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Towers' Wind-Force Design Questioned - June 19th, 2004


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."


© Copyright 2004, The New York Times

Sally Regenhard,

P. O. Box 70
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Monica Gabrielle,

P. O. Box 70
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