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| Reference Material |
| How the towers collapsed From The New York Times May 8, 2002, Today The federal investigation into the collapse of the World Trade Center towers on September 11 has been unable to pinpoint what engineering elements were critical in the disintegration of the building or how best to resist a recurrence. Even so, the six-month study � conducted by the American Society of Civil Engineers and the Federal Emergency Management Agency � has revealed some disturbing facts about modern skyscrapers that are potentially worrisome for those who work or live in high-rise buildings around the country. The most encouraging finding was that the impact forces of the huge jets that rammed into the towers would not have been enough, by themselves, to cause the collapse. Although the twin towers were designed to handle only the crash of a Boeing 707 flying at low approach speeds, the study says, when put to the test on Sept. 11 they absorbed the shock of slightly heavier Boeing 767�s flying at much higher speeds. Had no other stress been imposed on the structures, the study concluded, they could have remained standing indefinitely. Unfortunately, there was added stress, in the form of extremely hot fires that resulted when jet fuel ignited the contents of the building and planes. The flames softened the structural steel, triggering events that allowed the upper floors to cascade downwards���. The experts were unable to determine whether the fires alone, without the impact of the airplanes, could have brought the towers down. But it is disquieting to learn that an adjacent 47-story building collapsed completely as a result of a fierce fire fed by diesel oil on the premises, and that another building suffered a partial collapse from the fire. These are the first known instances of protected steel-frame structures collapsing from severe fire, suggesting that many modern buildings may be more vulnerable than anyone realized���. ***** See also Tony Abaya�s column of September 14, 2001 More cost-effective than Pearl Harbor. |