“Steel structures do not usually crumble because of an impact
such as that of the planes which hit the towers,” said Mike
Richardson, a San Francisco structural engineer.
Each tower was built with an inner steel framework and an
outer steel framework, which supported the floors. The floors
had been coated with a fire-retardant coating that was supposed
to resist the heat of a fire for about three hours, Richardson
explained.
“The hijacked planes were going coast to coast and were full
of fuel. That (fuel) was the primary weapon in disabling the
towers and eventually causing their destruction,” said Darell
Lawver, structural engineer in Los Altos, California.
The fire-retardant coating could not withstand the heat of
the fires because of its intensity. The fires were hot enough
to go through the coating which in-turn caused a weakening
of the inner framework.
“Steel melts at anywhere from 1000 to 2000 degrees,” said
Earl Sanford, a steel manufacturer. According to news reports,
the fires caused by the plane’s fuel produced temperatures
well in excess of 2000 degrees.
With the inner framework of the towers weakened, the floors
would have collapsed on top of each other like pancakes in
a stack. “The floors collapsing on one another would have
caused strain on the outer framework of the towers which would
cause them to collapse as they did,” said Richardson.
The towers were built to withstand the impact of a 707 airplane
not because of fear of terrorism but due to the amount of
air traffic in the area. Following the World Trade Center
bombings in 1993, John Skilling, one of the chief designers
said that “The firm’s analysis indicated that the biggest
problem from a plane crash would be the dumping of fuel into
the building.” Skilling died in 1998.
The engineering firm of Skilling Ward Magnusson Barkshire,
based in Seattle, designed the twin towers of the World Trade
Center. Tower One was built in 1970 and Tower Two was built
in 1972. Both towers were 110 floors. The same firm also designed
the Hawaii Convention Center.
