FIERY COLLAPSE Study assesses aircraft impact damage. (Photo courtesy of ArupFire World Trade Center Expert Report)

Federal officials say they have reached a milestone in the progress of the $16-million federal building and fire safety investigation into the destruction of the World Trade Center on Sept. 11, 2001. The U.S. Commerce Dept.’s National Institute of Standards and technology says it now has obtained access to "all of the essential information" needed to complete the two-year study by next fall. NIST also has completed awarding a total of $5.4 million in outside contracts to support its work.

The NIST team reported, in a public update on Dec. 2, that it has finished building comprehensive models for analyzing the most probable collapse scenarios from the moment each hijacked plane slammed into its 110-story target to the moment each tower collapsed. It also has completed the majority of the analysis of the recovered WTC structural steel and conducted nearly all of the analysis of the laboratory fire tests on mockups of typical WTC workstations, to provide data for fire dynamics computer models.
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Work has begun on fire endurance testing of typical WTC floor systems under both specified and as-built conditions. "NIST is developing a rigorous technical approach to evaluate the role of thermal insulation (i.e., fireproofing) in the collapse of the WTC towers," says a 41-page interim report on the WTC work, which is available on NIST's Website: wtc.nist.gov.

In addition, the team has analyzed data on active fire protection systems installed in the WTC towers and 7 WTC–a 50-story steel-framed building that collapsed after burning unattended for about seven hours–to evaluate their design, capabilities and performance. WTC study teams also have conducted 11 research and development projects, including work on fire safety design and retrofit of structures and the emergency use of elevators for rescue.

The analyses will take into account the several building systems–structural, life safety, mechanical and more–compromised by the planes. This includes the likelihood of a structure weakened not just because portions were obliterated but because of the possible weakening impact, through dynamic amplification, of the debris and planes as they "landed" on the remaining, but also likely weakened, floor structure below.

"We will also capture the as-built fireproofing condition and develop a probable extreme fire scenario and analyze it," says Shyam Sunder, lead WTC investigator for Gaithersburg, Md.-based NIST.

The study will account for zones where sprayed-on fireproofing on the structural steel frame was either missing, not specified in the original design or knocked off by the planes or flying debris. For example, there was no fireproofing specified for the tops of the dampers where the floor trusses met the perimeter structural tube, says Sunder. "Evidence suggests" that in a 3-ft region on floor trusses abutting perimeter columns, fireproofing may have been less than the minimum specified value, he says.

The second major aspect of the WTC fire study is to analyze the effect of the airplane impact damage to the structure "in an effort to estimate the region over which the fireproofing was dislodged on columns and floors," says Sunder.

There is some controversy over whether testing the fireproofing and reconstructing the exact collapse sequence make any difference. "Fireproofing is a red-herring issue because with any structural system, whether fireproofing was on floor trusses or wide-flange members or whether it was the concrete cover on concrete members, it would have been destroyed by the impact of the aircraft," said structural engineer Jon D. Magnusson in a presentation, Learning from Structures Subjected to Loads Extremely beyond Design, at a Dec. 4-5 Steel Building Symposium on Blast and Progressive Collapse Resistance in New York City. It was sponsored by the American Institute of Steel Construction and the Steel Institute of New York.

"The reality is that it doesn’t matter if the floor trusses, the columns or the connections went first [because] we can’t out-build the terrorists," said Magnusson, chairman and CEO of Seattle-based Magnusson Klemencic Associates. The firm is one of two successor firms to the structural engineer for the trade center, Skilling Helle Christiansen Robertson.

FIRE TEST Researchers are analyzing the effects of burning offices. (Photo courtesy of National Institute of Standards and Technology)

Under the National Construction Safety Team Act (NCSTA) of October 2002, NIST is authorized to investigate major building failures. The WTC probe is intended to result in recommendations, if deemed necessary, for changes in building codes and standards. However, no part of any report resulting from a NIST investigation into a structural failure or from an investigation under the NCSTA may be used in any lawsuit or action for damages arising out of any matter mentioned in a report.

There is controversy between the design and research communities about the need for and value of such a comprehensive and costly investigation into the WTC collapse. Though some are eager for any lessons to be learned that might improve building performance under extreme conditions, others think prevailing design codes for private-sector buildings are appropriate public policy.

"Many times people proposing code changes are misleading the public, through the media, into thinking we can out-build the terrorists," said Magnusson. "If we do not speak up as engineers and let our policymakers know the facts about this, we are failing in our professional obligations."