Structural steel of the twin 110-story towers of the World Trade Center was stripped of its fireproofing by debris from the aircraft impact and weakened by the resulting fires, eventually causing the towers to collapse, according to an interim report by the National Institute of Standards & Technology. The report says the region of dislodged fireproofing was determined from the predicted path of the 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 in the Oct. 19 release of another interim report of its $16-million study of the WTC destruction on Sept. 11, 2001, by terrorists. “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,” says the report.

Many experts familiar with the twin towers design are not surprised by the findings. But they are worth noting, say sources, because there are others, both structural engineers and fire experts, who have questioned whether the design by Skilling Helle Christiansen Robertson in some way contributed to the collapse.

According to S. Shyam Sunder, NIST’s lead investigator for the study, an ordinary office fire would likely have resulted in burn-out, not collapse.

Sunder says the working hypothesis for a conventional fire, where sprinklers are not working, is that it IS more likely TO have burn-out without collapse. "If the sprinkler system were not operational in a multifloor fire, our current working hypothesis, without having done the calculations," is that the building would not have collapsed, he says. On 9/11, the building collapsed because the fireproofing on the steel was dislodged by direct debris impact. The steel heated up and softened and lost strength.

In addition, NIST has determined that the majority of the steel was stronger than minimum requirements. “The safety of the towers was most likely not affected by the small percentage of steel below the minimum,” says the report. “Building designs routinely allow structures to withstand greater loads than are expected by including significant factors of safety. Moreover, the structural loads on Sept. 11, 2001, were well below this design level. “

In fire tests in August, NIST also determined that the floor systems in the towers met the New York City building code of the time (ENR 9/13 p. 16).

The findings include an explanation for the time delay between the collapses of the two towers. (The south tower, Two WTC, survived for 56 minutes; the north tower, One WTC, for 103 minutes). NIST says the difference was primarily due to five items: the asymmetrical structural damage of the aircraft impact to Two WTC compared to the aircraft damage to One WTC; the time it took for heat to soften, buckle and shorten core columns that had fireproofing dislodged by debris impact; the structure’s ability to redistribute loads as the core columns shortened; the time it took for fires to traverse from their initial location to the face of the towers where perimeter columns were bowing inward (as seen only minutes before the collapse of each tower); and the time it took for heat to soften and buckle those columns.

NIST plans to release its final draft of the twin towers’ report in December or January. A four to six-week public comment period will follow. The final release is expected in May. The draft report on Seven WTC is set to be released in May. The final report is expected out in July.