Duke Engineer: World Trade Center Disaster May Halt Construction of Supertall Buildings

September 13, 2001

With the tragic coordinated jetliner destructions of both World Trade Center towers in New York City Sept. 11, a Duke University engineering professor says "we very well may see the end of tall buildings of that magnitude for the foreseeable future."

"I think its going to be very difficult to make a proposal that financiers, the people that supply the money to invest in these buildings, are going to embrace," said Petroski, Aleksandar S. Vesic Professor of Civil Engineering at Duke's Pratt School of Engineering. He has written several books on big engineering projects such as skyscrapers and bridges, as well as on failures of structures large and small.

"The history of bridges I have written about tells us when there have been massive bridge collapses, bridges of that kind very often became obsolete and were no longer built," he said.

Petroski explained how buildings that had previously weathered a massive basement terrorist bombing in 1993 could collapse so quickly and completely this time after an airborne assault.

"Buildings like that are built so they can withstand fire, but not indefinitely," he said. "The idea is that the fire will either burn itself out or cool off before there is any structural damage. In this case, with all the jet fuel, my guess is that the fire began to soften the steel.

"That's why the building that was hit second probably collapsed first," he continued. With the second jetliner hitting that tower much nearer the ground, "there was more weight over the fire area, so the steel was softening and there was this great weight at the top of the building bearing down on it.

"Eventually these columns are going to buckle, and as one column gives way the next one has to take more load, and of course it can't because it's also been softened. So once the chain reaction begins there is virtually no stopping it."

Petroski noted that the Empire State Building survived a full-speed impact from a blinded-by-fog bomber in 1945 with "virtually no significant structural damage." While that's a testimonial to the construction standards of the past, "the reason higher skyscrapers can be built today is in part because they're built lighter," he said.

"You can always build things stronger, but then how strong? For absolute safety you would want no windows because that provides the greatest strength. But then, who's going to rent out a 100th floor office when they can't see the surrounding area?"