...when One WTC fell. The theory “suggests” a “classic progressive collapse,” says the report. An initial local failure was at the lower floors, below 13, due to fire and/or debris-induced structural damage of a critical column, which supported a large-span floor bay covering 2,000 sq ft. Click here to view image

NIST estimates 17,400 occupants, plus or minus 1,200, were present in the WTC towers at the time of the attacks, which killed nearly 3,000 people. Some 87% of the WTC tower occupants, including more than 99% of those below the impact floors, were able to evacuate. NIST estimates “about 20% or more” of those trapped may have still been alive just prior to the collapse.

For the twin towers, NIST is still studying “how and why” One WTC stood nearly twice as long as Two WTC. NIST also is exploring whether any factors related to “normal” building and fire safety considerations, not unique to the terrorist attacks, could have delayed or prevented the collapses. Another question, says NIST, is whether the undamaged towers would have remained standing in a “normal major” building fire. Click here to view image

The study has determined, based on photographs, that perimeter columns bowed inward on One WTC’s south face and Two WTC’s east face, “some” minutes prior to collapse. “Initiation of global collapse was first observed by the tilting of building sections above the impact regions” of both towers, says the report. One WTC tilted to the south and Two WTC tilted to the east and south and twisted in a counterclockwise motion.

The report also finds “a lack of technical basis” in the selection of fireproofing thickness to meet the two-hour fire rating of the composite floor system.

Sources critical of the investigation say its goal to improve building safety and building practice sounds noble enough but question whether it is achievable (ENR 12/15/03 p. 10). There are concerns that findings may somehow be used indirectly in pending lawsuits. No part of any report resulting from an NIST investigation into a structural failure or from an investigation under NCSTA may be used in any lawsuit or action for damages arising out of any matter mentioned in a report.

“This study has its value in coming up with information that will help us resist unanticipated attacks for particular buildings,” says Tomasetti. “Anything learned from this study does not indicate previous designs or previous codes were in any way deficient.”