Wtc collapse inquiry rules out design flaw
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Posted on Wed, Oct. 20, 2004
WTC collapse inquiry rules out design flaw
Crashes fatally crippled support columns, investigators say
BY ERIC LIPTON
New York Times
WASHINGTON — After the most sophisticated building analysis in U.S. history, federal investigators have largely ruled out a design flaw as a central factor in the collapse of the World Trade Center towers after the Sept. 11 terrorist attacks.
Since the twin towers fell in 2001, questions have reverberated among families of victims and fire-safety experts about whether insufficient fireproofing or an unusual weakness in the innovative, lightweight floors played a critical role in the collapse.
Instead, investigators tentatively conclude in documents released Tuesday, the towers failed because the structural columns at the buildings' core, damaged by the impact of the two airliners, buckled and shortened as the fires burned, gradually shifting more load to the towers' trademark exterior pinstripe columns. The exterior columns ultimately sustained 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 build the towers and 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 expected in December or January.
In interviews Tuesday, the lead investigator and other engineers who have studied the collapse said the evidence increasingly suggested that the structures — given the extreme conditions they faced, including temperatures that reached 900 or so degrees — performed relatively well on the day of the attack.
"We always said we had no preconceived notions, and that we would look at the failure information dispassionately," said S. Shyam Sunder, lead investigator at the institute, a division of the Commerce Department, which 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 — might still have resulted in fewer deaths, engineers said Tuesday.
For Leslie E. Robertson, the structural engineer who helped design the twin towers in the early 1960s, the latest findings buttress his assertion that the towers were fundamentally sound. His wife, Saw-Teen See, who is a managing partner at Robertson's New York design firm, said the report "validates the way we thought the structure would have performed."
But 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 design were not fundamentally at fault.
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 1960s, 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; 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 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 reports released Tuesday 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 reports 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, the exterior columns 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.
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 2 feet, adding to the inward pulling that already had started because of the buckling of the core columns.
But the investigators say that if a typical office fire had occurred — without the plane impact — "it is likely that burnout would have occurred without collapse."