RESTON, Va. --The World Trade Center towers would likely not have collapsed if the spray-on fireproofing had not been dislodged by the impact of the aircraft, concludes a report issued today by the National Institute for Standards and Technology (NIST). NIST urged the technical community to examine changes to design, materials and techniques- including possible changes to codes and standards-that could improve building performance and increase the safety of occupants and first responders, focusing largely on the effects of fire in relationship to other structural loads.
The NIST recommendations are largely consistent with the findings of the assessment report released in May 2002 by the American Society of Civil Engineers (ASCE) and Federal Emergency Management Agency (FEMA). The ASCE/FEMA report recommended six items- including improvements in fireproofing, sprinkler systems and egress design-in the design and construction of buildings deemed likely targets of terrorist attacks.
Gene Corley, PhD., P.E., team leader for the ASCE/FEMA study team, concurred with the NIST recommendation for further research, and welcomed NIST's plan to host a conference for key standards- setting organizations to review and discuss the findings and recommendations. "The broader scope and more detailed report provides the comprehensive analysis needed to progress from determining the cause of the collapse to finding ways to improve building performance," stated Corley. "The structural engineering community and the public at large will benefit over the long term from the NIST study."
"We've certainly learned a lot from studying how the towers and the surrounding buildings performed when subjected to extreme forces. But we also have decades of experience with the performance of other structures subjected to intense fire," Corley said. "Do we know enough to change the building code and should those changes apply to all buildings, or just to tall buildings or buildings considered 'high profile' targets? We've got to reach a consensus on those questions, and we most certainly need further research on some of the key issues raised in this report."
Representatives of the Structural Engineering Institute of ASCE supported further study into the specific kinds of events and environments that compromise fireproofing materials, and called for the development of testing facilities capable of studying the effects of fire on long-span steel beams. Experience at the World Trade Center and the Pentagon, which was also the subject of an ASCE performance assessment following the terrorist attacks of September 11, suggests that common fireproofing methods used in both steel-frame and reinforced concrete construction are vulnerable to being 'scoured' by the force of the debris field following a significant impact. Less clear is the magnitude of the force needed to cause widespread damage to the fireproofing, and whether the type of spray-on fireproofing material used at the World Trade Center remains intact in conditions of normal wear and tear, or even when subjected to the force of a major earthquake. Building performance studies conducted following the Northridge earthquake of 1994, for example, found that seismic motion damaged many of the connections in the steel beams and columns of buildings, yet the fireproofing remained in place.
The engineers cautioned that the NIST findings should not be interpreted as a "how to" guide for preventing a building's collapse when hit by an airplane.
"The lessons learned from this tragedy can help improve the ability of buildings to resist more routine fires," stated Jeremy Isenberg, Ph.D., P.E., president of ASCE's Structural Engineering Institute. "Instead of revising building codes to address extreme events such as the impact of a large jet airliner, resources should be focused on improving fire resistance methods for the conditions more likely to affect the types of buildings where most of us live and work. For extreme situations, it is best to direct resources toward preventing the attack."