October 20, 2004
Study Suggests Design Flaws Didn't Doom Towers
By ERIC LIPTON
WASHINGTON, Oct. 19 - After the most sophisticated building analysis in United States history, federal investigators have arrived at the clearest picture yet of the sequence of events that led to the collapse of the World Trade Center towers, largely ruling out a design flaw in the buildings as a central factor in the catastrophe.
Since the twin towers fell, questions have reverberated among families of victims and some fire-safety experts about whether insufficient fireproofing or an unusual weakness in the innovative, lightweight floors played a critical role in the collapse.
Instead, the investigators tentatively conclude in nearly 500 pages of documents released Tuesday, the twin towers failed because the structural columns at the buildings' core, damaged by the impact of the airliners, buckled and shortened as the fires burned, gradually shifting more load to the tower's trademark exterior pinstripe columns. The exterior columns ultimately suffered such extraordinary stress and heat that they gave way.
The investigation - based on an analysis of thousands of photographs and videos, an examination of nearly every element used to construct the towers and meticulous computer-enhanced modeling of the plane impacts and spreading fires - is not yet complete. A final report by the National Institute of Standards and Technology is not scheduled to be issued until December or perhaps January.
In interviews Tuesday, the lead investigator, as well as other engineers who have studied the collapse, said the evidence increasingly suggested that the giant structures - given the extreme conditions, including temperatures that reached more than 1,000 degrees - performed relatively well on Sept. 11, 2001.
"We always said we had no preconceived notions, and that we would look at the failure information dispassionately," said S. Shyam Sunder, the lead investigator at the institute, a division of the Department of Commerce, which has conducted the two-year, $16 million inquiry at the request of Congress. "The buildings performed as they should have in the airplane impact and extreme fires to which they were subjected. There is nothing there that stands out as abnormal."
Elements of the design and construction of the towers, investigators said, certainly played a part in how long the buildings stood. Buildings designed differently - with more robustly protected and spread-out emergency stairwells, for example - engineers said yesterday, might still have resulted in fewer deaths.
But the most severe shortcomings identified at the World Trade Center in the institute's comprehensive review do not pertain to how the buildings were conceived or built. Instead, the failings on Sept. 11 were chiefly found in the response by the New York City Fire and Police Departments, which was hampered by inadequate command, unreliable communications equipment and an overwhelmed dispatching system.
For Leslie E. Robertson, the structural engineer who helped design the twin towers as a young man back in the early 1960's, the latest findings buttress his longstanding assertion that the towers were fundamentally sound. His wife, Saw-Teen See, who is a managing partner at Mr. Robertson's New York design firm, said the report "validates the way we thought the structure would have performed."
The findings by the institute, however, still do not exonerate Mr. Robertson or the building's owner, the Port Authority of New York and New Jersey, which, in defending the trade center project from critics in the 1960s, boasted that the design was so robust that the towers could be hit by a jet traveling at 600 miles per hour without collapsing or endangering the lives of occupants beyond the impact zone. In retrospect, such a claim was unjustified because the engineers had failed to consider the added stresses caused by the resulting fires.
Sally Regenhard, who founded a group called the Skyscraper Safety Campaign in honor of her son, Christian Regenhard, a probationary firefighter who was one of 2,749 victims, said she was not ready to concede that the towers' design were not fundamentally at fault.
"It is far, far too premature to come to any conclusions that it wasn't the fault of the building, or nothing was wrong with the building," she said after listening to a daylong presentation in Gaithersburg, Md., about the latest findings.
The investigators have examined just about every possible factor that could have contributed to the collapse, including the steel used in the columns. Computer models were used to calculate, as accurately as possible, where different airplane parts traveled, and what kind of damage they did. Then, intricate models were built, essentially recreating the resulting fires.
Through all this, particular attention has been focused on the innovative floors that were central to the design of the twin towers. The floors were particularly critical in the trade center because in office buildings built before the 1960's, structural columns and beams were generally spread throughout - holding up the enormous weight and allowing the tower to resist the force from wind.
In the trade center, only the building's exterior and core had structural columns, and in between them were wide-open floors - relatively lightweight, decklike structures. Some engineers have wondered if insufficient fireproofing on the floor trusses led them to fail, undermining the structural integrity of the towers.
The federal investigators found, after conducting a test with a reconstructed section of the floor, that the original fireproofing on the floors, as built, was sufficient to ensure that they met the New York City building code under standard testing parameters.
Instead, the report released yesterday say, the Boeing 767 planes ripped through a swath of exterior steel columns, resulting in an immediate redistribution of the load to adjacent perimeter columns and, to a lesser extent, to the core columns.
As the planes penetrated the towers, they destroyed sections of the floors, knocked off spray-on fireproofing and severed three to 10 of the core columns in each tower.
The report found that the towers were able to stand, despite the initial assault, as "loads on the damaged columns were redistributed to other intact core and perimeter columns mostly via the floor systems and to a lesser extent, via the hat truss," a steel structure at the top of the towers that was connected to the core and perimeter columns.
The infernos that erupted in the two towers are to blame for the ultimate collapse, the report found. As temperatures rose in the buildings, the remaining core columns softened and buckled, shifting much of the burden to the building's exterior. The floors, which largely remained intact outside the impact zone, reacted by pulling the exterior columns inward, adding to the extreme stress on the exterior columns.
In the north tower, as fires consumed office furniture and other debris, softening the steel in the exterior columns, they gradually started to bow inward and then buckle. Ultimately, the entire upper section of the building above the impact zone tilted to the south, sealing the fate of the tower and anyone who remained inside.
"The buckled columns exceeded the strain energy that could be absorbed by the structure,'' the report says. "Global collapse then ensued."
The floors played a more significant role in the collapse sequence in the south tower, the investigators said. Fires there caused them to sag by as much as two feet, adding to the inward pulling that already had started because of the buckling of the core columns.
But the investigators say that without the plane impact - which weakened the structure and knocked fireproofing off the floor trusses and columns - if a typical office fire had occurred, "it is likely that burnout would have occurred without collapse."
The tentative conclusions by the federal investigators conflict with an earlier report by a team of structural engineers organized by the Federal Emergency Management Agency, who had asserted that the collapse of the north tower started in the core, not in the outer columns. But W. Gene Corley, a structural engineer from Skokie, Ill., who led that inquiry, said Tuesday that the new hypothesis was feasible, given that the federal team had the money to more closely dissect what happened.
But James G. Quintiere, a professor of fire protection engineering at University of Maryland, said he questioned the tentative conclusions, as his analysis showed that in the fires created by the impact, the lightweight floors rose to a temperature high enough to make them separate from the exterior columns. "They have not presented enough evidence," he said.
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