NEW YORK - US experts said Tuesday they will call for major changes to the way skyscrapers are built and managed after an exhaustive inquiry into why the World Trade Center towers collapsed in the September 11 attacks in 2001.
The experts said the two towers were felled by the way the hijacked jets dislodged key supporting columns, stripped away fireproofing and set ablaze masses of office material that burned long after the jet fuel had been consumed by fire.
Osama bin Laden's Al-Qaeda hijacked the jets which brought down the New York towers and crashed into the US Defence Department headquarters in Washington. About 2,800 people were killed in New York and about 180 in Washington.
The Commerce Department's National Institute of Standards and Technology (NIST) said its report was "the most detailed examination of a building failure ever conducted."
The NIST said it had found a number of safety practices and technologies that could have saved lives on September 11 and would be included in the agency's full report to be released in June.
It said it would recommend improvements in the way people design, construct, maintain and use buildings, especially high-rises. It said this would range from the design of elevators to the way office workers are trained.
"While the buildings were able to withstand the initial impact of the aircraft, the resulting fires that spread through the towers weakened support columns and floors that had fireproofing dislodged by the impacts. This eventually led to collapse as the perimeter columns were pulled inward by the sagging floors and buckled," said Shyam Sunder, lead investigator for the agency's building and fire safety investigation into the WTC disaster.
The report said each aircraft severed perimeter columns, damaged interior core columns and knocked off fireproofing from steel as the planes penetrated the buildings. The weight carried by the severed columns was distributed to other columns.
Fires caused by the jet fuel were fed by the building contents and the air supply resulting that came from the breached walls and windows.
"These fires, in combination with the dislodged fireproofing, were responsible for a chain of events in which the building core weakened and began losing its ability to carry loads.
"The floors weakened and sagged from the fires, pulling inward on the perimeter columns. Floor sagging and exposure to high temperatures caused the perimeter columns to bow inward and buckle -- a process that spread across the faces of the buildings."
04/05/2005 15:15 GMT