Towers' Wind-Force Design Questioned
NYTimes.com - June 19th, 2004
By ERIC LIPTON
Engineers who designed the World Trade Center may
have significantly underestimated the force of the winds that the twin towers
needed to withstand in the worst of possible storms, federal investigators said
yesterday, an oversight that could have led to weaker-than-needed exterior steel
columns.
That design decision, although inconsequential for three decades, perhaps
shortened the time that tenants and rescue workers had to evacuate the towers
before they collapsed on Sept. 11, 2001.
The finding, which the engineers dispute and that investigators acknowledge
needs more study and may turn out to be unfounded, was among dozens of interim
conclusions released yesterday as a two-year study on the collapse of the twin
towers neared a conclusion.
The report, released yesterday by the National Institute of Standards and
Technology in Boulder, Colo., includes detailed photographs that identify, for
the first time, the exact spots where the exterior steel columns gave way. These
critical zones were around the 81st floor in the south tower, near the offices
of Fuji Bank, and near the 96th floor of the north tower, where Marsh &
McLennan was based.
For 7 to 10 minutes before the towers fell, the photographs show exterior
columns bowing inward, almost like pieces of cooked spaghetti, a sign that they
are about to give way.
Even so, the institute investigators have not pinpointed why the two towers
collapsed after they were able to withstand the initial impacts by the Boeing
767 planes. But two primary theories have been identified.
As the fires ignited by the planes burned out of control, the exterior steel
columns in the towers, designed primarily to resist wind, might have given way.
Alternatively, the innovative lightweight floors that connected the exterior of
the towers to their cores might have sagged, pulling the exterior columns inward
and starting the collapse, the interim report says, adding that it might have
been a combination of both factors.
The investigators also released yesterday, for the first time, an estimate
that put the number of people in the towers at the time of the attack at about
17,000, about a third of their capacity given the early-morning hour. For that
reason, the report said, at least in part, almost all the tenants below the
floors of impact were able to flee before the towers fell. Those who died -
1,560 in the north tower and 599 in the south tower - were with few exceptions
at or above the impact zone, or they were among the 433 firefighters, police and
security guards involved in the rescue effort.
If the towers had been fully occupied, with 50,000 tenants and visitors, the
evacuation would have taken about four hours, the report says. The unstated
implication is obvious: many thousands more would have died, given that the
south tower fell in 56 minutes and the north tower in 102 minutes.
A crucial remaining question, investigators said, is whether the design of
the towers - their structure, their emergency stairwells and countless other
safety features - might have contributed to the number of deaths.
When the inquiry is completed at the end of the year, the investigators
expect to recommend changes in building and fire codes nationwide that would
better protect tall buildings against fires or other calamities, although not
necessarily an airplane crash.
"It is going to move forward the state of the art and improve the safety of
buildings in the future," said Dr. S. Shyam Sunder, the acting deputy director
of the Building Fire and Research Laboratory at the National Institute of
Standards and Technology, and the leader of the investigation.
Something as simple as fireproof wall partitions, which were not required in
the twin towers, might have limited the spread of the fires, Dr. Sunder said
yesterday.
Corporate tenants typically prefer open floor plans - they were a major
selling point for the twin towers - but the partitions might have slowed the
spread of fires that were ignited by jet fuel, he said, perhaps allowing the
fires to burn themselves out.
The questions about the wind tests conducted in the early 1960's as the twin
towers were being designed surfaced after institute investigators examined data
collected in 2002, when two laboratories conducted separate wind tests, as part
of an insurance lawsuit. The tests both came up with different results. But they
predicted a wind load on the towers during an extreme storm that was 20 to 60
percent greater than an engineer for the Port Authority of New York and New
Jersey had reported using during the original design calculations. The
structures are designed to handle a storm that might happen once every 500
years.
On the day of the attack, the winds were light, and the pressure on the
exterior columns was so modest that they were using about 16 percent of their
capacity, studies have shown.
When the planes hit, the towers swayed as much as 20 inches at their tops.
Even though a large swath of outer columns in both buildings' facades had been
blown out, the stress on the remaining columns rose to about 50 percent of their
capacity in zones at the top of the towers, and to more than 90 percent in spots
next to the impact holes, one study done for the insurance lawsuit has
shown.
At this point, the towers were strong enough to remain standing,
investigators said, but fire took its toll, weakening the steel columns and
consuming any available reserve. Institute investigators are now trying to
determine whether the original wind tests were accurate, or whether different
results would have led to stronger columns, thus increasing their reserve
capacity, and possibly preventing or delaying a collapse. Jack E. Cermak, a wind
engineer in Fort Collins, Colo., who did the tests in the early 1960's, said
that he remained confident that the original tests accurately predicted how much
force an extreme storm would have applied to the towers. In fact, during the
wind tunnel test - which used scale models of the towers - the vortex conditions
created by the high winds were so extreme that the twin towers had to be
redesigned to make them stronger. The twin towers were the first super tall
buildings for which wind tunnels were used to estimate wind loads.
"The initial studies in 1964 were conducted using the most sophisticated
modeling," Mr. Cermak said, "and it's an approach that is still being used at
wind tunnels today."
Leslie E. Robertson, a primary structural engineer on the project, said
yesterday that he had not read the interim report, but said it was unlikely that
he and others underestimated the potential wind forces.
"That is nonsense," Mr. Robertson said. "It is a totally implausible position
to take."
Among the reasons for the differences, Dr. Cermak said, is that data
predicting how strong the worst possible winds might be in Lower Manhattan might
have changed in 40 years.
Dr. Sunder said the matter was still being investigated, in part by trying to
determine what kind of wind loads the towers, as built, could have sustained,
instead of relying on a published statement from a Port Authority engineer at
the time.
Regardless, he said, the findings suggest that there is a need for national
standards governing wind tunnel testing to ensure that engineers are accurately
estimating how strong skyscrapers and other buildings need to be in order to
withstand monster storms, or other cataclysmic events.
"We want different laboratories to come up with the same results," Dr. Sunder
said. "These results dictate the design of these tall
buildings."
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