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Engineers Analyze WTC Collapse

Wed May 01, 2002 - National Edition
Construction Equipment Guide

Fireproofing that hardens steel beams and emergency stairwells "to withstand the impact of a plane should be included in high-risk buildings in the wake of the World Trade Center collapse, a federal study says.

The report from the Federal Emergency Management Agency and the American Society of Civil Engineers also said the trade center’s steel supports, called trusses, may have played a role in allowing the buildings to collapse as they did. But the report said more study was needed before a final conclusion could be drawn.

It said investigators detected no substandard structural problems at the trade center. In fact, the towers exceeded building code requirements in some areas, it noted.

The report obtained by The Associated Press confirmed the consensus that barring a windstorm or an arthquake the twin towers could have withstood the impact of the two hijacked Boeing 767 airliners that plowed into the trade center on Sept. 11. The towers succumbed to the ensuing fire, fed by thousands of gallons of aviation fuel, that softened the buildings’ steel framework.

Still, it was unclear whether engineering ever could protect buildings from fast-moving aircraft. Professor Jonathan Barnett of Worcester Polytechnic Institute, one of the study’s investigators, said the study proved overall the trade center performed well on Sept. 11. But he conceded that the overall findings "could have a chilling effect on construction" of tall buildings.

The National Institutes of Standards and Technology (NIST) is poised to take over the investigation, which could take two years and cost $16 million. NIST could eventually make broad recommendations to the building and fire codes using the trade center findings.

One key question will revolve around the trusses, the bracket-like steel supports which held up each of the 110 floors and provided lateral support to the skyscrapers’ vertical columns.

Another will be whether fireproofing and stairwell safety recommendations for buildings "evaluated or designed for extreme events" should apply to structures generally.

The jets’impact is believed to have blown off the fluffy fireproofing material on the trade center’s steel columns, making them susceptible to the intense heat from ensuing fire. Fireproofing able to withstand such impact is used in U.S. Navy destroyers but the cost could be prohibitive, Barnett said.

In theory, the team found, occupants in floors above the impact could have escaped had the stairwells been strengthened and had emergency escape routes not been so close together.

Much of the jet fuel in each plane was quickly consumed in massive fireballs that caused structural damage. But it was the remaining fuel that spilled across floors and down elevator shafts, setting ablaze furniture, computers, paper files and the planes’ cargo over multiple floors and igniting an inferno.

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