Femr writes:
"1) South face failure, followed rapidly by...
2) South to North core and East/West perimeter failure, followed rapidly by...
3) North face failure."
That is the official version of the sequence of failure events for WTC1 and 2. In that order.
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In their own words:
1-6D, p 312:
"Table 5–1. Summary of main events that led to the collapse of WTC 1.
Event Number........ Event
1 .......................Aircraft impact
2 .......................Unloading of core
3 .......................Sagging of floors and floor/wall disconnections
4........................Bowing of the south wall
5 .......................Buckling of south wall and collapse initiation"
1-6D, pg 314:
"Bowing of South Wall
The exterior columns on the south wall bowed inward as they were subjected to high temperatures, pull-in forces from the floors beginning at 80 min, and additional gravity loads redistributed from the core. Figure 5–6 shows the observed and the estimated inward bowing of the south wall at 97 min after impact (10:23 a.m.). Since no bowing was observed on the south wall at 69 min (9:55 a.m.), as shown in Table 5–2, it is estimated that the south wall began to bow inward at around 80 min when the floors on the south side began to substantially sag. The inward bowing of the south wall increased with time due to continuing floor sagging and increased temperatures on the south wall as shown in Figs. 4–42 and 5–7. At 97 min (10:23 a.m.), the maximum bowing observed was about 55 in. (see Fig. 5–6).
Buckling of South Wall and Collapse Initiation
With continuously increased bowing, as more columns buckled, the entire width of the south wall buckled inward. Instability started at the center of the south wall and rapidly progressed horizontally toward the sides. As a result of the buckling of the south wall, the south wall significantly unloaded (Fig. 5–3), redistributing its load to the softened core through the hat truss and to the south side of the east and west walls through the spandrels. The onset of this load redistribution can be found in the total column loads in the WTC 1 global model at 100 min in the bottom line of Table 5–3. At 100 min, the north, east, and west walls at Floor 98 carried about 7 percent, 35 percent, and 30 percent more gravity loads than the state after impact, and the south wall and the core carried about 7 percent and 20 percent less loads, respectively. The section of the building above the impact zone tilted to the south (observed at about 8°, Table 5–2) as column instability progressed rapidly from the south wall along the adjacent east and west walls (see Fig. 5–8), resulting in increased gravity load on the core columns. The release of potential energy due to downward movement of building mass above the buckled columns exceeded the strain energy that could be absorbed by the structure. Global collapse ensued."
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Basically, they are saying that the OOS south area floors sagged so badly it pulled in and destabilized the south wall. Then, apparently, the whole core gets pulled down as the top portion tilts 8 degrees or less than 1 degree, nobody seems to know which.
