Intense heat melted steel supports in Trade Center
Wednesday September 12, 2001
Towers were constructed to survive plane crash, not extreme fire
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.
impact of the jetliners was strong, it was the heat from the explosion
that most likely caused the buildings to collapse, experts say.
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
An exposition like this, he said, probably caused fires that burned at temperatures between 1,500 and 2,200 degrees.
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.
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.
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
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.
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.