source: ICSD 9/11 Mar 11, 2011
Weak poorly combusted fires not exceeding 120* C at the building exterior where the oxygen supply is greatest – High pressure gas exhaust and flame rollout – Sparks and white smoke.
In the second half of this video the timing sequence is not in order, it shows the temperature and what we actually see. For time accuracy the thermographic camera and the footage shot from a similar angle are shown, both with original timestamp. FLIR Image of heat source in the structural 75th floor of the North Tower – This is a view of one of the mechanical floors (they were the only floors for which the prefabricated perimeter wall units were not staggered).
The mechanical floors where not supported by trusses but by solid steel beams. Composite action between these beams and the concrete slab was by welded shear studs. The concrete slab was apparently considerably thicker than that of your average floor and specially reinforced with steel beams. Such floors were necessary to enable the towers to resist the significant lateral force of hurricane force winds. We have the following quote from Engineering News-Record, January 1, 1970.
On the 41st and 42nd floors, both towers will house mechanical equipment. To accommodate the heavy loads, the floors are designed as structural steel frame slabs. All other floors from the ninth to the top (except for 75 and 76, which will also carry mechanical equipment) have typical truss floor joists and steel decking. Typical office floors have 4-in. thick slabs of composite construction using top chord knuckles of the joists (trusses), which extend into the slab, as shear connectors. On mechanical floors, composite action is provided by welded stud shear connectors. The bright energetic fires seen after the South Tower demolition are on the 75th structural floor of the North Tower. Did NIST ever mention the source of this heat or the fact that it even existed?