1. Debris from WTCs caused damage to the perimeter moment frame and some interior core framing on the south side of the building. This debris caused the severing of about 25-33% of the south side perimeter columns.
2. Reports indicate that Debris caused damage to the bottom 15-20 floors (GF to F15-20).
3. Extent of structure and fireproofing damage, caused by debris, to the core framing, is unknown.
4. Damage to elevators occured at columns 69-78 at floors 8 and 9.
5. Fires were observed after 2PM on Floors 7,8,9,11,12,13, and possibly 5.
6. A Vertical collapse appears to have occured after interior columns 79,80, and/or 81 failed. The failure progressed vertically upward from initiation to the roof level.
7. The debris from this 40 Storey collapse (Floor 7 upward), upon coming down upon the remaining floors, would have created lateral forces that would have caused the remaining core columns to fail. This failure occured approximately 5 seconds after the penthouse failed.
8. The failure of the core columns caused too great a load, even with redistribution, causing failure of the cantilever girders on the north side. The remaining perimeter support framing (columns) were left unsupported, or already taken down via the interior collapse.
The above resulted in a global collapse with few external signs prior to system failure. It is suggestive of a classic progressive collapse.
