Thumbnail of NIST's 9/11 scenario
Here's a thumbnail of the scenario used by the National Institute of Standards and Technology to justify its computer modeling:
When the plane collided with a tower, a spray of debris "sandblasted" the core columns, stripping them of fireproofing. Some fireproofing may have come off because of the vibrations on impact.
Some jet fuel immediately burst into flame, in a fireball seen from the street, with some of the fiery fuel dropping down the core elevator shaft and spilling out onto random floors, setting a few afire.
Jet fuel, whether fueling an explosion or an ordinary fire, cannot produce enough energy to critically weaken naked core columns in the time alloted, the NIST found.
Hence, the NIST presumed that fires were accelerated by the jet fuel but got hot enough from office materials and furnishings, which the NIST variously put at five pounds per square foot or four pounds per square foot on average.
Though the heat energy actually detected via infrared analysis and visual inspection of the photographic record was far too low to have caused "global collapse," the NIST conjectured that the fires were hotter and more intense closer to the core.
The NIST's mechanism for collapse required sufficiently energetic fires to send heat to the ceiling -- which the NIST modeled as retaining its fireproofing -- and that heat was then convected along the ceiling and sideways into the load-bearing columns and connectors.
Had the fireproofing also come off the ceiling, the heat would have dissipated upward through the floor slab and, the NIST's computer models showed, the building would have stood.
Though there were fires on lower floors, the columns would have retained most of their fireproofing, and so critical weakening wasn't possible. No witnesses reported fireproofing off bare columns on lower floors.
The NIST said collapse was initiated by the shortening of core columns -- whether from buckling or from severing. Presumably this shortening would have occurred on the impact/fire floors, since that's where the fireproofing would have come off.
The NIST realized that blown-out windows supplied oxygen to keep the fire going, but also permitted much heat to dissipate out (with the smoke), making less available for the "blowtorch" scenario.
The NIST said that the impact zone fires seemed to progress around a floor in fits and starts until it had come all the way around the core.
Here are some concerns:
In Tower 2, which fell first, the observed fire energy was very low, the NIST agreed. The NIST attributed Tower 2's fall to structural damage from plane impact with some input from the fires. The plane that struck Tower 2 clipped the corner, meaning the collision force with the core would have been lower than in Tower 1, not more. Yet it is the core that supported the weight of the structure, not the exterior.
Tower 2's top block collapsed at a 23 degree angle. What this means is that Tower 2 had much less "hammer-down" force available for the lower part of the building than some have suggested.
On the other hand, Tower 1's top block came essentially straight down. What this implies is that the core columns on the opposite side of the elevator shaft from the plane were damaged roughly equally with those on the impact side. Otherwise, the top would have tilted over, as if on a hinge. Hence, one is forced by the NIST to accept the idea that a lot of fireproofing was shaken off the opposed columns, but not off the ceiling.
Also, in order for the fires -- which dimmed and flared as oxygen lapsed or became available when the heated air blew out windows -- to damage the core columns in a manner that initiates global collapse, not only must a specific percentage of the 47 columns be critically damaged, but the critical damage must be roughly evenly spaced.
That is, if a set of closely spaced columns lost strength, the building is less likely to give way, since the load is, by design, transferred to the remaining columns.
In the case of Tower 1, the damage to the core columns must be equivalent on the opposing side of the shaft, implying that the "blowtorch" acted symmetrically with respect to energy.
The NIST, in order to get the simulation to work, must have required that on each floor the fire maintain a specific rate of progress and a specific average ceiling energy. That is, if the fire moved through the debris too slowly, it would burn out or simply fail to do enough damage to other columns. But if it moved too rapidly, the circling fire would fail to bring enough heat to bear on any particular column to bring about critical weakening.
Though massive blasts were witnessed from outside just prior to collapse, the NIST felt obliged to discount these explosions as immediate collapse causes -- because the jet fuel simply wouldn't provide enough energy to do critical structural damage.