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December 2001: WTC Collapse

Referencing the World Trade Center disaster on September 11, 2001, an article in the November/December 2001 issue of the NFPA Journal addressed the issue of why buildings collapse. As tragic as that day was, analyzing the events that lead to the collapse may provide some clues that will benefit future building safety and design.

Several engineers felt that the tower could have withstood the impact of a single large aircraft, as they were designed to do. In fact, the towers were designed to withstand a direct hit from a Boeing 707, a state-of-the-art jetliner in the 1970s. This was factored into the design as a result of lessons learned 56 years earlier when an Army Air Force B-25 hit the Empire State Building between the 79th and 80th floors in heavy fog. Fourteen people died and damage to the building came to $1,000,000. The overall structural integrity was not affected.

However, who could have ever anticipated the introduction of 24,000 gallons of aviation fuel exploding into flames. The heat generated by the burning fuel, estimated to be 2000ºF, was well above temperatures that can weaken structural steel holding up concrete floors. The result was the collapse of both towers.

Although the towers were provided with full automatic sprinkler protection, the fire protection systems were designed for an office occupancy, which would normally consist of paper, office furniture, partitions, rugs, computers, etc. Introducing aviation fuel created a fire well in excess of the design capabilities of the sprinkler system.

There are fire suppression systems designed to cope with the fire load presented by aviation fuel. These systems are a combination of foam and water and are routinely installed in aircraft hangers. They are however, very elaborate and not practical for a high-rise structure. Water can have some of extinguishing effect on a fire in jet fuel, but not nearly as effective as a foam-water system.

Can we design terrorist-resistant high-rise structures? Probably, but they would resemble fortresses and would be prohibitively expensive. This leaves the option of increased security. It is possible to use reinforced structural frames and perimeters, Kevlar curtains or bulletproof glass, fewer windows and more secure entrances. Several buildings now incorporate restricted vehicle access and no underground parking in addition to increased security screening for both workers and visitors.

You might say that none of the above additional security measures or construction enhancements would have prevented the damage caused from a modern jetliner with 24,000 gallons of fuel intent on crashing into a building. You would be right. However, those enhancements combined with the planned improvements in air traffic control and the military presence both in the air and on the ground may provide an adequate deterrent against a similar event in the future. It's to soon to tell how the lessons we learned from the attack will affect future building construction. But we will learn. And those lessons will not only change future building design but the way we conduct our everyday lives.