This series includes technical reports prepared by faculty, students and staff who are associated with the John A. Blume Earthquake Engineering Center at Stanford University. While the primary focus of Blume Center is earthquake engineering, many of the reports in this series encompass broader topics in structural engineering and materials, computational mechanics, geomechanics, structural health monitoring, and engineering life-cycle risk assessment. Each report includes acknowledgments of the specific sponsors for the report and underlying research. In addition to providing research support, the Blume Center provides administrative support for maintaining and disseminating the technical reports. For more information about the Blume Center and its activities, see https://blume.stanford.edu.
Real strength and ductility characteristics are critical determinants of the seismic behavior of structures. Overstrength, which is defined here as member or structural capacity greater than that assumed in design, may be beneficial or, under certain circumstances, detrimental to seismic behavior. When the yield level under lateral loading of a structure is increased by overstrength, the inelastic seismic demands on the structure are decreased. If relative member overstrengths are not well balanced or tuned, inelastic structural capacity will be reduced. Overstrength for several structural steel systems is estimated and the effect on structural response evaluated.
Inelastic demand and structural capacity can be partially characterized by ductility. Local ductility is the basic measure of member performance and relates to detailing considerations, low cycle fatigue, and a large body of laboratory test data. Global ductility is the basic measure of structural response and offers a convenient design parameter. Overstrength models are implemented to relate local and global ductilities.
Current SEAOC recommendations are evaluated in the context of overstrength and ductility demands. The current approach leads to inconsistent designs with, in some cases, inadequate protection against overload of nonductile elements.
Satisfactory seismic performance requires, in part, selection of sufficient structure strength and ductility capacity. A design approach for single degree of freedom systems employing overstrength concepts developed herein and extensions of previous ductility studies is suggested. Concepts presented are illustrated with analytical studies of structures damaged in the 1985 Mexico earthquake and prototypical structures designed to current SEAOC recommendations.
Osteraas, JD and Krawinkler, H. (2013). Strength and Ductility Considerations in Seismic Design. John A. Blume Earthquake Engineering Center Technical Report 90. Stanford Digital Repository. Available at: http://purl.stanford.edu/cb239mm7088
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