[911] 11 September 2001 - WTC / Pentagon tråden

"When you slam an aluminum aircraft at high velocity into a concrete structure, it's going to do exactly what we saw happen at the Pentagon on 9/11," Partin said. "If you look at a frontal mass cross-section of the plane, you see a cylinder of aluminum skin with stringers. When it impacts with the exterior [Pentagon] wall at 700-800 feet per second, much of the kinetic energy of the plane converts to thermal energy, and much of the aluminum converts to vapor, burning to aluminum oxide. That's why on the still photos from Pentagon surveillance camera, you first see the frame with that brilliant white luminescent flash just before the frame of the orange fireball, the jet fuel burning. The aluminum cylinder ? the plane fuselage ? is acting like a shaped charge penetrating a steel plate. It keeps penetrating until it is consumed. The Boeing 757 is over 150 feet long, so it's going to penetrate quite a ways before it's spent. The wings have a much lower mass cross-section and are loaded with fuel besides, so there is little left of them except small bits and pieces."

The cores of the towers were connected to the outer walls/skin by long (60 to 70 feet) open-web truss type joists that supported the concrete floors. Such trusses have long been a source of concern for firefighters because their rapid weakening and failure in fires can produce early and unpredictable collapses. Failure of the trusses and their connections at the core and outer walls isolated the core from the skin, causing extreme overloading and loss of lateral support that fostered column buckling. The only way for the trusses to survive structurally in fire is for them to be very well thermally protected, and this was not the case in the towers.

Structural members in high-rise construction must be thoroughly covered (i.e., encased) by fire-resistive insulation of sufficient fire rating (typically four hours) to prevent or minimize loss of strength resulting from the elevated temperatures that occur in fires. This is required by fire codes. That the various steel structural members of the towers were not sufficiently encased in thermal protection was known beforehand.

Certainly on the impact floors, large portions of the insulation that was in place were torn and stripped off (and would have been the case no matter how substantial and well-attached the insulation was). The ongoing raging fires presented extreme high temperature environments that pushed any locally remaining and adjacent floor insulation to or beyond its limits. But the fact that there likely was insufficient insulation on undamaged columns in the core and on the floor trusses would have caused these members to weaken and fail in relatively short time periods, certainly shorter than expected. These members were already bearing higher-than-design loads because other parallel members had been damaged or destroyed.

Since there was no formal inspection and oversight during construction and after, the true nature of the thermal protection was not well known. For example, the state of the steel surfaces at the time of insulation application was not documented; and, if there were already corrosion (rust or scale) on the surfaces, the bonding would not have been proper and firm. The floor bar joists (trusses) are particularly vulnerable because the sprayed-on material adheres to only a small surface area. Weathering and handling during construction can cause damage and stripping. At the WTC, there was evidence of problems resulting from inferior materials and workmanship, documented in photographs that showed areas that were not even sprayed. It is not known how much inferior insulation was uncorrected.