My video about freefall of Building 7 is #18 in the series (see my signature below). JSanderO's explanation above is interesting. I'm not sure I've heard this. I did think that the outside perimeter wall was more than just a "curtain," that it was attached to structurally relevant perimeter columns. And the theory I put out is not so much a rotation theory as a theory of leveraging, much as explained above by LSSBB. And it's certainly not "my theory," as I am a journalist not a scientist/engineer. Without a lot of help from other people here I would NEVER have come up with an explanation f faster-than-freefall on my own.
Chris,
7wtc *facade-skin* is a rather common assembly. This is very different from the twin towers.
A typical high rise building has a curtain wall. It's non structural, non load bearing and is attached to the frame... whether the frame is concrete or steel. Curtain wall are usually aluminum and glass w/ assorted sold panels of aluminum, painted glass, thin gauged stone, even thin "brick".
Masonry such as brick... self supporting and load bearing is too heavy for high rise construction. When it's used it is supported at various elevations by steel which is connected to the frame. So it plays no role in load bearing and is in fact carried by the frame.
Same is true for concrete. It would me a very thick wall at the base to support the concrete all below. I worked on a 10 story brick and steel building recently where the brick was over 24" thk at the lowest level and 12" at the roof parapet... stepping back in thickness as you go up.
7WTC was a non structural curtain wall connected to the steel spandrel beams, which in turn were connected to the columns. The connection system is with steel angle clips with slotted bolt hold for positioning to create a plumb and true facade.
The inward bowing of the north facade is a tell tale indication that the floors and north south beams were no longer connected to the columns and spandrels. There was a floor beam connected to the spandrel and one to each column. On top of the floor beams was the metal deck with concrete.
For the entire north facade to fold inward creasing between column lines 47 & 46 - the *kink* would mean that all the beams and the slabs were not there and unable to resist the inward bowing. There was an addition floor beam 24wf55 connecting the column line 76 which was connected to the west end of TT#1 running out the the spandrel beams on EACH floor. This 24wf55 is what PULLED column line 46 and 46 at the building's perimeter inward.
Running west from column 76 was a series of beams connected to col 73 then 70, then 67, then 64, then 61, then 58. Col 58 sat on the north end of TT3 which was cantilever.
When TT1 collapsed... it pulled col 58, 61, 64, 67, 70 and 73 off the same columns at floor 5-7. All 7 columns... 59-76 collapsed as TT1 did. When those 7 columns collapsed they beams, and the E-W girder collapsed with them TOWARD the core. I believe the collapse proceeded east to west and this caused the kink at the line between col 76 and col 47.
A similar inward pull was created going east of the line of col 76-47. This included perimeter column 42 (NE corner) 43, 44, 45, 46. Those 5 columns were north of TTI and the girders connecting TT1 to columns 79, 80 and 81.
TT1's collapse caused cols 79, 80 and 81 to be pulled off the same columns below floor 5 and this was what we see at the collapse of the East penthouse right through the entire building.
Sequence:
TT1 collapses
TT1 pulls via 24wf55 at col 46 and 47 breaking the breaking their connecting spandrel in the process. This separates the north frame into two sections... one west of 47 and one east of 46
TT1 pulls the e-w girder connecting 76 to 58 and the 7 columns off alignment and they drop... east to west progression.
Floor slabs break and drop east to west progression beginning at line of 76-47... and west to east at line 76-46
TT2 collapses and all the floors above it and those supported on col 79.80 and 81 collapse down
TT1 pulls TT3 off the columns which supported it. All the floors above TT3 out to the west facade break free and collapse east toward the core.
The collapse of TT1 and TT1 pull the east braced frame loose from the columns 28-41 above at floor 7
The collapse of TT3 pulls the west braced frame loose from the columns 1-15 above at floor 7
The core collapses and it pulls the beams and floors to the south of it supported at the south perimeter on columns 16-27.
The entire floor system has collapsed inward to the core. The frame below floor 7out to the curtain wall has collapsed inward separating it from the 58 perimeter columns above flr 7 which then comes down as a hollow shell...with curtain wall attached.
It's likely that the collapsing floors were just a but ahead of the descending curtain wall. The time being the time it took for the kink to manifest. Watch the vid...as the north facade goes from flat to bowed inward is the time of the collapse/separation of the floor system behind it.
No it was not the collapse of a BUILDING seen, but a collapse of the curtain wall and the frame it was connected to.
)