By 10:30 a.m. Tuesday, the New York City skyline had been changed forever.
Slabs of steel, glass and concrete rained down on the streets below, while dense clouds of ash and dust coated the city with debris when both main towers of the World Trade Center complex collapsed within two hours of being hit by hijacked planes.
"To say you design for something like this in the first place is a rather general statement given what people witnessed on television today," said William Faschon, a partner with Leslie E. Robertson Associates. Robertson was involved in planning the original design of the World Trade Center. "These events were enormously violent and powerful, and what's going to happen under those circumstances is very hard to predict."
About 45 minutes before the collapse of the towers, a third hijacked plane slammed into the Pentagon outside Washington, D.C., causing damage to the army section, which led to partial buckling of the infrastructure.
"If someone had asked me if something like this could happen, I would say that it could. But I didn't think it would happen in reality," said Henry Petroski, A.S. Vesic Professor of Civil Engineering at Duke. "I am extremely shocked because everything doesn't seem real. I had to pinch myself."
The World Trade Center towers, which together have a daily capacity of 50,000 people, are both supported by steel and aluminum frames rising an impressive 110 stories high. The 1,300-foot buildings, completed in 1974, were originally designed to take the impact of a Boeing 707, and most recently, they withstood an attempted terrorist bombing in the basement eight years ago.
And the towers, in fact, were able to withstand the impact of the larger Boeing 767 crashes Tuesday morning. But the fires that followed are what Petroski believes eventually caused the towers to cave in.
Since the hijacked planes were all cross-country flights, the large quantity of fuel created intense fires, which heated and softened the columns of steel that supported the infrastructure. When the weight of the top-most floors could no longer be supported, they collapsed and created a downward fall that progressed in an unstoppable chain reaction.
"What happened today was not due to a plane crash, but rather to the aftermath of the crash," said Petroski. "Theoretically, if the ensuing fire had been put out immediately after the crash, then I wouldn't be surprised if [the towers] were still standing."
A third building in the World Trade Center complex collapsed at 5:20 p.m. The 47-story building had been damaged from other debris, and flames had consumed the south side of the building.
The Pentagon, the world's largest office building, was completed in the early 1940s with structural steel and nearly 435,000 cubic yards of molded concrete.
"I've heard people express surprise that the plane was able to get through all those walls, but I'm not terribly surprised that a jetliner could cause the collapse of a structural wall like [the one at the Pentagon]," said Henri Gavin, assistant professor of civil and environmental engineering. "It's hard to know exactly where a force will be felt the greatest after the building is constructed."
The Pentagon's partial collapse has raised questions about the building, which is considered by some to be one of the world's most protected buildings.
"There are parts of the Pentagon that are secure like bomb shelters, and they were obviously not affected by the crash," Petroski said. "The upper stories above ground, though, were obviously not built to withstand an airplane crash."
Some professors predict these events will lead to building design changes that will take into account various security precautions.
"I would say that the mass and the energy included in a [large plane] flying at hundreds of miles an hour would be far in excess of any of the loads that a designer would consider in planning a building," Gavin said. "However, I think new considerations of terrorist attacks will definitely be included in designing buildings in the future to protect our facilities."
More immediately, though, Petroski believes these events will change city skylines and halt skyscraper construction.
"The most significant, immediate effects will be those that affect skylines for decades to come. We may start building down rather than straight up," he said. "Either way, New York City is going to look different. It literally looks like a war zone."
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