Fire that swept through the World Trade Center's twin towers likely led to their collapse about 90 minutes after being hit by hijacked airliners, structural engineers say.

        Although stressing that they can only speculate, engineers said the intense heat caused by the explosions could have weakened the structural steel, causing floors to collapse on each other like pancakes.

        Planes crashed into the upper floors, causing gaping holes. But the impact itself would not have caused the collapse, experts say.

        “One thing that was clear was ... the purpose was to start a big fire in there,” said Richard Miller, associate professor of civil and environmental engineering at the University of Cincinnati. “When structural members are heated, they do lose strength.”

        Leakage of jet fuel through the building would have intensified the fire, contributing to the failure of structural steel, said Oliver McGee, chairman of the civil and environmental engineering and geodetic science department at Ohio State University.

        “Then the upper floors begin to collapse, because connections fail,” said Dr. McGee, a Woodward High graduate. “One floor collapses on another. It's not designed to carry two floors, so it collapses on another one, and there's a domino effect.”

        Both Dr. Miller and Dr. McGee said that engineers don't design buildings to withstand such an attack. But the Chicago Tribune reported that the structural engineer who designed the towers said last week that their steel columns could remain standing if they were hit by a 707.

        Dr. Miller took some solace in the fact that the Trade Center “took that impact and explosion, and it stayed up (temporarily). At least we bought ourselves an hour. How many people did we get out of there in an hour?”

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