Wednesday September 12, 2001

Towers were constructed to survive plane crash, not extreme fire

Despite the impressive construction of the 110-story World Trade Center, the glass and steel structure did not manage to survive yesterday's unexpected terrorist attack.

Although the impact of the jetliners was strong, it was the heat from the explosion that most likely caused the buildings to collapse, experts say.

Richard Ebeltoft, a structural engineer and University of Arizona architecture lecturer, speculated that flames fueled by thousands of gallons of aviation fuel melted the buildings steel supports. Ebeltoft said steel loses half its strength when heated to temperatures of 700 to 1,000 degrees.

An exposition like this, he said, probably caused fires that burned at temperatures between 1,500 and 2,200 degrees.

"In very tall buildings like this - called a tube structure - the outside structure behaves like a steel tube," Ebeltoft said. "With this (explosion) it's like you drove holes through the building. When you lose that kind of support, any lateral loads, like winds, will cause the structure to lose balance."

Once the top of the building lost balance, he said, the floors appear to have begun shredding downward.

The Twin Towers, which were once the world's tallest buildings, were designed by architect Minoru Yamasaki, who worked with engineers John Skilling and Leslie E. Robertson.

In his 2000 book "Building Big," architect David McCaulay described the towers' engineering as "a series of load bearing exterior columns spaced three feet apart and tied together at every floor by a deep horizontal beam, creating a strong lattice of square tubing around each tower."

Sawteen See, a managing partner in Robertson's engineering firm, said the twin towers were originally designed to sustain a direct hit by a large jetliner, but that such construction couldn't make them fire- or bombproof.

He said that the two towers have staircases in all four corners of the buildings and were designed to be evacuated in an hour, but it appeared that since the planes crashed into the corners, escape was cut off for those on the floors above.

"I could never conceive of anybody being able to bring down those two buildings," Brown added.

-The Associated Press contributed to this report.