. Core core was hollowed put structurally above the plane strike and the process caused the perimeter columns above the strike zone to fail and pull the core side of the slabs with it and they in turn broker free from the perimeter columns and that was the mass that drive all the way through the building.

    The effect of the fire was more likely to distort the core causing columns which had lost bracing to move laterally and destroy axially alignment and failure.

    The notion that scores of sagging double truss pulled the facade in at about the same time from fire is almost laughable and defies credulity.
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  31. Mick West

    Mick West Administrator Staff Member

    I know, my point was that you said
    I then invited you to go and try to bend (apply a moment) that that section of cable from the Golden Gate Bridge

    I think you'll find that it can take quite a moment before it bends.

    Of course it's all a matter of scale. It can take a large bending force, but it's going to sag (and pull in) over longer spans.

    If pull-in is not possible then one wonders then why a gradual localized pull-in was observed on the floors affected by fires. If this pull-in was due to core failure, then why did it not apply to all the floors above that point? Are you suggesting all the core columns buckled inwards from some careful application of nano-thermite on the floors affected by the plane impact and fires?

    NIST explains that the did not fully model the floors in a way that the exact observed sagging would result, simply because it was far too complex to do on anything other than a single model of a truss.

    But again here it's a matter of degree. The point of my demonstration is that a sagging system pulls in. You are simply arguing that it did not pull in enough, despite the observed pulling-in only in those regions where there was both fire and damaged insulation. The demonstration is valid.

    You modify your position somewhat with:
    Really? The catenary force jumps from zero to non-zero when the moment resistance of a structure goes from just below near-total to near-total? How does that work?

    I would like to improve my physical models though. I'm not sure if you intended "flat bar" as a criticism, but it was actually a 5/8" square tube, which is a lot stiffer
    Metabunk 2018-03-09 08-35-31.
    That seems an unlikely assertion. Zero catenary force? If the metal bar sagged when heated to 700C in one spot, then surely it's going to have even LESS resistance to sagging when heated to 700C over its entire length?

    That's perhaps something of a moot point. I was thinking of building a fire pit, but unless I do it's impossible to heat the entire length to 700C (although I suppose I could buy ten propylene torches).

    However, it occurred to me that there's something of a middle ground between steel and iced chains: aluminum. Could I perhaps make an analogous structure from aluminum (and tiling grout for concrete). I could then load it appropriately and heat this to around 250C with continuous torch movements. The aluminum would weaken and sag.

    • Agree Agree x 1
  32. Mick West

    Mick West Administrator Staff Member

    I just knocked up a quick proof of concept.
    Metabunk 2018-03-09 09-05-12.
    An aluminum tube is suspended between two blocks. It's loaded with a a 2x4 that is weighed down with some bricks and rocks.

    4x video:

    Source: https://www.youtube.com/watch?v=U967Ld5QPzs

    The aluminum exhibits the same gradual sagging of the ice chain, yet is clearly still highly moment resisting. At the end it pulls in the the two blocks, even thought it's not attached to them.

    This seems quite promising.
  33. Tony Szamboti

    Tony Szamboti Active Member

    Mick, you need concrete on top which is composite with a metal frame below for your simulation to have any validity.

    You say you used a 5/8" square tube earlier. Was it steel? What was the wall thickness? What was your span?

    If you give me that I can tell you what an equivalent thickness of concrete on top of it would be to allow you to reasonably mimic the trusses and slab in the Twin Towers. You can put wires through it to simulate the welded wire fabric and screws through the top of the tube that would jut into the concrete to make it a composite beam and provide the effect of shear studs (knuckles from the trusses in the case of the Twin Towers).

    I wasn't trying to imply anything by using the term "flat bar". That is what I thought it was. A flat bar can be quite stiff as moment of inertia for a prismatic beam is 1/12 x base x height^3. The width or base is less significant. It is the vertical depth or height that counts the most, as it is cubed. A 5/16" wide x 1.25" high flat bar has the same cross section and weight per unit length of a 5/8" square bar, but is 4 times stiffer vertically than the square bar if the 1.25" is in the vertical.
    Last edited: Mar 9, 2018
  34. Mick West

    Mick West Administrator Staff Member

    Metabunk 2018-03-09 09-49-20.

    Looks like 3/32" (13 gauge).

    The problem again is heating it evenly to 700°C. This is why I'm suggesting doing it in aluminum, which only needs to be heated to 250°C. It should still be able to demonstrate the general principle, no?

    We might be able to heat the Al sufficiently with a row of candles.

    Simulating concrete on a small scale is going to be a problem. Any aggregate, even sand, is going to introduce structural weaknesses that would not exist on the large scale. It needs a degree of flexibility. I'll experiment with grout, as that's what I have to hand.
  35. Tony Szamboti

    Tony Szamboti Active Member

    I think it is a 3/64" wall thickness tube, not 3/32". Each mark is 1/32" and it looks like it is 1.5 times that wide which would be 3/64". It is probably 18 gauge which is .0478" and is close to the .0468" decimal equivalent of 3/64".

    I don't think you need any large aggregate in the concrete. Sand alone with Portland cement should work to provide the same reaction as concrete.

    I don't think you even need to heat it evenly. If you get a stretch of 6 inches of the metal frame to 700 C it should take the compressive resistance away from the metal enough.

    I will calculate how much cement and sand you need proportionately along with its height dimension to stay 5/8" wide. Give me a day.
    Last edited: Mar 9, 2018
    • Like Like x 1
  36. Tony Szamboti

    Tony Szamboti Active Member

    Here is the basic concrete mix a material science teacher at the University of Illinois uses for lab experiments. He doesn't make them use aggregate and has them use just water, cement, and sand.

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  37. Mick West

    Mick West Administrator Staff Member

    Ah yes, my mistake. 1.5/32, 3/64

    The other factor would be how much additional load is required to scale. This steel is from an old fence, so I've got quite a few sections I can cut to 48" long.
  38. Mick West

    Mick West Administrator Staff Member

    I still think aluminum might be the way to go. We are not going to get accurate numbers with steel anyway due to various scale issues. So aluminum should still demonstrate the principle.

    I only had this 16"x1"x1/8" piece laying around. But let's do some investigatory experiments
    Metabunk 2018-03-09 10-56-38.

    #6x3/4 wood screws (it's all I have!) every 2" in 1/8" holes, as shear studs.
    Metabunk 2018-03-09 10-57-24.
    Metabunk 2018-03-09 10-58-33.

    Add grout as a concrete substitute. Note I drilled some holes on the end for anchoring. Not sure how yet.
    Metabunk 2018-03-09 11-00-35.

    I'll let that set, then see it it takes a load without cracking, then try heating it.

    This all is just investigation, not intended to demonstrate anything yet. The grout might just crack. A trip to Home Depot might be in my near future.
    • Like Like x 2
  39. qed

    qed Senior Member

    This is rather exciting.

    I would like to be clear about what @Tony Szamboti is predicting.

    What do we see happening in the following experiment that will no longer happen with the addition of the concrete (according to Tony)?

    [... I understand that this is still an investigation towards a more properly agreed on experiment, but what in principle is Tony predicting ...]
    Last edited: Mar 9, 2018
  40. Mick West

    Mick West Administrator Staff Member

    Metabunk 2018-03-09 13-38-55.
    Metabunk 2018-03-09 13-54-32.
    This shorter piece of the Al was heated by seven candles. It reached thermal equilibrium at 75°C. Thinner candles would increase the heat flow in (more flames). I guess I could also make a mega-candle with two rows of wicks down the middle.

    So: possibly viable for heating aluminum, but would require lots of candles, and the proximity of the candles would limit deflection.