ignacio
From MIT Civil Engineering dept .... who are scientists:
Quote:
three-step failure mechanism that occurred at different scales of the two WTC towers.
1. The impact of the plane at one or several floors of the WTC: The building was designed for the horizontal impact of a large commercial aircraft, and the towers did indeed withstand the impact and the following explosion which may have destroyed one or more floors. This destruction should have locally reduced the resistance of the columns in the core (and eventually also at the perimeter) to buckling. Even so, the resistance was strong enough to carry the loads of the upper floors for approximately one hour.
2. The failure of the impacted floor system: The explosion following the impact may have destroyed some of the thermal insulation around the steel columns. The 60 tons of flaming jet fuel easily raised temperatures to 1,500° F or more. Exposed to such intense and prolonged heat, structural steel loses rigidity and strength. This may have caused a reduction of the buckling force of the connected 2-3 floor structural system. The column-bracing system was subjected to the load of the top floors. After about 60 minutes, local buckling of the columns became inevitable. Compared to a ductile failure, the floor system failed in a brittle way, explosively releasing the energy stored in the system. The tower with the higher load above the impacted level collapsed first, but both towers showed almost the same failure mechanism.
3. Dynamic crash of the remaining structure: The failure of the 2-3 floor system set a 25- to 30-floor mass free-falling onto the 80- to 85-floor structure below. The enormous kinetic energy released by this 2-3-floor downfall was too large to be absorbed by the structure below. The impact of this upper part onto the lower part was surely much higher than the buckling load of the columns below, which to this point may had been largely undamaged, and may have ultimately caused the explosive buckling, floor after floor, of the WTC towers. Similar to a car smashing into a wall, the towers crashed into the ground with an almost free-fall velocity.