The World Trade Center was designed by architect Minoru Yamasaki to withstand a jet crash better than other skyscrapers, according to authors of books about the Trade Center and a professor of architecture.
"The thought was the buildings could take a pretty heavy hit anywhere and survive," said author Eric Darton, author of Divided We Stand: A Biography of New York's World Trade Center.
"Obviously no one anticipated what this was," he said.
Experts said exterior bracing of the towers provided more protection against collapse than traditional design.
Kenneth Carper, Washington State University architecture professor and author of several engineering books, including Why Buildings Fail, said after watching video footage of the attack, he sees "two scenarios" that may have led to the collapses.
"One, eventually the upper levels collapsed, causing millions of tonnes of rubble to come down. That could lead to a progressive collapse all the way down," he said. "In the second scenario, the impact could have displaced the building a foot or two, causing it to lose its vertical stability. Then gravity takes over.
"The building is a bit like a drinking straw -- there's a lot of structural integrity, but only when the tube hasn't been violated," he said. Still, he said the design may have helped delay the collapse.
"It was amazing to see the structure was able to span above the rupture" for so long after the initial impact, he said.
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