The south wall at the beginning of collapse. Building features tilted less than 1 degree to the south the core and all 4 perimeter walls failed completely.
Complete failure has already occurred by these moments:
The dust emerges when the northwest corner has only fallen a few feet.
The tilt angle varies from 0 to 2 degrees over this interval, meaning the south wall has fallen only about 12 feet by this time in the region of floor 98. The ejections emerge from around the 89th floor. They are light grey, not darker colored like the smoke and ejections above. South Wall Model
1) The wall fails through the red line.
2) The wall above the failure line falls out and over the wall below. This means that large perimeter pieces from above the failure line go into freefall and cover the floor-by-floor ROOSD progression from view.
3) Sheets from the middle section are not visible, but there was no large continuous sheet falling away as there was on the north face. The middle section must have fragmented somewhat because it doesn't fall far away from the building. It drops relatively near to the base of the building.
4) The middle fragments down to the upper edge of the lower MER level. The Lower section then falls out as a sheet.
Drop of the Upper Section
Things to consider:
There is a large piece of the west wall just above the failure line that goes into free-fall. Can you see it emerge from the dust yet? (No.)
Using this image you can already know that WTC1 south is nothing like WTC2 east. Recall, in WTC2 east you could see smooth ROOSD progression clearly because the top section folded behind the middle section.
Can you see a clean unobstructed ROOSD progression? (No.)
Do the west and south ejections look similar? (Yes.)
Why? Because there are large sheets of perimeter in free-fall within the dust blocking any view of a smooth ROOSD progression. You cannot see them emerge yet but they are there all the same.
The large west piece in free-fall will fall together with the large pieces in free-fall from the south wall. How is that possible? Because they both broke away from the building at the same time from the same elevation (just above the failure line).
This motion is impossible unless the sheet above the failure line falls out and over the lower sheet during the initial failure sequence.
Perimeter sheets emerging from the dust
Original here Drop of the Middle and Lower Sections
2 sets of high resolution photographs provide the best record of movement of the middle and lower sections
Notes on first set:
Middle section clearly destroyed by photo #6
Drop of lower wall must be during photos #6 to #13
Smoke movement is consistent with a large sheet dropping
WEST VIEW - Jennifer Brown
Images DSC_0065 through 0084 are missing. They show probably the first half of the collapse. Either not received from original source or not released by NIST.