In my opinion the fires had a less important role to play in the collapse of both towers than the damage from the initial impact. It took both to cause the collapse, however the fire was in no way severe enough to have caused the collapse on its own. The reasons for this opinion are as follows:
1. If the temperatures inside large regions of the building were in the order of 700+ deg C, then these regions would have been glowing red hot and there would have been visible signs of this from the outside. Also there would have been visible signs of flames. If one looks at the photos of the Cardington fire tests, the flames and glowing of the steelwork is clearly visible even in the large enclosure test where the maximum fire temperature was only 700 Deg C. In contrast, the pictures of the towers after the impacts and prior to the collapses show signs of severe burning over only relatively small regions of the tops of the towers, even pictures taken from the air looking horizontally into the impact region (e.g. Fig.9).
Photos of the First Interstate Bank fire in Los Angeles in the early 1990s? show what appears to be greater heating effects and over larger regions than were apparent in either tower.
This does not mean that there were no regions subjected to severe heating. It is likely that temperatures in some parts of the impact region would have exceeded 700 deg C for some or all of the time between impact and collapse, especially on the South side of the North tower. However, the extent of impact damage would have been such as to leave the residual vertical load carrying system within the core regions of both buildings vulnerable to further weakening at temperatures lower than 700 deg C.
In contrast, had the columns in the core and the perimeter frames remained intact and protected ( an impossible scenario given the magnitude of the impact) then it is expected that the building would have remained standing, with significant floor damage, even when subjected to fire temperatures of 1000 deg C and having suffered the loss of the fire rated suspended ceiling to the floor slabs.
2. When fully developed fire conditions ( temperatures of over 700 deg C) are reached within a region of a building, this results in the breaking of glass in any external windows within that region. This continuous breakage of glass as the fully developed fire spread through the floor of the First Interstate Bank, for example, was the most hazardous feature of the fire to those at ground level around the building.
In contrast, once the blast and fireball effects of the impacts had subsided, there appeared to be little ongoing window breakage from either tower, either as evidenced from pictures/video footage or as reported from the ground. Significant areas of window even remained intact within the impact region ( see e.g. Fig.9). This is further evidence that fully developed fire conditions did not spread much through and beyond the initial devastated region, following the impacts. 3. If there had been severe fires burning in the core regions of the building due to the fire load from the plane combining with the fire load from the buildings, this would have adversely impacted on the conditions in the stairwells below the impact region. This would have especially been the case for the North Tower, where the core was destroyed by the impact, leaving the regions within the core below fully exposed to fire conditions within the impact region, such as the ingress of burning fuel and other combustibles. However the stairwells below the impact region on the North Tower were sufficiently clear to allow some occupants close to the impacted floors to escape and to allow firemen to reach at least the floors around the 70th level, as reported by survivors from the building. In the South Tower, at least one stairwell remained operable past the impact region after the impact. Given the damage that must have been done by the impact to the walls surrounding this stairwell, the resulting fire is unlikely to have been “incredibly severe”, otherwise the few survivors from the South Tower above the impact region would not have been able to escape.
