Crashes Destroy Engineering Wonders

Sept. 11, 2001 — -- They survived powerful hurricane gusts, even a bomb explosion, but this morning's two intentional plane crashes reduced the twin towers of the World Trade Center to rubble.

The 110-story towers, the tallest buildings in the city and the fifth and sixth tallest in the world, collapsed in billows of debris following two plane crashes to their sides.

Despite initial damage from the crashes, the two twin towers remained standing for just over an hour this morning and appeared to be a testimony to the abilities of structural engineering.

But experts say structural damage, caused mostly by fires following the impacts, was evidently severe enough to overburden the lower sections of the towers and eventually cause both towers to topple.

"The World Trade Center was designed as a very large tube with steel columns on the perimeter of the building," said John Cryan, president of Severud Associates, a structural engineering firm that provides consulting to skyscraper architects. "What must have happened is the top part of the buildings probably collapsed and that put too much weight on the lower halves and that had a domino effect on the entire towers."

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The World Trade Center hosted an estimated 50,000 employees and received an average of 1.8 million visitors annually.

Part of the severity of the damage, Cryan believes, was the place of impact by the planes. The lower the crashes, the greater the damage to the towers' overall integrity. The first tower to crumble, the southern tower, was the one that had received the lowest strike by an oncoming aircraft.

Rich Behr, a professor of architectural engineering at Pennsylvania State University, further points out that the approximate one-hour delay in the towers' collapses suggest the main damage was likely caused, not by the plane strikes, themselves, but by fires that burned inside the buildings for more than an hour following the crashes. These fires, fueled by the aircrafts' fuel tanks, likely caused the steel beams to melt and lose their stiffness.