NIST Statement on Progressive/Disportionate Collapse Provisions

The United Kingdom has a progressive collapse standard Standards to avoid progressive collapse Large panel Construction since 1968. The standard lists two methods for mitigating progressive collapse: (1) by providing alternative load paths, assuming the removal of a critical section of the load bearing system, and (2) by providing stiffness and continuity to the structural system to ensure the stability of the building against forces liable to damage the load supporting members. The standard also specifies an accidental static pressure of 5 psi and minimum tie forces for continuity. These provisions are based on engineering experience and judgment. Similar provisions have been adopted in the Eurocode. Currently, there is no field evidence to indicate that these provisions are not working or that the resulting building designs are less safe.

In the United States, the American Society of Civil Engineers (ASCE) has in its standard (ASCE/SEI 7), structural integrity requirements for progressive collapse mitigation. ASCE plans to develop guidance for the prevention of progressive collapse. A technical committee has been recommended, but has not yet been formed. It will be some years before a guidance document is developed and made available for code adoption.

NIST has an ongoing multi-year research project on the development of criteria for prevention of progressive collapse and is currently assessing best practices in current use. The NIST best practices document is intended to provide owners and practicing engineers with the current best practices to mitigate progressive collapse, including methods similar to those adopted in the U.K., and those used by federal agencies such as GSA, DoD, and the State Department. The draft of the document will be made available for broad review in conjunction with training seminars to be conducted by ASCE in 2006. The final document will be available by the end of 2007.

In the course of its Investigation into the collapse of the World Trade Center Towers, NIST has not found any evidence that well-tied buildings performed unfavorably (or collapse earlier) than buildings that are not well-tied. In fact NIST has found that, had the major structural subsystems of the WTC towers not been tied together, the core of the towers would have collapsed earlier. The hat-truss tied the core to the perimeter walls of the towers, and thus allowed the building to withstand the effects of the aircraft impact and subsequent fires for a much longer time enabling large numbers of building occupants to evacuate safely.