"No building is designed to withstand the impact of a very large aircraft and then the fire. I am not even sure you could do that," said Ed Galea, head of Greenwich University's Fire Safety Engineering Department. "The main line of defense - the building design - is inadequate to cope with such contingencies. We need to look again at building evacuation procedures."
Charles Moran, managing director of Controlled Demolition Group, specializing in building demolition worldwide, said the buildings had in effect been hit by a flying bomb.
"You've got to imagine a (Boeing) 737 weighing 100 tons, full of fuel and traveling at hundreds of miles an hour. You've got in effect a very large bomb."
Hyman Brown, who teaches civil engineering at the University of Colorado and oversaw construction of the World Trade Center for seven years, believes the 24,000 gallons of burning jet fuel melted the building's steel girders and triggered the collapse.
Brown says the Twin Towers should be rebuilt. He says that sends a message to terrorists that America can withstand tragedy and overcome it.
Gordon Masterton, of the Institution of Civil Engineers, said that, short of building a nuclear bunker in the sky, little could be done to withstand such an attack.
"You could have thick concrete and steel walls, barrage balloons and fighter jets patrolling. But little else would prevent what in effect was an act of war," he said.
Galea said the usual procedures for high-rise buildings were to evacuate three floors below and one floor above the fire floor, but to leave everyone else in place.
"If they were the procedures in place, and I don't know if they were, and people stayed in place that would have been not the right thing to do in this incident. You would want to get out of there as fast as you could," he added.
Thousands of people, including several hundred rescue workers, are feared crushed or burned to death after two hijacked civil airliners were crashed into the twin towers of the 110-story icon of world capitalism.
Barely an hour after the attacks, both towers collapsed. "Typically in a fire situation in a high-rise building, even in the very worst situations, you would expect the building to withstand half a day before it collapsed," Galea said.
But Masterton said that, while the unprotected steel skeleton of the World Trade Center did cope with the impact of the planes, it had no chance against thousands of gallons of aviation fuel burning at 1,830 degrees Fahrenheit.
"Steel loses half its structural strength at 500 to 600 degrees centigrade (930 to 1,100 Fahrenheit), so it would have simply collapsed," he said.
But even if the people who could do so had immediately begun to evacuate the buildings down th staircases, the fate of most was sealed the moment the aircraft crashed.
A study after the 1993 bombing of the same building, which killed six people and injured 1,000, showed only 40 percent of people got out in under an hour, with 52 percent taking one to three hours and 8 percent more than three hours.
"The buildings collapsed within an hour, so, going back to the 1993 bombing, I imagine a good deal of the people would have been caught in the building," Galea said.
The only way out of the building during an evacuation is down the stairways, a difficult task for even healthy individuals asked to quickly descend 60 floors or more.
Galea said the main thrust of modern building design was to make the structure as safe as possible and that evacuation was kept only as the last resort.
"But when you are faced with a cataclysmic event like yesterday's, then these type of strategies are inadequate.
"We need to look again at building evacuation procedures and safety measures to cope with these types of situations," he said. "It is something we have to look at... but I am not sure there is a lot we can do. This was a cataclysmic event."
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