Damage to fireproofing took down Trade Center

Towers could have stood if shields held, report says

April 06, 2005|By Stevenson Swanson | Stevenson Swanson,CHICAGO TRIBUNE

NEW YORK - Neither the impact of two passenger jets nor the raging fires they ignited were enough to bring down the World Trade Center towers by themselves, according to a comprehensive study of the towers' collapse released yesterday.

But when the planes hit the twin towers on Sept. 11, 2001, they dislodged the fireproofing protecting the steel columns and trusses that held the buildings together. That proved to be the key factor, according to the National Institute of Standards and Technology.

Stripped of protection, the steel sagged after prolonged exposure to temperatures of at least 1,000 degrees, triggering the towers' pancaking collapse that killed 2,749 people.

But lead investigator Shyam Sunder said the failure of the fireproofing was not as a result of improper installation or lax maintenance.

"You would not expect fireproofing to be installed to withstand the impact of an airplane," Sunder said, speaking at a news conference at a Times Square hotel. "We are not suggesting here that people should design buildings to withstand airplanes."

Based on mathematical modeling that pushed the boundaries of computer science, the agency's conclusion is the most authoritative statement likely to emerge about the sequence of events on Sept. 11, 2001.

The institute's wide-ranging investigation is examining not only the engineering issues behind the collapse but also the evacuation of the towers, the response of police, firefighters and other emergency personnel, and many technical issues. The study is intended to find out whether current building codes, construction practices and emergency procedures need to be revised to take account of the threat of terrorism.

At yesterday's conference, the agency released three reports on which it has completed work, including a study of how occupants evacuated the building and how communications hampered firefighters.

To re-create the impact of the two fuel-laden Boeing 767 passenger jets, investigators created extremely detailed mathematical models of the planes and the towers.

In both cases, the planes broke apart into thousands of fragments within one second of striking the towers, Sunder said. The impact was so powerful that the south tower swayed for four minutes.

The jet fuel burned up in a few minutes, but it ignited the contents of the floors, such as papers and furniture. Even so, the towers likely would have remained standing if the impact had not also dislodged the sprayed-on fireproofing material that covered the towers' steel columns and floor trusses, the study found.

Since the attacks, several theories have been put forward about the sequence of events that caused the towers to collapse. Questions arose about the reliability of the towers' design, an innovative arrangement of exterior columns linked by floor trusses to an inner core of columns. Some architects and engineers speculated that the trusses separated from the outer columns in the fire and led to the collapse.

Sunder said the institute's investigation concluded that, apart from the impact areas, the trusses mainly stayed in place. But subject to high heat and stripped of fireproofing, they sagged and pulled the fire-weakened outer columns inward.

The Chicago Tribune is a Tribune Publishing newspaper.

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