Note, this was previously posted at 911blogger here. 

The Seattle Times

Twin Towers Engineered To Withstand Jet Collision

Eric Nalder

Engineers had to consider every peril they could imagine when they designed the World Trade Center three decades ago because, at the time, the twin towers were of unprecedented size for structures made of steel and glass.

"We looked at every possible thing we could think of that could happen to the buildings, even to the extent of an airplane hitting the side," said John Skilling, head structural engineer. "However, back in those days people didn't think about terrorists very much."

Skilling, based in Seattle, is among the world's top structural engineers. He is responsible for much of Seattle's downtown skyline and for several of the world's tallest structures, including the Trade Center.