There are two distinct ways to view the physics of a ROOSD-type process, as a progressor or as an initiator.


Obviously, this structure is composed of 3 main components:




The perimeter
The core
Flooring


The initiation of collapse, in general, can be classified as

1) Perimeter failure initiated
2) Core failure initiated
3) Floor slab failure initiated



One would think that the NIST would carefully try to find out which of the three cases actually occurred. The ROOSD process is vital to understanding the collapse progressions of WTC1 and 2, but it is also vital to understanding the initiation sequence.

Progressive floor collapse initiated by failed floor slabs has always been one of the 3 basic logical possibilities for the initial failures of WTC1 and 2.


In fact, the FEMA report released in 2002 described ROOSD as an initiator. This was apparently an accepted explanation until the NIST disagreed and changed the official explanation to being perimiter-led collapses.


From the FEMA report:


FEMA 2002 concept of WTC1 collapse initiation:





The (in)famous early "pancaking" theory.


FEMA on the WTC2 collapse initiation and progression:

2.2.2.5 Initiation of Collapse

The same types of structural behaviours and failure mechanisms previously discussed are equally likely to have occurred in WTC2, resulting in the initiation of progressive collapse, approximately 56 minutes after the aircraft impact. Review of video footage of the WTC2 collapse suggests that it probably initiated with a partial collapse of the floor in the southeast corner of the building at approximately the 80th level. This appears to have been followed rapidly by collapse of the entire floor level along the east side, as evidenced by a line of dust blowing out of the side of the building. As this floor collapse occurred, columns along the east face of the building appear to buckle in the region of the collapsed floor, beginning at the south side and progressing to the north, causing the top of the building to rotate toward the east and south and to begin to collapse downward (Figure 2-32). It should be noted that failure of core columns in the southeast corner of the building could have preceded and triggered these events.