published September 12, 2001
NEW YORK -- Built to withstand earthquakes and hurricane-force winds, and equipped with enhanced security after a 1993 terrorist bombing, the twin towers of New York's World Trade Center were supposed to last.
Their architect boasted that they could withstand the impact of a jumbo jet.
But when two hijacked commercial jetliners crashed into the 110-story structures within 15 minutes of each other early Tuesday 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," added 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 aviation gasoline softened or perhaps even melted the steel strength members supporting 50 floors of undamaged skyscraper above the point of impact in 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 Tuesday 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, for while 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 gasoline-filled guided missile 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 gasoline 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.
For the people trying to escape, what followed was a macabre race against time, and the odds weren't 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.
Hyman Brown, a University of Colorado civil engineering professor and the Trade Center's construction manager, said it appeared the attack was meticulously planned.
"If they did it lower in the building the fire department could have gotten to it sooner. In its simplicity, it was brilliant."
He said that the two towers have staircases in all four corners and were designed to be evacuated in an hour, but it appeared that since the planes crashed into the corners, escape was cut off for those on the floors above.
The end came when the fire had softened the girders to the point 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 for a bit longer.
Angus Kress Gillespie, author of the 1999 book Twin Towers: The Life of New York City's World Trade Center, said architect Minoru Yamasaki 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 issued a statement Tuesday saying the firm was in contact with authorities and had offered assistance.
Cesar Pelli, designer of the Petronas Towers in Kuala Lumpur, Malaysia, the world's tallest buildings, suggested that while "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."