(The buildings trusses were BOLTED to the outside towers with 5/8 and 7/8 inch bolts---NOT WELDED. The trusses were the only steel cross-members that held the outside, vertical steel columns--the shell-like walls of the towers. This design was the primary reason the collapse occured. The towers had no large girders as cross members, to provide "lateral support"---hold the columns vertical---as do most high rise buildings.)

The bolts, on these critical supports of the vertical columns--walls--of the Towers, pulled loose and the floors pancaked down, one on top of the other, as the top floor came down on the lower.

ENGINEERS STUDY WTC COLLAPSE By SHANNON McCAFFREY

WASHINGTON (AP) - Fireproofing that sticks to steel beams and emergency stairwells "hardened" to withstand the catastrophic 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 unique steel supports - called trusses - "may have played a role in allowing the buildings to collapse in the manner that they did." But the report said more study was needed before a final conclusion could be drawn.

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

The report, which was obtained Tuesday by The Associated Press, confirmed the consensus that barring a windstorm or an earthquake 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.

"The fact that the structures were able to sustain this level of damage and remain standing for an extended period of time is remarkable and is the reason that most building occupants were able to evacuate safely," according to the report, which was to be released Wednesday by the House Science Committee.

Still, the report said it was unclear whether engineering ever could protect buildings from fast-moving aircraft. "Reliably designing a building to survive the impact of the largest aircraft available now or in the future may not be possible," the report said.

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.
"We didn't find a glaring blunder," Barnett said.

Still he conceded that the overall findings "could have a chilling effect on construction" of tall buildings.

The National Institutes of Standards and Technology is poised to take over the trade center collapse investigation, which it expects to 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 impact of the jets 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 the floors above the impact could have escaped had the stairwells been strengthened to withstand the attack and had the emergency escape routes not been placed so close together. The two Boeing 767s slammed into the twin towers on the morning of Sept. 11. The north tower, struck first, stood for 102 minutes. The south tower fell 56 minutes after impact.

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