Consulting engineers reflect on the World Trade Center disaster

A week after the terrorists’ attacks in the U.S., Canadian consulting engineers were as shell-shocked as everyone else about the horrific event. Structural engineers felt especially stunned that the mighty twin towers of the World Trade Centre in New York City had tumbled to the ground in seconds.
At Quinn Dressel in Toronto, a firm that has designed towers like the 68-storey Scotia Plaza in Toronto, and Liberty Place in Philadelphia, the tragedy kept them preoccupied and poring over the media and other reports. “We could talk for days,” said Ben Burke, P.Eng. a partner of the firm when asked what kind of questions it raises for engineers.
Burke was on his way to a ceremony for a building opening on that fateful morning of September 11, when he began to hear the radio reports of a hijacked Boeing 767 plowing into the north tower’s 73rd floor, and a second plane hitting the south tower 20 minutes later. He was afraid for the people, but says, “Even at that point I was thinking, Well, planes crash, but buildings should not collapse.” He was wrong, of course, and in events that have become legendary in their horror, within an hour and 45 minutes, by 10.30 a.m. both towers had crumpled into the ground. Meanwhile a third terrorist pilot had hit the Pentagon in Washington, D.C., and a fourth plane had plowed into a field in Pennsylvania, the hijackers reportedly thwarted from taking aim at the White House.”When the aircraft hit the building, I’m sure it was literally shredded,” Burke says. He sounds as though his mind has spent a lot of time racing over what might have happened. “But those elements that couldn’t be shredded like engine parts, undercarriages and very stiff elements probably made out of steel continued to batter their way through.” Like everyone else, he’s seen the repeated television footage. “In the second aircraft that hit the tower you can see that the pilot banked the plane to get maximum impact on as many floors as possible — four or five floors. Part of the fuel tanks are in the wings, so there was a lot of fuel deposited on many floors.”
With the towers reduced to “ground zero,” it is going to be a long time until investigators know exactly how the structures collapsed — if ever. The American Society of Civil Engineers has two task forces going in to investigate the centre’s collapse and the Pentagon damage, and the Steel Institute of America is also investigating. Until then, engineers continue to speculate.
Burke doesn’t think the World Trade Centre steel structure was an obvious target for saboteurs: “If you were to pick a building to hit from a structural engineering point of view, I’d be confident that one would survive. It’s a core within a core, a tube within a tube — it’s very, very stiff. And even if you puncture the columns on the outside, which you could see happened where the first aircraft went in, there was no immediate collapse. The structure itself withstood the impact and stayed. It didn’t even lean.” Designed by architect Minoru Yamasaki and Emory Roth & Sons and built between 1969-1973, the aluminum-clad box towers reached 417 and 415 metres, with about 4.7 million square feet of space.
Like most commentators, including the American Society of Civil Engineers, Burke believes that it was the intense heat from the impact explosion and thousands of gallons of burning aircraft fuel that weakened the steel structure, and then the floors progressively collapsed like a concertina. One fortuitous aspect, as Burke sees it, is the fact that there was a six or seven level underground parking structure to the towers. “The building imploded into itself,” he says, and “it had somewhere to lodge itself — that hole in the ground.” Otherwise the towers may have toppled over and done more damage laterally. As it was, a third building of 57-storeys (still seven storeys taller than Vancouver’s new Wall Center) burned, and collapsed later that day, possibly because its foundation was weakened by the fall of the other towers.
Burke wasn’t surprised that there were hardly any survivors underneath the fallen World Trade Center. “Between those two buildings there was 150,000 tonnes of steel alone,” he says. The site looked like a war zone with debris still piled 10 storeys high in some places. Local structural engineers were working in shifts to direct its safe removal by four construction firms (one was AMEC).
More than 5,000 people died in the disaster. Some office workers were trapped in the floors above the flames, and perished or jumped to their deaths. Others undertook the long and treacherous descent in the stairwells but didn’t make it in time. Firefighters and rescue works entered the building to help only to meet their deaths