In a nutshell ☝️ (report p. 22)
HOW DID WTC 7 FALL?
Diagram 1 – Typical WTC 7 floor showing locations of columns (numbered). The buckling of Column 79 was the initiating event that led to the collapse of WTC 7. The buckling resulted from fire-induced damage to floors around column 79, failure of the girder between Columns 79 and 44, and cascading floor failures.
This article gives a rare, two sentence answer, which I’ve interspersed with some more explanations:
“The heat from the uncontrolled fires caused steel floor beams and girders to thermally expand [NP: this “thermal expansion” was an exposed weakness of the construction or materials], leading to a chain of events that caused a key structural column to fail [NP: this was the second majorly exposed weakness, an Achilles heel apparently for this building (see below)]. The failure of this structural column then initiated a fire-induced progressive collapse [NP: this one column’s collapse triggered two adjacent columns to collapse, which left the entire East side of the building without any interior support, so the dominoes toppled thereafter] of the entire building.”
Was the collapse of WTC 7 a unique and unexpected event? Yes. But man-made structures have been failing expectations since the Tower of Babel. There were indeed a couple of major design defects in the structure of WTC 7 that were exposed by the uncontrolled fires that day: One was Long-span floor systems, on which see the section: “What improvements to building safety [have been learned] … ” to thermal expansion. Secondly, it was found that there were key columns in this structure which, if they failed, would lead to a progressive collapse of the whole structure (which is certainly a design defect as stated below).
Look at the diagram below at the left (East) side of the picture and locate support column 79, as well as columns 80 and 81 behind it (South of it). NIST’s conclusion is that the floor in the vicinity of that ‘loner’ column 79 collapsed at the 13th floor, and that this triggered a series of floor collapses below it down to the 5th floor. As NIST states: “This collapse of floors left Column 79 insufficiently supported in the east-west direction over nine stories. The unsupported Column 79 then buckled and triggered an upward progression of floor system failures that reached the building’s east penthouse [at the top of the building].”
This is not just some arbitrary description. Any casual viewing of the tower’s collapse reveals that a major fault or ‘kink’ travelled all the way up the building, right in the vicinity of column 79. Progressive collapse events continued from that point on.
“NIST studies have shown that because of the large floor areas, the failure of just one (key) column on any one of the lower floors would cause a vertical progression of collapse so that the entire section would come down. (NIST, S. Shyam Sunder lecture) This single-column’s failure initiating progressive collapse is a design defect noncompliant with the NYC codes. The buckling of two or more of these three key columns (79, 80, and 81) would have removed support for all columns directly above, putting all upper floors in immediate longspan suspension with eventual collapse. The breaking of the widows on the east side of the north face simultaneously with the buckling of the roof shed was evidence of this tension as the north wall was pulled and leaned in.” (source)
Does this really look like a controlled demolotion?!
The video of the collapse we’re all used to seeing is always shared as if it somehow strongly supports that a controlled demolotion occured. I get it: It’s a stunning event, and untrained eyes (most of ours, mine included) don’t recognize the difference. Adding to this is that once the entire structural collapse began, the lowest floors we can see is between floors 30-40, so we are most definitely not seeing the whole thing. It again makes it easy to think the whole building just collapsed from the floor level like you would expect to see in controlled demolotions. But in fact, when we focus on the momentus events that occured many seconds prior to global collapse, we can see that this looks nothing like a controlled demolition! (Ok, I’m no expert either, but I don’t think this is the common way to demolotion buildings!)
The video clearly shows what the NIST report finding states: that column 79, or at least in that general vicinity, first collapsed many seconds before a progressive collapse began to propogate. It’s also obvious, now that you’re looking for it, that a downward kink forms in the vertical area of that initial collapse, which propogates vertically across the whole building, which strongly illustrates that a collapse of support pillars in that part of the building had occured. According to NIST’s reconstruction, at this point (after the penthouse sank out of view), the entire interior of the east quarter of the building had completely collapsed (the penthouse fell because everything under it fell).
“The exterior facade on the east quarter of the building was just a hollow shell” (cited from the full report referenced above, page 22 (PDF page 64)).
But what about that explosion sound? Though I am of course not qualified to give an engineer’s answer, this seems to fit the NIST reconstruction nicely. Namely, floor 13 fell within the vicinity of Column 79; that collapse caused the progressive collapse of the floors below it on down to floor 5 (which was a much stronger floor, which is why it did not give). And thus we have significant portions of 8 floors collapsing – is that not going to cause an explosion? This must be emphasized. Would not the objectors claim that the NIST proposed reconstruction is clearly bogus, if they were to claim that major portions of 8 floors collapsed, 8 FLOORS!, but that this didn’t cause an explosion to be heard? The collapse of these floors was immediately followed by the buckling of column 79, which led to the collapse of columns 80 and 81 (this is the essense of a progressive collapse, where one dominoe’s fall causes the next dominoe to fall, and so on). “The downward movement of Column 79 led to the observed kink in the east penthouse, and its subsequent descent” (NCSTAR 1A, p. 22).
Undamaged? Minor office fires?
Here are some snapshots of the ‘undamaged’ and ’not hit by a plane’ World Trade Tower 7. Read the firefighter interviews to hear it in their own words, which thoroughly corroborates that the building was actually quite damaged, and that they were even fearful it too would collapse. If it had been designed better it shouldn’t have been in danger of collapse, but…
