Material is clearly moving as 2 distinct groups. The large group of
objects in free-fall is the west wall from just above the failure line
along floor 98.
All 3 main features of the drop of the west wall are seen in this image.
1) The upper wall falls out and over the lower wall during the initial failure (falling upper wall is seen in the next image).
2) The lower wall falls outward as a large unit. The building opens like a tulip.
3) The collapse front is uneven along the west wall. The south side
leads the north side by 10 to 20 stories. This means the south part of
the wall is stripped from the building considerably earlier than the
same wall on the north side. THis uneven stripping creates a twisting
in the wall as it falls away.
West wall above floor 98. Notice the straight break along the lower spandrel (fl 99):
These falling west face objects are tracked in detail here and here.
Slow motion clip showing the west wall falling outwards as a large sheet.
The effect of the perimeter peeling is obvious in this image of the region to the west of the WTC1 footprint:
This image
shows people standing on the angled sheet, including the photographer.
You can see that the sheet has its outside facing upward.
Dropping of the west wall.
The 3rd diagram shows how large sheets fell away from the building and fragmented.
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1) The failure line goes through floor 98 (red line)
2) The upper wall fell out over the lower wall in a few large pieces
3) Collapse propagation travled down the south side (right) ahead of the north side by 10 to 20 stories
4) Bottom part opened as a tulip down to the lower MER green stiffening
section. It breaks under its own weight while opening. and it twists
as a result of uneven stripping.
Photo archive of debris in the area west of WTC 1 here
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