Burning jet fuel probably melted steel, causing towers to collapse

It likely wasn't the impact of two passenger aircraft that brought down the twin towers of the World Trade Center, according to the chief executive of the engineering firm that designed the structures.

Instead, the heat of burning jet fuel probably melted structural steel, sending one floor crashing on top of another, said Jon Magnusson of Skilling Ward Magnusson Barkshire, a Seattle-based firm that worked on the World Trade Center three decades ago, when the firm was named Worthington Skilling Helle Jackson.

Magnusson, who did not participate in the project, said most office buildings are designed with sprinkler systems and specially treated steel beams to prevent catastrophic fires from spreading.

Because neither tower collapsed within an hour after impact with the aircraft, Magnusson said the structures likely buckled when fires caused temperatures to exceed 1,600 degrees Fahrenheit, the point when steel begins to bend.

Jet fuel burns much hotter than paper or drywall. As a result, aircraft hangars are designed with systems that spread foam instead of water to suffocate fire. Office buildings, however, aren't equipped with such systems.

"It looks like the buildings survived impact," said Magnusson. "With most fires, you don't expect jet fuel. This is an event that buildings are not designed for, planes burning inside them."

Both World Trade Center towers seemed to fall on themselves, a pattern consistent with what's known as "progressive collapse" — when floors fall through the length of the building, said Magnusson.

Even though both planes seemed to hit the upper half of the towers, the weight of the top floors collapsing on the lower floors could have caused both buildings to come down, he said.

The towers didn't topple over, but crumbled into a relatively confined space, which likely saved hundreds of lives.

"If you crash an airplane into a building, anything could happen," said Magnusson. "If it had to fall down, this is the best way it could happen."

At the time the World Trade Center opened for business in 1975, there was buzz that it could withstand direct impact from a Boeing 707.

Magnusson said he doesn't know whether that precise scenario was part of the design specifications, but a possible strike by an aircraft might have been considered because a plane had crashed into the Empire State Building decades earlier.

Alex Fryer can be reached at 206-464-8124 or afryer@seattletimes.com.