A very large piece of perimeter is hidden in just inside the dust about to emerge. The northeast corner has broken away from the building and gone into free-fall (left).
The piece emerges below, about 11 stories tall.
This piece is from above the failure line. The upper wall fell out and over the lower wall. The north wall dropped in 4 stages:
1) Wall above failure line falls out and over the lower wall.
2) Upper wall below the failure line falls outward in pieces down to the stiffer MER panels.
3) The middle section falls outward from fl 78 down to fl 44 in long pieces.
4) Lower section falls.
Model of the North Wall
The wall drops in at least 3 noticable stages
1) Initial failure is along the red line
2) The pieces above the failure line fall outward and over the pieces under the failure line. They are quickly stripped from the building and go into free-fall The yellow rectangle is largest piece shown above. northeast corner is light blue, northwest corner darker blue.
3) Large pieces from below the failure line break away and fall down to the upper edge of the 75-77 fl MER stiffening belt (purple rectangle).
4) The middle section falls outward as long sheets, seeming to pivot outward from above the 41-43 MER stiffeneing belt. One of these long pieces seems to strike WTC 7 (marked as large red rectangle).
5) The lower section was obscured by dust, but the northeast corner was seen to break down to just above the 41-43 MER level (as marked).
The large leading north sheet is marked in yellow.
The large west and northeast corner pieces are marked in red.
Notice how their relationship to the red line is maintained and the north leading north piece always trails these two pieces.
Also, the northwest corner from above the failure line is marked in blue.
Dropping of the upper section
The large piece from the north face above the failure line is still obscured in the dust. The 75-77 MER level is visible with an ejection emerging from the center. Everything above that MER belt is the upper wall.
The upper wall has failed and is falling at this moment.
In the next image the northeast corner path is shown by the red arrow. The leading large piece falling from the north face follows the pink arrow.
The paths are marked by smoke, so one can verify that these 2 pieces were next to each other in the wall but fell outward in different directions.
The sheets following the blue arrow are of the wall just below the failure line. They fall trailing the leading piece. The original position is marked by a purple rectangle in the model diagram.
The initial failure line along the north face is visible in the following video. The large piece can be seen to break away from the northeast corner section during the earliest moments of north wall failure:
Dropping of the Middle Section
The upper sheet drops as pieces down to the upper edge of the stiffer belt of MER panels along the 78th fl.
The middle section then falls outward as a few long sheets, seeming to pivot near the upper edge of the 41-43 fl MER belt. One large piece pivots toward WTC 7.
The process is shown in 2 different sets of high resolution photographs.
You will notice the perimeter fold outward exposing tall core columns. The tallest column marks floor 77. It is easy to see middle wall folding outward from the elevation of floor 77 in the photographs.
Notes on the first set of photographs:
The upper wall is failing from photographs #1 to #4
The middle wall is failing from photograph #4 to #7, It pivots outward exposing a standing column. That column is 77 stories high and can be used to estimate the height of the north wall in photographs #4 to #7
The upper wall is failing from photographs #1 to #3
The middle wall is failing from photograph #4 to #9, it pivots outward exposing a standing column. That column is 77 stories high and can be used to estimate the height of the north wall in photographs #4 to #9
Another excellent example the north and east walls opening up from the 77th fl level in this photo The tallest column extends to the 77th floor. This column can be used as a reference mark to verify that the walls indeed open from around the 77th floor.