PBS Nova Why the Towers Fell (2002)

This documentary uses footage of the erection of the World Trade Center, the 1993 bombing, Sept 11 and Ground Zero, graphics of the towers and the planes hitting them, and testimony from survivors, engineers, firemen, and the builder Leslie Robertson. The WTC was built to overcome what was previously seen as drawbacks to older buildings like The Empire State Building, with a new approach to provide more usable floorspace. This was to make a steel outer skeleton which would bend in the force of the wind, a steel inner core of girders, and steel floor trusses that braced the skeleton to the core. However, the heat-resistant foam that coated the trusses and the drywall plasterboard are what ultimately weakened the towers to the attack, since they both were practically blown away by the jet fuel fire, which weakened the steel. What is interesting is that while both towers eventually suffered "progressive collapse", as the floors pancaked, each tower fell for a specific different reason. The south tower (which fell first) fell because the outer skeleton snapped, but the north tower fell because the inner core collapsed. The differences were also due to the angles of the planes and the height of the floors that were impacted, and the comparative time that each tower endured their fires. Although experts disagree as to whether different truss connections in the design might have helped the towers withstand the impact, they agree that the length of time the towers stood is remarkable.

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