'Magnitude Beyond Anything We'd Seen Before'

But when two hijacked commercial jetliners crashed into the 110-story structures within 20 minutes of each other early yesterday morning, experts flinched, for "what we saw today was several orders of magnitude beyond anything we'd seen before," said the National Academy of Sciences' Richard Little, who has overseen several studies on how to protect buildings from terrorist attacks.

"We were hopeful at first," said Pennsylvania State University architectural engineer Kevin Parfitt, who teaches a course in building failures. "But the longer the fire burned, the more we feared the outcome."

With justification. In just under an hour, a raging fire from burning jet fuel softened or perhaps melted the steel strength members supporting 50 floors of undamaged skyscraper above the point of impact in the South Tower. The top floors slumped to the damaged area, and the impact of the dead weight caused the entire building to pancake to the ground. A half-hour later, the North Tower collapsed in the same way.

By late afternoon, the 47-story Building 7, another of the center's seven buildings, had also fallen after burning all day. Building 6, the U.S. Customs House, was a smoldering, soot-blackened hulk.

Experts agreed that collapse of the two towers was almost inevitable; although their "tube structure" design was their greatest source of strength, it was also an Achilles' heel. For someone who wanted to bring them down, a guided missile filled with jet fuel was perhaps the only way.

The towers were built like "rectangular doughnuts," Parfitt said. Strength came from a central steel core and from steel columns spaced closely around the perimeter of each building. There was no structural support between the core and the outer walls.

"When the planes come through, they cut through a number of those [perimeter] columns," Parfitt said. "At the same time, the planes are starting transcontinental flights, and they have full tanks of aviation fuel. You get a massive explosion and a fire."

The initial jet fuel explosions most likely blew the insulation off the towers' girders, Parfitt suggested, incinerated easy combustibles and gave the ensuing fires free access to the unguarded steel. "Sprinklers aren't going to do too much in that situation," Parfitt said.

For the people inside the buildings trying to escape, what followed was a macabre race against time, and the odds were not good. Each of the Trade Center towers had 250 elevators, but only three stairwells. Between 20,000 and 25,000 people had to get out of each building as rapidly as possible.

In 1993, after terrorists set off a bomb in a basement garage, it took four hours to evacuate the towers, but Dennis Wenger, director of the Texas A&M; University Hazard Reduction and Recovery Center, said half the occupants got out in the first hour.

"There was no panic, and a lot of cooperation," even though the stairwells were "quite narrow" and smoke was wafting upward as people climbed down, said Wenger, who studied the 1993 evacuation.

In some ways, yesterday's scenario might have seemed better -- the fires were above most of the occupants. But what they didn't know was that -- in the South Tower -- they had less than an hour to get out. The North Tower was not much better.

The end came when the fire had softened the girders so that the weight above the crash sites became unsupportable. The South Tower, hit lower down, fell first beneath the greater weight. The North Tower, with less weight above the explosion, held out a bit longer: "The whole thing just imploded," said Melvyn Blum, 55, a real estate executive who was watching through a telescope from his 44th-floor office a few miles away on Manhattan's Seventh Avenue, "just like you see when they take buildings down with dynamite."

Angus Kress Gillespie, author of the 1999 book "Twin Towers: The Life of New York City's World Trade Center," said architect Minoru Yamasaki had designed the towers to withstand the impact of a jumbo jet, "but planes have become bigger" since the center was built in 1972. Minoru Yamasaki Associates (MYA) issued a statement yesterday saying the firm was in contact with authorities and had offered assistance. "We believe that any speculation regarding the specifics of these tragic events would be irresponsible," the statement said. "For obvious reasons, MYA has no further comment at this time."

By late Tuesday, few were criticizing Yamasaki for misplaced bravado. After the 1993 bombing, the center's towers "were probably among the half-dozen strongest buildings in the world, but it couldn't withstand that kind of insult," Little said.

Cesar Pelli, designer of the Petronas Towers in Kuala Lumpur, Malaysia, the world's tallest buildings, suggested that although "it will take structural engineers a long time to figure out exactly how" the towers collapsed, he agreed that "no building is prepared for this kind of stress.

"I feel a tremendous sense of loss, but this is insignificant when you think of the horror of the loss of life," Pelli said. "The grief is just unimaginable."