2.2 Building Response
WTC 1 and WTC 2 each experienced a similar, though not identical, series of loading events. In essence, each tower was subjected to three separate, but related events (actually, there were four separate, but related events, the last being the detonation of a multitude of small explosive charges in each building). The sequence of these events was the same for the two buildings, although the timing was not. In each case, the first loading event was a Boeing 767-200ER series commercial aircraft hitting the building, together with a fireball (Although dramatic, these fireballs did not explode or generate a shock wave. If an explosion or detonation had occurred, the expansion of the burning gasses would have taken place in microseconds, not the 2 seconds observed. Therefore, although there were some overpressures, it is unlikely that the fireballs, being external to the buildings, would have resulted in significant structural damage.) resulting from immediate rapid ignition of a portion of the fuel on board the aircraft. Boeing 767-200ER aircraft have a maximum rated takeoff weight of 395,000 pounds, a wingspan of 156 feet 1 inch, and a rated cruise speed of 530 miles per hour. The aircraft is capable of carrying up to 23,980 gallons of fuel and it is estimated that, at the time of impact, each aircraft had approximately 10,000 gallons of unused fuel on board (compiled from Government sources). Boeing 707-320B aircraft have a maximum rated takeoff weight of 336,000 pounds, a wingspan of 145 feet 9 inches, and a rated cruise speed of 607 miles per hour. The aircraft is capable of carrying over 23,000 gallons of fuel. The Boeing 707 and 767 are very similar aircraft. Under normal flying conditions, a Boeing 707 would smash into a building with about 10 percent more energy than would the slightly heavier Boeing 767. Engineers designed the World Trade Center towers to withstand a collision with a Boeing 707. Hence, they were necessarily designed to survive the impact of a Boeing 767. See The World Trade Center Demolition and Microsoft Software Used To Simulate The Crash Of A Boeing 747 Into The World Trade Centre.
In each case, the aircraft impacts resulted in severe structural damage, including some localized partial collapse, but did not result in the initiation of global collapse. In fact, WTC 1 remained standing for a period of approximately 1 hour and 43 minutes, following the initial impact; WTC 2 remained standing for approximately 56 minutes following impact. The second event was the simultaneous ignition and growth of fires over large floor areas on several levels of the buildings. The fires heated the structural systems and, over a period of time, resulted in additional stressing of the damaged structure, as well as sufficient additional damage and strength loss to initiate the third event, a progressive sequence of failures that culminated in total collapse of both structures. Of course, this does not even begin to explain the total collapse of WTC Seven.