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Please try to avoid associating this question with baseless controlled demolition theories

enter image description here

I suppose the former 7 World Trade Center and it's fate does not need an introduction.

The progressive collapse started with the failure of 2 or 3 columns in the east end of the core at the lower levels. This caused the floors supported by these columns to cave in due to the bean connections being unable to redistribute loads. This is the official version, and is quite intuitive to the layman. It can also be deduced from the observed motion of the eastern penthouse.

What is confusing is what happened next. After 5 seconds of nothing happening, the whole building suddenly collapsed.

We can tell that all the remaining core and exterior columns failed near simultaneously by observing the motion of the western penthouse. This penthouse, just like the other one, was supported by the core and had no connection to the exterior framed tube other than the floor beams. Yet it moved almost in sync with the facade.

The official version holds, and common sense suggests, that these two events are causally connected. But how?

What caused all of these columns to buckle at once?

  • The fire was not hot enough to weaken any them at any point in time, according to the official version.

  • The floors bracing them remained mostly in place, as can be seen in the NIST simulations.

enter image description here

  • They were not all hit by falling debris because nothing was falling around them.

Then why did the the columns shown in the above screenshot buckle?

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3 Answers 3

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Bolts attaching gussets to columns failed in shear. The horizontal beams were attached to vertical columns with bolted gussets. Those bolts on the upper floors were heated by fire until they failed in ductile shear, it could be called stress rupture. The first floor to drop doubled the stress on the next floor. When the second floor failed, the stress on the third floor bolts was three times design and they were heated causing reduced strength. By the time this process repeated about five times ,the weight of the upper floors put so much load on the bolts that they failed even if they were not weakened by heat.

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  • $\begingroup$ Thanks, but: This is the initial local collapse of floor 14 through 7 in the east which lead to the buckling of columns 79, 80 and 81. This is beyond the scope of the question. $\endgroup$
    – Abdullah
    May 25, 2022 at 2:59
  • $\begingroup$ @Abdullah Then what are you looking for in an answer? The computer modeling that was done in the official report showed a viable process for the reaction of the structure. This answer helps to show how a small part of the structure would behave. At some point as support is lost, members will be loaded past any mechanical strength that they have and fail "instantly". It sounds like you are looking for one specific answer. $\endgroup$
    – hazzey ♦
    May 25, 2022 at 12:39
  • $\begingroup$ @hazzey see the edited OP $\endgroup$
    – Abdullah
    May 25, 2022 at 13:01
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The simplest explanation of "progressive collapse" is depicted below. Depending on the resistance of the remaining columns, the complete collapse (due to loss of an edge column) may occur instantaneously, over a long time, or never occur.

For the case in question, the impact of the airplane, the explosion, and the heat from the burning fuel, all have significant contributions to the final fate of the building. But, I would claim that the design and construction are rather impressive, otherwise more fatalities would be the result.

enter image description here

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  • $\begingroup$ This is about WTC7 $\endgroup$
    – Abdullah
    Jul 22, 2022 at 9:14
  • $\begingroup$ This explains "progressive collapse" for all structures, what is WTC7 special about? $\endgroup$
    – r13
    Jul 22, 2022 at 13:38
  • $\begingroup$ This explanation assumes that the total gravity load is constant, which was not the case with WTC7 $\endgroup$
    – Abdullah
    Jul 22, 2022 at 13:48
  • $\begingroup$ Unless it is not on the earth, disregard the minor differences, how could gravity is not constant? $\endgroup$
    – r13
    Jul 22, 2022 at 14:57
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    $\begingroup$ I suggest to read an engineering mechanics book to gain some basic understanding on structural equilibrium and stability. Good luck. $\endgroup$
    – r13
    Jul 23, 2022 at 10:56
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one month and no answer that even seems to comprehend the question? Okay, let me give it a shot.

The failure mode was "crippling" of the framed tube due to loss of rigid connection with the east wall.

Consider Euler's buckling equation: slenderness² × strain = π².

Both perimeter and core columns were made mostly of W14 sections with similar depths to the width, with web plates or cover plates up to 8 inches thick. This means that the columns were up to 30 inches wide at most. This sets an upper limit for the gyradius at 15 inches or 1.25 foot. enter image description here

The columns were mostly made of steel of 36ksi and 50ksi. So let's assume an average stress of 15ksi. This gives a strain of 1/2000. So a slenderness of 140.

The above gives us a buckling length of 185ft in a 630ft building. This form of the equation is valid for pinned columns (which is not applicable) and fixed-guided columns (which could be applicable for the tower twisting bodily).

This is exacerbated by the fact that the tensile strength of the welds holding the core columns together was low.

(See here and here for details)

Therefore, it can be said that after the caving of the east floor area, WTC7 behaved as a box column which had lost one of it's flanges. That this crippling was the buckling mode is supported by the NIST simulations, which show the tower twisting south by "over 1m".

enter image description here

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  • $\begingroup$ You missed addressing your bean connections. Many of us are waiting. $\endgroup$
    – Solar Mike
    Oct 3, 2022 at 19:26

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