That's why I'd like to see comparative load-path diagrams. My brain isn't very interesting. It's the mechanical processes in the buildings that we're talking about.
Would the WTC Twin Towers have collapsed from fire alone, without plane impact?
- Thread starter econ41
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benthamitemetric
Senior Member
And I'd like to see you make an actual argument in one of these threads. We can't always get what we want.
My thinking is as follows:
A structure can be built to sway by a foot or more with a lot of weight at the top or very little. An antenna, for example, can sway a great deal and doesn't have a lot of weight up there. If you put a big mass at the tip, however, then you don't want it swaying too much or the antenna will snap. You build something to restrain the swaying. Something like a tower.
So basically, all this mass that we think would overwhelm the lower structure of a WTC Tower if it moves downward a foot or two has been moving around in all directions laterally by about the same distance for thirty years.
So I'm saying the lower section was in fact designed to manage a lot of dynamic forces. I don't know exactly how they compare to the dropping scenario. You think the latter are obviously way off the scale. I'd like to see some math.
As you know, I'm always happy to drop it when people think it's tiresome. Just stop asking more questions.
Your image isn't loading for me. However, I found this bit regarding how long the jet fuel lasted:NIST said no.
Article: 
See NIST NCSTAR 1-5, section 6.15.
Oystein
Senior Member
Do you have a source for that?
Ideally one that gives us some quantities (such as "30% of columns severed" or "all columns reduced by 50%", something to that effect)
In the meantime, I want to remind you that Danny Jowenko, in the 2006 interview so often quote-mined by Truthers, also expressed zero surprise that the towers would collapse completely once the top gets going, for that is what gravity does also in controlled demolitions. No mention is made of a requirement to pre-weaken the structure below.
Yikes.
Seriously?
A gravitational load applied dynamically exerts a force AT LEAST TWICE as large as the same mass does statically.
The factor 2 only applies if the mass is applied coming from a vertical distance (or, equivalently, an initial velocity) of ZERO.
Once the mass already is moving down as it loads a bit of structure, the forces multiply quickly, and easily far beyond static capacity.
Let me elaborate the concept of dynamic loading from a height/velocity of zero:
Imagine WTC1 being built up to the 95th floor, and the top 15 floors being assembled separately, then lifted by a giant crane to be placed onto the lower 95 floors.
Case 1: You carefully make the column ends touch, with just a wee bit of force experienced by the lower structure. Then slooooowly lower the crane, such that the load is applied (increased) linearly, until the elastic starin of the lower 95 floors are equal to the weight of the 15 upper floors, at which point the crane no longer bears any weight: The 95 floors now "feel" the static weight, that is mass times g (gravitational acceleration) of the top.
Case 2: Again, you gently lower the crane until the top column ends juuust touch the bottom column ends, but almost no force between them, the crane still carries the weight of the top, it's just hanging at a height of zero above the target. Now cut the cables between crane and top in one instance: The weight of the top will start to bear on the bottom, which therefore starts to "feel" the weight; in reaction to this, both structures get compressed elastically - strain energy builds up, columns shorten, the top moves down and accelerates - until the recoil force is as large as the simple weight of the top - this is exactly the point where the static whole tower is in equilibrium and not moving - but our case 2 dynamically applied top is at this point still moving downward, at its maximum velocity - from this point on, as the bottom gets compressed even more and recoil force increases beyond the weight of the top, the top now starts to get decelerated, until it comes to a stop. At that point, elastic compression (strain) is at its maximum - and the force experienced by the columns at the intersection between top and bottom happens to be TWICE the weight of the top, hence TWICE the force of a statically applied top
Case 3: The top is lowered by the crane, with columns perfectly vertically aligned, but not all the way: This time, we cut the cables simultaneously an inch or a foot or twelve feet before column ends meet. This way, the top already has some velocity before the structures get loaded and elastically compressed, the columns end up getting loaded MORE THAN TWICE when they reach the point of maximum compression - and there really is no limit to the factor by which load is multiplied through dynamic loading. Bazant&Zhou's result in 2001 was that 12 feet would very definitely result in an overload / buckling.
Case 4: Like case 3, except that columns no longer align vertically, instead the top is leaning out of plumb, and column ends either meet imperfectly or miss each other entirely, such that load bearing capacity is nowhere near where it was before. In this case, the multiple load of Case 3 is compounded by a fraction of capacity.
----
I hope you now will never again pretend that it hasn't been explained to you why the fact that the towers were able to stand as built (static loading) in no way predicts "no total collapse" once a 15-floor top part has begun to fall (dynamic loading, compounded by deteriorated alignment/loss of capacity).
Oystein
Senior Member
But - why?
Your brain is interesting here, because it is the only one that thinks that a comparison between the forces involved in swinging in the wind vs. collapsing under gravity would be enlightning. So: why? Please explain your thought processes!
Oystein
Senior Member
Okay, here it goes:
The swaying is very slow compared to falling: I don't remember with any precision, but the towers were naturally swinging at a rate of something like once in 10 seconds. Not sure if that is half a circle (going from right-most to left-most position) or full circle (right-left-back right).
Now let's say amplitude was several feet - 5 ft from right to left?
So average velocity would have been 5 ft in 5 or 10 seconds (0.5 to 1 ft/s or .15 to .3 m/s). Average acceleration thus anywhere from 0.15m/s / 5 s to 0.3 m/s / 2.5 s - the latter works out to (approximately) 0.1 m/s^2. That is (approximately) 1% of gravitational acceleration. And less (by a factor of 3 or so) than that than the acceleration the structure could exert vertically on account of there being excess capacity on a factor of 3 or so.
The two scenarios are thus separated by about two orders of magnitude. And more, when you consider that the falling top would not nearly encounter full vertical capacity to slow it on account of misalignment.
benthamitemetric
Senior Member
Ok... so, if you understand that not all dynamic forces are of the same magnitude or applied to the same elements of the building equally, what is it specifically about the the ability of the building to sway in wind loads that you think makes its floors, which were connected to the shells with specially designed viscoelastic dampers that allowed for the sway in question, capable of arresting a collapse of 6+ floors of the building? And how does the fact that we already know based on NIST's calculations that the floors were not capable of arresting such a collapse, which calculations build in any inherent strength added to the floors by the elements of the building that permit it to sway slightly under high wind loads, not render this whole thought experiment moot?
deirdre
Senior Member.
maybe a better question to ponder is WHY do they design buildings to sway. ??
Mendel
Senior Member.
they don't, the same way they don't design buildings to be vulnerable to fire
it happens naturally, and they have to design against it
econ41
Senior Member
Actually @Thomas B is not the only person to think that way. It was proposed by "psikeyhackr" with this model back in 2007:
Source: http://www.youtube.com/watch?v=z0kUICwO93Q
And for a couple of years the many 9/11 sites then active were flooded by "psikey's" obsession with the distribution of mass/weight of steel and concrete.
We were sparring partners for several years and I enjoyed some diversions into discussing the sonic aspects of oscillation of a flexible vertical cantilever with mass/weight concentrations distributed in a variety of arrangements. (uniform, tapered as per WTC Towers, big lump at top, big lump halfway up etc etc...the harmonic resonances being the point of interest.)
AND - the analogies to organ pipes and quarter wave ground plane radio antennas.
Much of the "wave mechanics" physics is common to all three. "psikey" was active on the Richard Dawkins Net Forum at the time I was a moderator. I think he has made occasional posts on Metabunk. And he is still focused on the oscillator aspects AKA "swinging in the wind".
But - yes - it is a derail.
econ41
Senior Member
That is actually a good question. Because it goes right to the foundation issues underpinning many of these misunderstandings of the applied physics of the WTC Twin Towers. In both "normal" operating mode and under the collapse trauma of 9/11.
Source: van Jaarsveldt, W.J. & Walls, R.S., 2016. Predicting the failure load of steel columns weakened to facilitate demolition of a structure. In A. Zingoni, ed. Insights and Innovations in Structural Engineering, Mechanics and Computation. Cape Town: Taylor & Francis, pp. 1190–5. https://www.researchgate.net/public...kened_to_facilitate_demolition_of_a_structure...there are safety risks when structures are weakened prior to demolition. If structures are weakened too much they may collapse prematurely killing workers, if not weakened enough they may not collapse when the demolition team requires them to.
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econ41
Senior Member
OK - let me respond to a few insightful comments that I have missed mostly due to Tme Zone differences.
This one by @Mendel is the first one that I consider significant
@Mendel quoted this part of my OP:
The second issue is far more important and adds a whole extra layer of confusion. We cannot know if the buildings would have survived the fires they were designed to resist. Because neither Tower has ever bveen tested under such a fire scenario, So I hinted at the second issue.
Now your scenario @Mendel incorporates two factors which are "otherwise changed nothing about the tower (structural steel and its fire-proofing remain intact)": That scenario is still well in excess of what the buildings were designed to resist. I suspect (Engineers gut feel guesswork) that the towers would still have collapsed if the fires had been as per 9/11 BUT without the initial impact damge. An interesting moot scenrio but one that I regard as impossible. To support that gut feel remember that the actual collapse started from the side oposite the initial entry and perimeter column failure was the probable trigger of the cascading failure of the initiation stage.
The additional complication is "were the desing rules at the time of building appropriates" - a complex topic which has already been subject of some unresolved discussions. One complication that I did not want to incorporate into this thread merely to get around the ambiguity of the original question.
This one by @Mendel is the first one that I consider significant
@Mendel quoted this part of my OP:
.. and asked me this:
I am not assured of the answer because it throws the scenario in the middle of the confusions I hinted at in the OP. There are two relevant confusing factors. Which I identified in my final comments. The first one explicitly sated:
Remember that the original question by @Henkka did not explicitly define the scale of fire. I chose to treat it as implied in the question i.e. the same fire as actually occurred minus the aircraft impact. And, since that fire is near enough "impossible" to create by other means I said NO. @Henkka has implicitly accepted that his "hypothetical" is not plausible when he invoked "magic".
The second issue is far more important and adds a whole extra layer of confusion. We cannot know if the buildings would have survived the fires they were designed to resist. Because neither Tower has ever bveen tested under such a fire scenario, So I hinted at the second issue.
Now your scenario @Mendel incorporates two factors which are "otherwise changed nothing about the tower (structural steel and its fire-proofing remain intact)": That scenario is still well in excess of what the buildings were designed to resist. I suspect (Engineers gut feel guesswork) that the towers would still have collapsed if the fires had been as per 9/11 BUT without the initial impact damge. An interesting moot scenrio but one that I regard as impossible. To support that gut feel remember that the actual collapse started from the side oposite the initial entry and perimeter column failure was the probable trigger of the cascading failure of the initiation stage.
The additional complication is "were the desing rules at the time of building appropriates" - a complex topic which has already been subject of some unresolved discussions. One complication that I did not want to incorporate into this thread merely to get around the ambiguity of the original question.
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Answering this will take us off topic, so I'll consider opening a separate thread. The short answer is that the NIST calculations makes it seem like the floor-to-column connections were the only thing resisting the collapse and therefore the only structural elements that needed to be destroyed. (The rest of the structure then collapsed under its own weight due to slender-column effects.) I need a better explanation for how the wind-resisting capacity (among other strengths) was rendered completely "moot" during the entire process. I think of the buildings as a tightly-bound, integrated structural system, not a stack of jenga blocks.
econ41
Senior Member
A good idea.
Confusion will continue whilst ever members keep treating NIST as the ultimate authority on aspects that have been better and more comprehensively explained.... The major factor limiting the "speed" of the collapse was momentum changes resulting from the accumulating mass of debris. The connection shearing forces were much less significant.
I know I'm wasting my effort saying so BUT ---- Step #1. Understand the actual collapse mechanism. What actually happened is easily understood. AND, since it is what happened, it must be incorporated into explanations whether or not CD is being claimed.
Mendel
Senior Member.
I understand NIST to disagree with you. NCSTAR 1-5 explicitly addressed the question, "Could an extreme but conventional fire, occurring without the aircraft impact, have led to the collapse of a WTC tower?" Their answer was, first, that a conventional fire would have less heat output than 9/11; and second, that the WTC wouldn't have collapsed from the 9/11 fire, had it not been damaged.
I do not think such a determination could have been made back when the WTC was being designed.NIST said no.
Article: 
See NIST NCSTAR 1-5, section 6.15.
econ41
Senior Member
Great. That is part of the reason I have never relied on NIST, Bazant et al as ultimate authorities, And I'm accustomed to other members ignoring me - especially when I explain things.
<< which is the main point of my comment. Those four factors. They seem to recognise that fuel concentration, accelrated start up over several floors would cause "more heat output". Without explicitly saying so. Remember also that NIST's explanations were not targeted at the detailed types of claims that truthers pursue. So I needed to make the issues explicit here for this audience.
... I doubt that is correct but I cannot prove it. It looks like a possibly "PR" comment.
It's part of that whole scenario of the evolution of design standards. Not just the "lack of computer capacity" for the structural design. But the problem that the regulatory rules will always be behind the leading edge of practice. Including learning by 'mistakes'. That other relevant thread seems to have died.
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Mendel
Senior Member.
Note also that the windows being blown out provided ventilation to the fire; it would have been air-limited, had that not happened.The jet fuel sharply shortened the time to involvement of all accessible combustible surfaces and, thus, the time to the peak HRR [heat release rate].
NIST set up their computers to run a fire simulation, and then to translate the results of that into mechanical values for their finite element analysis package. With this setup, it was easy for them to keep the simulated fire going past the collapse (and unaffected by it), so they did.
I believe their assessment is well-founded, inasmuch as their fire simulation reflects reality.
I still don't understand this objection... Even if it's implausible that the floors would be doused in fuel through some other means, we can still think about what would happen if they were.
Like for example, in the other thread I posed a hypothetical about a giant crane lifting up the top six floors of one of the towers, and then dropping them. Obviously, that's totally cartoonish and it would be implausible to build such a crane. But it's just a hypothetical to think about what would happen if the floors were lifted a bit and then dropped. You can switch out the crane for a wizard using magic to lift the floors, and the thought experiment is still the same.
Mendel
Senior Member.
this would be worth more if I didn't expect you to pivot back to "designed to be vulnerable" (omitting the "designed to be vulnerable by magic") forthwith
what happens to a real building in dreamland is not interesting to me
NIST's experiment is interesting because it shows that the mechanical abrasion of the spray-on fireproofing was necessary to bring the buildings down
Mendel
Senior Member.
econ41
Senior Member
Yes. Already answered. IF you can create the same fire scenario as happened on 9/11 without the structural damage I think collapse would occur. And I'm possibly out on my lonesome on that opinion. BUT the scenario is as near impossible as I can claim given the limitations of the protocols of the "scientific method".
Yes that impossible hypothetical can be discussed because it leads to a true dichotomy. The TWO issues that caused so much confusion viz (1) Did the Top Block drop to impact and (2) did the top block land with column ends aligned. Neither of those two occurred in the real event as has been explained many times. So the impossibility of the hypothetical has no effect on the reasoning. The columns neither "dropped to impact" NOR "impacted in alignment" which negates the effect of the hypothetical because it is irrelevant.
Au contraire BOTH extremes (and all the grey areas between) arising from your "magic fires" hypothetical are plausible. If the created fire was equal to or bigger than the real event fire - YES collapse would occur. BUT that "equal or bigger" depends on all four of the characteristics I identified PLUS the one I left out for simplicity of argument - the question(s) of insulation stripping. Remove any one of those four (or five) and the scenario becomes too hard for realistic speculation. THEREFORE the impossibility of setting up the scenario becomes the deciding criteria. So "NO" fire alone would not cause the collapse because you cannot achieve that fire as a deliberate effort. And I covered the issue by my repeated comment to the effect that ANY steel tower will collapse if the fire is big enough. And Oystein has stated that last point in agreement with me but in more hyperbolic language.
Oystein
Senior Member
For what it's worth: You are, if memory serves, and I am pretty certain it does even if details are a bit hazier, not alone. Eminent experts on this topic have opined on this, stating that the fires alone, unaided by mechanical damage from the plane crashes, would have brought the towers down:
James Quintiere opined in 2002 that the fireproofing (as built, not post crash) would have been insufficient: https://www.washingtonpost.com/arch...ollapse/9ce89208-8ef8-444c-8524-d65f8459e2b9/ (There are more experts names in this article who say similar things)
Wikipedia knows of some more experts who think fires alone could do the job:
Oystein
Senior Member
Thanks for the link, but doesn't answer much of the question at hand. The paper doesn't say whether this pre-weakening is
- done always, often, sometimes or rarely
- necessary (always, often, sometimes, rarely, or not at all) to facilitate a total collapse (or merely to save resources, or to control collapse sequence and where debris falls)
- done to facilitate collapse initiation, or collapse progression (we are here interested in that only), or both
In the other thread, the consensus seemed to be that an upper block of six floors was the minimum required to cause a runaway collapse. And this expert says that collapse could have been started by a major fire on three floors, even without any impact damage. So if we combine these two, we get a kind of minimum scenario to cause the total collapse of the building... The upper block would be floors 105-110. And below that, there would be a major fire on floors 102-104. That would cause the total destruction of the building, supposedly.
I guess everyone just has to decide for themselves if they find that believable or not.
benthamitemetric
Senior Member
There were two questions in that post. As is par for the course for one of your trolling thread diversions, you ignored the first and then deflected on the second. In any case, it should be obvious that an "integrated system" that is designed for one thing (e.g., absorbing wind loads) may not be suited for another thing for which it is not designed (e.g., absorbing massive impact damage to floor trusses). And, in any case, integrated systems by definition rely on individual components, which, if separately damaged, can damage the system in disproportionate/nonlinear ways. You don't even pretend to put forward an actual argument as to how or why the ability of the building to withstand windloads would also prevent the floor trusses from being broken in a collapse of 6+ floors; you are just in word salad mode at this point.
Back to the OP, though none of us can run simulations like NIST or ARUP, I think there is reason to be skeptical that the building would be able to survive 3+ floors of fully involved fire at a time. That's not to say it couldn't (and NIST provides a reasonable basis to believe that it could survive at least some such fires), and it may just depend on the exact parameters of the fire, but such a fire would be well outside the envelope of what the building's passive fire resistance was designed to handle. In such a scenario, the middle floor would be heated far more than in the typical compartment style travelling fire, which was the type of fire upon which its ability to continue to perform in a fire was premised.
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benthamitemetric
Senior Member
We're now talking about a hypothetical fire scenario that is so unlikely without an airplane crash (or a missile strike) as to basically be impossible: at least three floors being simultaneously fully involved in fire at the exact same time. That's not a common scenario at all and one that 99.9999+% of all tall buildings on the planet have never faced, and likely will never face. So it's very reasonable that the towers were not designed for and may not have been able to survive such a scenario (though, as Mendel has pointed out, there is good reason to believe the towers were nevertheless robust enough to survive at least some such scenarios).
As for the runaway collapse, you're just stuck on incredulity on this point, but perhaps you can think about it in reverse. How far down do you think the failed floor would need to be in order for a collapse to run away and why that floor and not the floor above it?
Mendel
Senior Member.
No. It's sufficient. It's not the minimum.
And it includes the damage from the aircraft.
Mendel
Senior Member.
Are there 3 million tall buildings in the world? ;-)
cue CD
benthamitemetric
Senior Member
Currently there are about 40k tall buildings registered with the CTBUH, so I was leaning heavily on the "likely will never face" when extending my decimal places, but point taken.
I don't know if a "runaway" collapse makes sense to me in general, if by that you mean a collapse that would have continued all the way to the ground no matter how many floors were below. I think it would decelerate and stop eventually... But that made think of another interesting hypothetical:
WTC 1 had 12 floors in the "upper block", and was overall 110 floors. Those 12 floors falling on the rest caused a runaway collapse that destroyed the entire building. But what would have happened if the upper block was the same 12 floors, but the tower itself was much taller? Let's say it was twice as tall, at 220 floors. If you dropped 12 floors on 208 floors, would that have destroyed this "Super WTC" tower in its entirety? Or would the collapse have decelerated and stopped eventually? In other words, we know that the upper block of 12 floors could destroy 98 floors below, but is there an upper limit? Or are you saying the 12 floors could theoretically destroy an infinite number of floors, since it was a "runaway" collapse?
benthamitemetric
Senior Member
There was nothing in the building capable of decelerating the floor collapses once they began with the critical mass of 6+ floors (based on NIST's *very conservative* calculation). Every subsequent floor below the collapse wave would fail, just like the floor above it. So, yes, so long as the mass falling within the original perimeter of the structure remained as large or became larger than the original falling mass that collapsed the first floor, it would propagate forever, just as dominos can keep going forever. I know this has been beaten to death, but there is no actual reason to think the collapse would decelerate. The height of the tower doesn't matter; given the apparent collapse mode, it's the strength of the floors that matter and that strength was constant all the way down. I think you would prove this to yourself in the abstract if you complete the thought experiment of thinking through how many floors it would take to cause a runaway collapse. And if you don't think there is any amount of weight that could destroy the structure if dropped upon it, then I don't know what to tell you.
Well as I said, I don't know if a "runaway" collapse makes much sense to me, but I know that in verinage demolitions they usually start the collapse in the middle. So if you dropped the top 55 floors on the bottom 55 floors, they would maybe destroy each other simultaneously. But in verinage demolitions, the initial drop is deliberately engineered to give the top block significant momentum, and then it comes crashing down. If all you had was fires slowly weakening the steel on floors 54-56, I don't know why the top block would come crashing straight down, rather than maybe tipping over or such.
Gamolon
Active Member
Do you agree that when the upper block of 12 floors impacted the lower block of 98 floors, the bottom floor of the upper block was destroyed along with the top floor of the lower block?
So that after that initial floor to floor impact, you come away with the upper block consisting of 11 floors and the debris from it's now destroyed 12th (lowest) floor and the lower block now consisting of 97 floors and the debris created from it's now destroyed 98th (top) floor.
At this point, the upper block becomes 11 floors + the debris from it's destroyed 12th (lower) floor + the debris from the lower block's 98th floor. The upper block has now become 13 floors (11 intact floors + 2 floors worth of debris) descending upon the top floor of the lower block which NOW is 97 floors instead of 98.
Gamolon
Active Member
Because when you weaken structural components the load they carry has to go elsewhere. The increasing load from failing/weakening components increases to a point that fails everything else in rapid succession.
Loose debris is not going to deliver the same kind of impact as an intact block... It's like, would you rather have an intact brick dropped on your head, or an identical brick that has been ground up into tiny pieces first?
In your scenario, there would be nothing but loose debris left after 24 floors had been destroyed. The remaining 86 floors would rather easily stop its downward progression.
benthamitemetric
Senior Member
This is an example of physics by intuition gone wrong. The collective mass of the floors will impart a similar force whether acting as a single piece or multiple pieces hitting nearly simultaneously. The total kinetic energy of the falling mass is the same. The floors may lose rigidity as a whole, and some energy will be lost to heat, etc., but the pieces of the floors are still transferring most of their kinetic energy straight down at each successive impact, which will continue to successively overload the floors below, all while the aggregate mass that is falling (and the force it can impart) actually increases on net with each added floor.
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Gamolon
Active Member
Tiny pieces?
You make it sound like everything that made up each floor was ground into powder. What about the steel columns and beams? Floor trusses? How about the electrical panels for the elevators? Piping/fittings?
How is my example of a brick wrong, though? A brick weighs about 2kg, and the same brick chopped up into pieces will weigh the same. Yet it's obvious which one you would rather have dropped on your head.
I think Bazant understood this problem, which is why he insisted the upper block stayed intact during the "crush-down" phase, and then got crushed itself during the "crush-up" phase... Like a car crash where one car becomes deformed first, and then the other one, haha.
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