...ask for "clear and convincing" evidence from NIST that "all the insulation was knocked off the core columns, since that is crucial to their conclusions."

Engineers involved with other studies of the WTC destruction also are weighing in on the latest NIST reports. "We think that rigorous efforts like these, taking a year or more, are needed…to counteract the kind of rash judgments that immediately followed the events of 9/11," says Najib N. Abboud, associate principal of Weidlinger Associates Inc. The New York City-based firm led a WTC probe sponsored by the WTC leaseholder, Silverstein Properties, which is among the firms being blamed for the WTC deaths in pending personal injury lawsuits. "Such premature judgments have misled victims’ families and unfairly besmirched the reputation of innocent parties," adds Abboud.

The interim report, released Oct. 19, says the towers’ structural steel was stripped of its fireproofing by the impact of the hijacked planes and the debris from the crashes. The heat from the resulting fires eventually sapped the strength of the unprotected steel. Eventually, the towers collapsed. NIST determined the region of dislodged fireproofing from the predicted path of debris.

"Had the fireproofing not been dislodged, the temperature rise of the structural components would likely have been insufficient to cause the global collapse of the towers," says NIST. "Fireproofing dislodged by debris left the components more sensitive to heat than any areas where there was missing or thin fireproofing before the aircraft impacts."

Adds Sunder: "If the sprinkler systems were not operational in multi-floor fires, our current working hypothesis is that the buildings would not have collapsed."

NIST has also determined that the majority of the WTC steel was stronger than minimum requirements. In recent fire tests, NIST found that the floor systems met the local building code of the time (ENR 9/13 p. 16).

NIST plans to release its draft for public comment by February. The final report’s release is expected in May. The final report on Seven WTC, which collapsed on 9/11 after burning unattended for seven hours, is expected in July.