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Since the release of the Final Report on the Collapse of the World Trade Center Towers in October 2005, NIST has received many questions from interested readers curious about NIST’s findings and the technical basis for them. The complexity of the investigation and the length of the final report (including all of the supporting volumes) have made understanding of the investigation a challenge for many interested readers. In response, NIST has prepared simplified answers to the most frequently asked questions.
The vertical capacity of the connections supporting an intact floor below the level of collapse was adequate to carry the load of 11 additional floors if the load was applied gradually and 6 additional floors if the load was applied suddenly (as was the case). Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings.
[...]
Since the number of floors above the level where the collapse initiated, exceeded 6 for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated.
Originally posted by bsbray11
Can someone explain how the first floor failed, and why the floor immediately beneath it would also fail when NIST just stated themselves that a floor could withstand about 6 under dynamic loading?
Yes, there was more than enough gravitational load to cause the collapse of the floors below the level of collapse initiation in both WTC Towers. The vertical capacity of the connections supporting an intact floor below the level of collapse was adequate to carry the load of 11 additional floors if the load was applied gradually and 6 additional floors if the load was applied suddenly (as was the case). Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings. Details of this finding are provided below:
Consider a typical floor immediately below the level of collapse initiation and conservatively assume that the floor is still supported on all columns (i.e., the columns below the intact floor did not buckle or peel-off due to the failure of the columns above). Consider further the truss seat connections between the primary floor trusses and the exterior wall columns or core columns. The individual connection capacities ranged from 94,000 lb to 395,000 lb, with a total vertical load capacity for the connections on a typical floor of 29,000,000 lb (See Section 5.2.4 of NIST NCSTAR 1-6C). The total floor area outside the core was approximately 31,000 ft2, and the average load on a floor under service conditions on September 11, 2001 was 80 lb/ft2. Thus, the total vertical load on a floor outside the core can be estimated by multiplying the floor area (31,000 ft2) by the gravitational load (80 lb/ft2), which yields 2,500,000 lb (this is a conservative load estimate since it ignores the weight contribution of the heavier mechanical floors at the top of each WTC Tower). By dividing the total vertical connection capacity (29,000,000 lb) of a floor by the total vertical load applied to the connections (2,500,000 lb), the number of floors that can be supported by an intact floor is calculated to be a total of 12 floors or 11 additional floors.
This simplified and conservative analysis indicates that the floor connections could have carried only a maximum of about 11 additional floors if the load from these floors were applied statically. Even this number is (conservatively) high, since the load from above the collapsing floor is being applied suddenly. Since the dynamic amplification factor for a suddenly applied load is 2, an intact floor below the level of collapse initiation could not have supported more than six floors. Since the number of floors above the level where the collapse initiated, exceeded 6 for both towers (12 for WTC 1 and 29 for WTC 2), neither tower could have arrested the progression of collapse once collapse initiated. In reality, the highest intact floor was about three (WTC 2) to six (WTC 1) floors below the level of collapse initiation. Thus, more than the 12 to 29 floors reported above actually loaded the intact floor suddenly.
Originally posted by CaptainObvious
NIST explains it here:
Since the number of floors above the approximate floor of collapse initiation exceeded six in each WTC Tower (12 and 29 floors, respectively), the floors below the level of collapse initiation were unable to resist the suddenly applied gravitational load from the upper floors of the buildings. Details of this finding are provided below:
Originally posted by Griff
Originally posted by CaptainObvious
NIST explains it here:
Not really.
Originally posted by Griff
How did 6 or more floors pancake together to fail the floor below it?
How did 6 or more floors suddenly fail when their connections could hold 6 or more floors of dynamic load?
Plus, it still doesn't answer how the columns would buckle.
I have lost faith in NIST.
Originally posted by MikeVet
wouldn't you say that the collapse would most likely happen in a chaotic manner that can't be imagined by thinking of a floor cleanly break off and smoothly fall onto the floor below?
Originally posted by bsbray11
If you want to think about it more realistically, then how does a whole floor fail at the same time anyway? Because you know the connections on both sides of most trusses were completely independent from each other, right? So one of them sagging and etc. will only result in a localized failure (and the technical definition of failure actually does not mean completely coming apart, but any kind of permanent deformation, for example, because the structure actually behaves as the bolted-and-welded unit that it is when heated), not a failure that spans across the entire floor simultaneously and includes apparent total failures at all the bolts on both sides of the trusses simultaneously (AND the core structure completely failed AND the perimeter columns started falling all at the same time in WTC1), unless the whole floor is expected to heat up/be damaged symmetrically and all reach some theoretical critical point at the same time. Does that not make sense? How else could it be, given that most of the trusses were totally independent on each floor? How can they be expected to behave together regardless?
Originally posted by MikeVet
The vertical loads are held by the columns, yeah? So.... if the buckling DID occur - by, as Griff says, some unexplained means - would it matter if floors failed or not at that point?
I don't understand why EVERYTHING would need to fail to initiate the global collapse.
Again as a layman, I envision that if they needed......say 20 intact columns out of 40
Also, I could envision the global collapse not necessarily happening on a single floor. I could see some core columns failing on this floor, and others failing on the above and below floors, etc. I just don't see it in that uniform, orderly, floor by floor collapse.
Originally posted by bsbray11
Can someone explain how the first floor failed, and why the floor immediately beneath it would also fail when NIST just stated themselves that a floor could withstand about 6 under dynamic loading?
Originally posted by bsbray11
Considering that the given global collapse theory is that the floors were what "domino'ed" all the way down to the base, yes. Columns do not telescope down into themselves.
No one said it did. I just pointed out that the core and perimeter columns in WTC1 did fall in unison at the collapse initiation, which is supposedly when the floors also started falling. You can tell the entire core structure failed at that instant because the antenna start dropping immediately, and it was supported by the core structure directly below it.
That would be a safety factor of 2, speaking very generally. It's equivalent to saying that you would have to double the load on each individual column before it would reach its design capacity. A safety factor of 2 is something you'd expect in a home or commercial building where everything is routine and well-understood. The WTC Towers were innovative and, at the time, the tallest skyscrapers on Earth. One of the engineering groups working on the project in the 1960s stated once that some of the perimeter columns in the structure had a safety factor of 20. That means, for that particular area of the building (which was probably in the higher floors, since the perimeter columns were about the same size at the top as at the bottom, but the bottom ones had to carry a LOT more weight) 1 column had the capacity to carry what was actually divided up amongst 20 columns. I've seen the factor of safety for the perimeter columns generalized at 5 (some kind of average taken from each floor?) and the core structure at something like 2.25, but these are both from NIST and they don't detail how they got those figures.
Then doesn't this symmetry strike you as odd?
Originally posted by MikeVet
1- the official theory says that the columns telescoped? I thought the big gripe is that it wasn't explained thoroughly. Isn't there some other way that the columns would fail if their vertical capacity is exceeded? Buckle? Bend? Snap? Some other term?
2- well, i would envision a global collapse involving all the core and perimeter columns, so i don't see anything unusual about that. Otherwise, it wouldn't be a global collspse, but some kind of local collapse.
Also, since the floors are attached to the core and perimemer columns, they would fall in lock step when the columns fell, yeah? Nothing unusual there either, IMHO.
3- so i think you agree that ALL the columns wouldn't have to be totally compromised.
Originally posted by bsbray11
Yes but none of those failures have anything to do with a straight-down collapse. They would just fail at one floor, everything above that would be unstable, and everything below the buckle would be intact. Tell me what happens to the columns to allow the building to just plummet straight down at such speed.
That's semantics. Tell me HOW it happened.
The core columns are what holds up the floors (on one side), not vice versa. If you yanked a floor out, only the CONNECTION to the core column would fail. Not the entire freaking core column. This has to do with which is stronger and which would reach its ultimate strength first. Which do you think? The connections were tiny compared to the columns. Under dynamic loading especially, the connection would shear right off.
A very large portion of them would have to be totally failed, which is unrealistic given the circumstances. The planes only took out ~
Originally posted by MikeVet
1- well, i would imagine that once the global collapse initiated, the "unstable" floors, columns, whatever else would crash down onto the "intact" floors, columns,whatever else , and game over.
2-how it happened? The load bearing capacity of the columns was exceeded through a combination of physical damage - planes - and weakening of the steel - fires. Then the scenario described above ensues.
3- i think you're misunderstanding me. I'm not saying the floors pulled down the columns. I agree with you about connections, etc. What I said is that when the columns failed and went down, they would pull the floors down.
4- i think you're basing your "very large proportion" statement on the floors load bearing capacity of 6/11?
Because I would think that if you were to gently place 11x the normal load on a floor,
6- by asymmetrical, I'm referring to columns failing on different floors,
Originally posted by bsbray11
The way you conceptualize this is so vague it's no wonder that you aren't sure about it. The core would not be falling weight. The perimeter columns would also not be falling weight, they would just be shifting loads. None of the floors above the first floor to break away would be falling weight. The only falling mass could only be the first floor to break away, by whatever means it breaks away.
Why would the core columns go man? I'm talking mainly about the core structure.
6/11? I'm talking about a safety factor of 2 meaning 50% of the columns would have to be compromised, and that's a low safety factor, but the planes only knocked out 15% of the columns across all of the floors they hit. The fires would have to do 4x the damage to make the buildings reach their critical point, IF they were built to that low of a safety factor.
11x the normal load? Where is that coming from out of nowhere to bear down onto a single floor?
Yeah well I shouldn't have to tell you where exactly to look for the symmetry. If you can't tell me which way WTC1 was leaning and give me a measurement in degrees for it then I would think we would have to agree that there is some symmetry there, no? The building dropped straight down at first, everything at the same instant.
Originally posted by eyewitness86
The CORE turned to dust exactly the same as the rest of the Towers did: Only steel was left in the rubble pile, the rest was turned to dust, basically.