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Why did the World Trade Center towers collapse? Are skyscrapers safe? How can we make our buildings--and cities--less vulnerable to attack?
In part to find refuge from Sept. 11 "in sense and logic," MIT structural engineers have begun to answer those questions. Their conclusions were sent to MIT Tech Talk in two essays now on the web (URLs below) and in e-mail replies.
The World Trade Center towers "were indeed designed to withstand the impact of a large commercial aircraft," wrote Professors Oral Buyukozturk and Franz-Josef Ulm of the Department of Civil and Environmental Engineering (CEE). "They were not, however, designed to withstand the prolonged effect of fire resulting from a bomb in the guise of a fully fueled aircraft." As a result, "a building designed to rocket toward the sky imploded into the ground."
CEE Professor Eduardo Kausel agrees. "Both buildings survived the initial assault, and did not give way for a remarkably long period of time after the crash. This extraordinary capability allowed many lives to be saved and is a major credit to the designers. Ultimately, however, the intense fire heated the structural steel elements well beyond the thermal limit of some 800 degrees F, which caused the steel to lose resistance or even melt, and as supporting members gave way, the final failure of the building was initiated.
"At that point, the upper floors began to fall wholesale onto the structure below, and as they gained momentum, their crushing descent became unstoppable."
Via two simple models, Kausel was able to determine that the fall of the upper building down onto a single floor must have caused dynamic forces exceeding the buildings' design loads by at least an order of magnitude (i.e., more than 10 times the weight of the upper floors).
"There was no way in the world that the columns below could have taken this large overload, and as these failed in turn and collapsed, a domino effect down the building ensued," he said.
Of skyscrapers in general, "these buildings are and will continue to be very safe indeed," said Kausel. He, Buyukozturk and Ulm stressed that damage as catastrophic as that sustained by the World Trade Center towers is very rare. "The vast majority of modern high-rise buildings are well-engineered and designed to resist office fires, though not jet-fuel fires," Kausel said.
Although existing skyscrapers should probably be retrofitted with some additional safety measures, the professors say it doesn't make sense economically or aesthetically to protect them all from similar catastrophes. "Retrofitting is very expensive and is therefore usually done only for monumental buildings," said CEE Professor Jerome J. Connor.
Kausel agreed. "Making [existing buildings] jet-crash-proof makes no sense whatsoever," he said. "The chance that any individual building out of the thousands in the nation might suffer a similar attack is so small that retrofitting them would not make economic sense."
What about holding new buildings to higher safety standards? Again, "that would be very expensive and probably not economical," said Connor. Further, a terrorist could simply increase the load, or forces, applied to the building.
Buildings that could withstand massive blast forces and fires caused by jet fuel would also be "unbearably ugly," said Kausel. "Nobody would wish to live or work in such fortresses."
TOWARD A SOLUTION
The key to decreasing our vulnerability in this new environment is built-in redundancy at the structural and municipal levels, say Buyukozturk and Ulm. If one system fails, another will stand. Indeed, "it appears that structural redundancy made the World Trade Center towers initially withstand both the impact of the aircrafts and the following explosion."
On a structural level, the professors suggest extending this principle of redundancy to areas such as fireproofing, evacuation planning and firefighter operations. For example, a new generation of extremely heat-resistant composite materials could be applied to critical structural components. "This would increase the time of dimensional stability of the structural components, thus increasing the time for evacuation."
Redundancies in escape routes may be needed "if for no other reason than to allay the concerns of people whose fear of a similar tragedy will persist for years to come," said Kausel. "I, for one, would not wish to live or work in a mousetrap with insufficient escape routes."
A version of this article appeared in MIT Tech Talk on September 26, 2001.