WSJ -- Heat of Jet Fuel Fire Likely Caused Collapse of World Trade Buildings
September 12, 2001
Heat of Jet Fuel Fire Likely Caused
Collapse of World Trade Buildings
By LEE GOMES
Staff Reporter of THE WALL STREET JOURNAL
The intense heat of a jet-fuel fire, rather than structural damage from the
impact of the airplanes, is probably what led to the collapse of the two World
Trade Center buildings.
"The mechanical hit takes out a good chunk of the building, but it will still
stand," said Larry Anderson, an expert in fire damage at Exponent Failure
Analysis Associates, a Menlo Park, Calif., consulting operation. But when you
spray thousands of gallons of fuel around, and then light it all at once, that
softens the building and leads to its collapse."
Structural engineers who watched television reports of Tuesday's catastrophe
noted that the World Trade Center towers remained standing about an hour
following the impact of the first plane. During that time, though, a fire was
raging out of control on multiple floors of the buildings. Jet fuel is extremely
combustible, and produces fires that can easily exceed 2,000 degrees
Fahrenheit, more than 500 degrees hotter than other kinds of more routine
office fires.
Steel, though, begins to weaken at 800 degrees Fahrenheit, and comes close to
melting at around 1,500 degrees. While steel in modern high rises is routinely
coated with fire proofing materials, those materials can't protect the steel from
prolonged, intense heat.
"Once the physical damage to the building was done, if the fire wasn't
extinguished in a very short period of time, the likelihood of collapse was
100%," said Charles Warren, chairman of Engineering Systems Inc., of
Aurora, Ill.
Rather than tilting over and falling, the towers appeared to "implode" on top of
themselves, in much the same carefully controlled manner as buildings that are
demolished.
That is most likely because once the steel at the points of impact could no
longer support the floors above them, those floors rushed straight downward,
creating an unstoppable force that went all the way to the ground, said Tom
O'Donnell, of O'Donnell Consulting Engineers Inc. in Bethel Park, Pa.
Engineers said the fact that the towers stood so long after the impact of the
airplanes was testimony to the engineering skills of the buildings' initial
designers. The impact of the airplanes certainly damaged, or even destroyed,
the pillars around the perimeter of the structures, which are a key part of the
buildings' overall support system.
But there was enough "redundancy" in the design, in the form of support pillars
at the core of the structure, to pick up the load and keep the building from
toppling, say engineers.
"It was a perfectly well-designed building," said Jim Wiethorn, of Haag
Engineering Co. in Houston.
While the fire was likely to be the principal cause of the collapse, whatever
damage was caused by the crashes themselves certainly weakened the
buildings and made them more vulnerable, say engineers.
The consensus among engineers Tuesday was that there is no way a building
could be economically designed to withstand the sort of devastation involved in
Tuesday's attacks. The worse calamity that skyscrapers are designed to endure
are massive hurricanes.
Similarly, said engineers, even the most advanced fire codes can't protect a
structure from the explosive power of a fully loaded airplane ramming into its
side.
Write to Lee Gomes at lee.gomes@wsj.com
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