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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.

The impact-echo method is a technique for flaw detection in concrete. It is based on monitoring the surface motion resulting from a short-duration mechanical impact. The method overcomes many of the barriers associated with flaw detection in concrete based on ultrasonic methods. The purpose of this paper is to provide an overview of the technique and to discuss the important parameters involved in this type of testing. One of the key features of the method is the transformation of the recorded time domain waveform of the surface motion into the frequency domain. The impact gives rise to modes of vibration and the frequency of these modes is related to the geometry of the test object and the presence of flaws. The principles involved in frequency analysis are discussed. The importance of the impact duration in relation to flaw detection and other factors affecting the smallest flaw that can be detected are also reviewed. The paper concludes with a summary of the ASTM standard governing the use of the impact-echo method for measuring the thickness of plate-like structures.