Report- WTC towers may have been weaker based on faulty tests

NEW YORK -- The World Trade Center's designers may have severely underestimated the forces that wind exerted on the twin towers, leading them to design skyscrapers less able to handle the Sept. 11 attacks, a federal report says.

Wind tunnel tests conducted as part of litigation over the buildings' collapse found wind loads 20 to 60 percent higher than those found in tests performed during the towers' design in the 1960s, according to the report released Friday by a federal institute examining the collapse.

The buildings would have been stronger and might have performed better during the 2001 attacks by terrorists in hijacked jetliners if the higher wind load numbers had been used, said Shyam Sunder, the lead investigator for the National Institute of Standards and Technology.

"It is relevant to evaluating the buildings' capacity to withstand an unanticipated event," he said.

The report also provided the first official estimate of the number of people in the twin towers on the morning the planes struck.

The agency estimated that between 16,200 and 18,600 people were in the mammoth buildings, based on interviews with more than 1,000 of the occupants and with first responders.

About 2,800 people were killed in the attack.

The institute's investigation began two years ago as an effort to determine why the buildings collapsed and apply the lessons to possible improvements in building codes and other standards. Its final report is due in December.

Investigators have found that north tower structural components that had been coated with thicker fireproofing during a 1990s upgrade had more than three times the ability to withstand fire than original components in the south tower.

The institute is examining whether fireproofing was a factor in the north tower's ability to stand twice as long as the south tower after it was hit.

The report also details the collapse of 7 World Trade Center, a 47-story building that was not hit by a plane but burned for hours before collapsing that afternoon.

The city's emergency operations center was located in the building, which housed diesel fuel to power the center and generators for other tenants. The NIST found that the collapse most likely originated around the fifth floor, near a distribution system for the fuel.

No one was killed in the collapse of 7 World Trade Center, but the Giuliani administration has been criticized for locating the city's emergency center in a building containing thousands of gallons of diesel fuel.