Structural and fire experts are gratified but not surprised by the latest "interim" findings of the National Institute of Standards and Technology on the behavior of the World Trade Center’s twin 110-story towers after the buildings were attacked by terrorists on Sept. 11, 2001. NIST has concluded, after completing most of its in-depth, $16-million investigation, that the towers would likely have withstood the heat of the jet-fuel-triggered fires had the hijacked planes and the debris from impact not knocked off the sprayed-on fireproofing.

Building design professionals are also not surprised by NIST’s "working hypothesis" that the frames’ sprayed-on fireproofing was adequate to maintain the structural integrity of the steel tubes in an "ordinary" office fire that was not fought, but left to burn out. Click here to view diagrams

"NIST confirmed what I, and others, have been saying for the past three years," says Edward M. DePaola, principal of structural engineer Severud Associates, New York City, and a member of the first team that investigated the disaster. "It was the fire, not the structural damage or any perceived design deficiencies, that caused the collapse," he says.

Shyam Sunder, Gaithersburg, Md.-based NIST’s lead WTC investigator, says the study, considered the most in-depth probe ever of a building disaster, is important because it provides documentation for what many knew all along.

Some sources think the entire exercise was a waste of taxpayer money. Richard C. Schulte, a fire protection engineer based in Evanston, Ill., has been strident in his criticism against those, including New York City-based Skyscraper Safety Campaign, who blame the 9/11 deaths on the structural integrity of the twin towers and their original designers and builders, rather than the terrorists (ENR 6/7 p. 48).

"NIST’s preliminary conclusions pretty much lay to rest the Skyscraper Safety Campaign’s allegations that the towers were improperly designed and constructed," says Schulte.

James Quintiere, a professor in the Dept. of Fire Protection Engineering at the University of Maryland, College Park, and an advisor to the Skyscraper Safety Campaign, declined to comment on the preliminary conclusions other than to...