September 12, 2001   http://query.nytimes.com/gst/fullpage.html?res=9B0DE4DD1238F931A2575AC0A9679C8B63 

THE BUILDINGS

Believed to Be Safe, the Towers
Proved Vulnerable to Jet Fuel Fire

By JAMES GLANZ

  he cause of the twin collapse
  yesterday of the World Trade
Center towers in downtown
Manhattan was most likely the
intense fire fed by thousands of
gallons of jet fuel aboard the two
jetliners that crashed into the
buildings, experts on skyscraper
design said.

The high temperatures, of perhaps
1,000 to 2,000 degrees, probably
weakened the steel supports, the
experts said, causing the external
walls to buckle and allowing the
floors above to fall almost
straight down. That led to
catastrophic failures of the rest of
the buildings.

The towers were built to
withstand the stresses of
hurricane-force winds and to
survive the heat of ordinary fires.
After the 1993 trade center
bombing, one of the engineers
who worked on the towers'
structural design in the 1960's
even claimed that each one had
been built to withstand the impact
of a fully loaded, fully fueled
Boeing 707, then the heaviest
aircraft flying.

No engineer could have prepared
for what happened yesterday, the
experts said. "No structure could
have sustained this kind of
assault," said Richard M. Kielar,
a spokesman for Tishman Realty
and Construction Company, the
construction manager for the
original project.

The enormous heat from the jet
fuel fire probably caused the steel trusses holding up
concrete-slab floors and vertical steel columns to bend
like soft plastic, said Jon Magnusson, chairman and chief
executive of Skilling Ward Magnusson Barkshire in
Seattle, a structural engineering firm that worked out the
original design.

The skyscrapers had two means of defense against normal
fire damage, Mr. Magnusson said. One, thick layers of
insulation sprayed onto the steel beams, could have been
breached by the initial crash, he said. Another, the
building's sprinkler system, may have been disabled as
well, or it may simply have been useless in the heat of the
jet fuel fire.

Although they resisted collapse immediately after the
planes' first impact, the hundreds of steel columns spaced
around the outer facing of each tower eventually failed.

"They buckled outward and then the floors came down,"
said Mr. Magnusson, who warned that no conclusions
could be reached yesterday since the information
available was so sketchy.

Other experts agreed that the extreme conditions caused by
the fire, and not unusual vulnerabilities of the buildings,
were the likely causes of the collapse.

"There isn't anything particularly vulnerable about it," said
Aine Brazil of Thornton-Tomasetti Engineers in New
York, a structural engineering firm that worked on the
Petronas Towers, the world's largest buildings, in
Malaysia.

Buildings are simply not designed to withstand "the
extreme levels of heat that would be found in the situation
with the amount of jet fuel and the explosion that
occurred," Ms. Brazil said.

Mr. Kielar, the Tishman spokesman, said it was too early
to piece together a precise train of events, but he agreed
that weakening by fire, followed by catastrophic collapse
of the floors, was the most likely possibility. "As the
structure warped and weakened at the top of each tower, it
— along with concrete slabs, furniture, file cabinets and
other materials — became an enormous consolidated
weight that eventually, progressively crushed each tower
below," he said in a statement.

The later collapse of the smaller 7 World Trade Center
could have been caused by a combination of falling debris
and a less intense fire — one not accelerated by jet fuel
— lasting several hours, said Brian McIntyre, chief
operating officer of Skilling Ward. Such a building is
"basically designed to resist heat buildup for three hours,"
he said. 

The structural design of the two towers, fairly common
now, was considered innovative in its day. Instead of the
heavy internal bracing and heavy exterior masonry of, for
example, the Empire State Building, the designers of the
trade center towers chose a light glass-and-steel facing
threaded by steel columns. Those columns, 61 on each
side, gave the towers most of their stiffness and largely
held them up, said John Schuring, a professor and
chairman of civil engineering at the New Jersey Institute
of Technology.

"The major strength of the building is in its skin," Dr.
Schuring said.

There was also a cluster of columns in the center,
supporting structures like the stairs and elevators, he said.
A network of steel trusses ran between the two sets of
columns, holding up each concrete floor and providing
further strength to the buildings.

A special set of plates on each floor ran among the
trusses, serving to dampen stresses on the buildings
caused by winds of up to 200 miles per hour, said Jack
Cermak, president of Cermak Peterka Peterson in Fort
Collins, Colo., the firm that did the wind-tunnel testing for
the design of the towers.

Dr. Cermak agreed that the impact of the crash itself
probably could not have collapsed the massively
reinforced building on its own.

"I presume, without knowing the details, that that collapse
was caused by weakening of the structure due to the heat,"
Dr. Cermak said.

Matthys Levy, an architect at Weidlinger Associates and
the author of "Why Buildings Fall Down" (Norton, 1992),
watched the first tower collapse while standing at Seventh
Avenue and Houston Street, some 20 blocks away. 

"I saw the beginning of the top moving down, and the
whole thing collapsed in a cloud of smoke," Mr. Levy
said. "From what I saw, it seemed to come straight down."

Mr. Levy said the situation was much different from the
one that occurred in 1945 when a much smaller plane
slammed into the Empire State Building. That plane, a
bomber with a smaller impact and less fuel, ripped a
20-foot hole in the structure, but the building remained
standing.

There was some disagreement yesterday about whether,
decades later, the trade center towers had been designed
to withstand an impact from an airliner filled with fuel.

The engineer who said after the 1993 bombing that the
towers could withstand a Boeing 707, Leslie Robertson,
was not available for comment yesterday, a partner at his
Manhattan firm said. 

"We're going to hold off on speaking to the media," said
the partner, Rick Zottola, at Leslie E. Robertson
Associates. "We'd like to reserve our first comments to
our national security systems, F.B.I. and so on."

But Anthony G. Cracchiolo, director of priority capital
programs for the Port Authority of New York and New
Jersey, which owned the buildings, said little thought had
been given to the possibility of a plane crash into the
towers.

"We never were asked to consider trying to protect the
building from such a threat," said Mr. Cracchiolo, who
was among those who coordinated the reconstruction after
the 1993 bombing. "As structural engineers, there is
nothing we could have done to protect the building from a
direct impact from a plane as large as these."

Melvin Schweitzer, a member of the Port Authority board
of commissioners from 1993 to 1999, said, however, that
the board repeatedly inquired about that possibility. "We
were just told that architects had explained that the
building was designed to withstand a jet," Mr. Schweitzer
said. "Frankly, when we raised that question, most of us
were thinking of a small plane." 

The architectural firm for the trade center, Minoru
Yamasaki Associates of Rochester Hills, Mich., declined
to answer specific questions about the collapse, and
issued only a brief statement.

"The company has been in contact with law enforcement
authorities, and we will provide any assistance we can to
aid the rescue efforts," the statement said. "In this time of
national emergency, we believe that any speculation
regarding the specifics of these tragic events would be
irresponsible."