To the OP:
I started a thread on this years ago, when NIST released their report on WTC7. I don’t have the thread handy but trust me when I say that we have done this topic repeatedly and in some detail. Trust me also when I say that you are not presenting anything new, nor are you asking questions that have not already been done to death here and elsewhere.
Before we get into this yet again, I’ll give you my “credentials” for being able to have a reasonable discussion; I have my MS in structural engineering from one of the top CE schools in the world. I don’t claim to be an expert but I have the background to be able to understand what’s going on.
What is your educational or work background in any relevant field such as structural engineering, fire safety, architecture or even (sigh) controlled demolitions? Taking a college physics class doesn’t make you an expert.
You talk about the NIST report but as others have said, it doesn’t sound like you have actually read it. I did, all 130-odd pages. The abstract that you cited does not include the committee’s summary so let me post that. It’s really simple to understand and it’s thoroughly discussed in the body of the report.
From p25 “The Leading Hypothesis”:
-The conditions that led to the collapse of WTC 7 arose from fires, perhaps combined with structural damage that followed the impact of debris from the collapse of WTC 1. The fires were fed by ordinary office combustibles.
-The fires on Floors 7 through 13 heated the building structure. Being lighter than the columns and with thinner SFRM, the floor beams, floor slabs, and connections heated more quickly and to higher temperatures than the columns. The elevated temperatures in the floor elements led to their thermal expansion, sagging, and weakening, which resulted in failure of floor connections and/or buckling of floor beams.
-Sufficient breakdown of connections and/or beams resulted in loss of lateral support and buckling of at least one of the critical columns supporting a large-span floor bay on the eastern side of the building on or below Floor 13. This was the initiating event of the collapse.
-The initial local failure progressed upward to the east penthouse. As the large floor bays became unable to redistribute the loads, the interior structure below the east penthouse collapsed into WTC 7.
-Triggered by damage due to the falling debris and the loss of lateral support to adjacent interior columns, the failure progressed westward in the region of Floors 7 through 14, where the floors had been weakened by fires. This ultimately resulted in the collapse of the entire structure.
Let me summarize this a bit:
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WTC7 caught on fire. Ordinary office contents fed the flames. WTC7 may have also suffered structural damage when WTC1 fell down nearby.
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Fires heated a number of the structural steel elements. The floor members in one area (7th through 13th floors) were thinner and had less fire resistant coating than the columns and so those horizontal members got hotter, faster compared to the columns.
2a. When steel gets hot it expands. The floor members expanded which weakened the structure in that area, caused failure in some connections and buckled floor beams.
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Enough stresses from (2a) resulted in the loss of lateral support (bracing) of one of the long columns that was supporting a large-span floor. When a column loses lateral support the buckling load of that column (the axial load which will cause the column to fail in a particular manner) drops noticeably. Support was lost and the column failed. This started a local collapse in that part of the structure.
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That local collapse spread upward. As the collapse continued and more of the structure was lost, the remaining parts of the building had a harder time supporting the additional load - no alternate load path. This resulted in a chunk of the east part of the building collapsing into WTC7 itself.
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Failure progressed to the western part of the building in the already-weakened floors 7-14 area and eventually the whole building was lost.
Do you have any questions about any of that?
I don’t know what the constant refrain of “free fall time! 6s!!!” is supposed to mean. There’s a table in the report (it’s on p43) showing observed times of various events in the collapse. When the east penthouse began to fall is time 0sec. At 9.3 sec the west penthouse fell below the roofline. First of all it’s not like the whole building fell down in one single splat and second, what do people think is going to happen when you drop debris? Of course some of it’s going to fall at “free fall” speeds, it’s falling free. Some of it may not because the structure underneath is collapsing which takes time (admittedly not a lot of time) - it’s not as if it vanished in the blink of an eye.