This document is intended to provide owners and practicing engineers with current "best practices" to reduce the likelihood of progressive collapse of buildings in the event of abnormal loading. The report includes a discussion of an acceptable risk approach to progressive collapse, which involves defining the threat, event control, and structural design to resist postulated event. Practical means for reducing risk for new and existing buildings are presented. An extensive review is provided of the design methods used to enhance a buildings resistance to progressive collapse. These include the indirect method (providing sufficient tie forces), the specific local resistance method (designing key elements to withstand abnormal loads), and the alternate load path method (allowing for redistribution of load in the event of the loss of a key member). Design considerations for different structural materials are summarized. The methodology for evaluating and mitigating progressive collapse potential in existing buildings is also discussed. Three appendices provide supporting information. Appendix A presents a worldwide review of progressive collapse provisions in various national design standards. Appendix B identifies knowledge gaps related to progressive collapse that require research. Appendix C provides case studies of progressive collapses. This document is not intended to provide step-by-step design guidance for practicing engineers; however, applicable design standards are referenced and summarized in Appendix A.

A progressive collapse is a structural failure that is initiated by localized structural damage and subsequently develops, as a chain reaction, into a failure that involves a major portion of the structural system. The collapse of the Ronan Point Tower in Canning Town, London, United Kingdom in May, 1968 prompted numerous efforts to develop structural design criteria for progressive collapse resistance. A series of papers and reports appeared in the decade following the Ronan Point collapse, and attempts were made to implement provisions addressing progressive collapse and enhancing general structural integrity in codes and standards. Improved building practices and design procedures to control the likelihood of progressive collapse now are receiving heightened interest by engineers, architects, and standards organizations in the aftermath of the tragedy of September 11, 2001. Advances in the science underlying structural design and the development of structural reliability and risk analysis tools now permit design strategies that would have been considered infeasible 30 years ago. This article provides a critical appraisal of key research products that provide a basis for designing a structural system to withstand local damage without the development of a general structural collapse, and identifies strategies that currently are feasible to implement in the General De- * sign Requirements in ASCE Standard 7, minimum design loads for buildings, and other structures. The article concludes with a discussion of research challenges to further enhancement of design practices, with particular emphasis on needed support for structural engineering analysis and design from advanced computation and information technologies.

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