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PREFACE:

This document is intended to provide owners and practicing engineers with current “best practices” to reduce the likelihood of progressive collapse of buildingsin 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 isalso discussed. Three appendices provide supporting information.

 AppendixA 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.This best practices document is prepared in response to one of the recommendations from the July 2002 industry workshop on prevention of progressive collapse, which was held in Chicago, Illinois. 

The preparation of this document was facilitated by the Multihazard Mitigation Council (MMC) of the National Institute of Building Sciences. The MMC contracted with the three principal authors to prepare an initial draft and organized a workshop in February2004 to solicit public comments on the initial draft document. The body of the document was prepared by three principal authors:

 •  Dr. Bruce R. Ellingwood (Georgia Institute of Technology),Chapter 2; 

•  Dr. Robert Smilowitz (Weidlinger Associates), Chapters 3, 4, and 5 and Appendix B. 

•  Mr. Donald O. Dusenberry (Simpson, Gumpertz & Heger), Chapters 3, 4, and 5 and Appendix C. Contributing authors to Chapters 3, 4, and 5 included Dr. Wilkins Aquino, Mr. John Dal Pino, and Mr. James R. Cagely. Dr. Dat Duthinh of the National Institute of Standards and Technology (NIST)prepared Appendix A. Dr. H. S. Lew of NIST and Dr. Nicholas J. Carino of KT Consulting Inc. contributed to Chapters 1 and 6 and served as co-editors.

 To aid the understanding and use of this best practices document and to provide an opportunity for technical exchange with the principal authors, NIST and the Structural Engineering Institute of the American Society of Civil Engineers organized jointly four workshops during September and October 2006. 

The workshops were held in Denver, New York City, San Francisco, and Chicago. In addition, the document was posted on the Web to solicit public comments. 

Comments received during and subsequent to the workshops have been incorporated into this final document. Individuals who submitted comments are acknowledged for their useful suggestions.

The Chicago workshop also identified a need for codeprovisions and design standards that will provide resistance to progressive collapse at reasonable additional design and construction costs.

 This need was reinforced by Recommendation 1 resulting from the NIST investigation of the collapse of the World Trade Center towers, which calls for development and adoption of code provisions and consensus standards to prevent progressive collapse in buildings. NIST is currently carrying out research in support of meeting this need.