The National Institute of Standards and Technology (NIST) recently issued a guide to help owners, engineers, and building officials avoid, through prudent planning and design of structures, progressive collapses such as occurred in 1995 following a terrorist attack on the Oklahoma City Alfred P. Murrah Federal Building. That collapse heightened concern about the vulnerability of multi-story buildings to the spread of an initial local structural failure by chain reaction that results in the collapse of an entire structure or a disproportionately large part of it.
The report, Best Practices for Reducing the Potential for Progressive Collapse in Buildings, argues that although no building system can be engineered and constructed to be absolutely risk-free, risk-informed assessment and decision-making can reduce the risk of progressive collapse. According to researchers, engineers must not simply work to the minimum requirements of the building code; they need to consider ways to improve structural integrity and robustness to accommodate local failures.
According to NIST engineers, hazards that increase the risk of local structural failures that, in turn, can lead to a partial or complete progressive collapse include design and construction errors, fire, gas explosion, transport and storage of hazardous materials, vehicular collision, and bomb explosions.
The NIST report cites a lack of continuity of support within a building system; a lack of ductility in structural materials, members, and connections; and lack of structural redundancy in providing alternate load paths as critical factors that limit structural integrity. The use of large-paneled or bearing wall construction, for example, can limit continuity and ductility. Such systems may be poorly suited to absorb or dissipate energy resulting from unforeseen events such as gas explosion and sabotage.
The guide catalogs a number of cost-effective engineering solutions for retrofitting existing structures, summarizes national and international best practices for designing buildings resistant to progressive collapse, and summarizes existing knowledge for use by engineers in making risk-informed planning and design decisions. But, NIST says, it is not intended to provide step-by-step guidance. Appendices describe applicable design standards from around the world, research needs relevant to progressive collapse, and case studies of progressive collapse.
Best Practices for Reducing the Potential for Progressive Collapse in Buildings (NISTIR 7396) is available for download at www.bfrl.nist.gov/861/861pubs/collapse.
Source: National Institute of Standards and Technology