Abstract
Strongback-braced frames employ an essentially elastic truss, or strongback, that defines an elastic load path to redistribute seismic demands and mitigate the formation of story mechanisms. However, unlike the forces from traditional capacity design—which assumes that inelastic response limits the earthquake-induced forces—design forces in the strongback arise from a nonlinear first mode and near-elastic higher-mode response, because the strongback is designed to remain elastic in every mode. A design method using modal pushover analysis, which combines the response from pushover analyses in multiple modes, can estimate the magnitude and distribution of the force demands in the first and higher modes, including inelastic response. The existing modal pushover analysis procedure is simplified and applied to the design of the strongback in strongback-braced frames. Accounting for only the first mode of response resulted in estimates below the 16th percentile of force demands extracted from nonlinear dynamic analysis. However, a modal pushover approach using multiple modes resulted in improved estimates able to represent both the distribution and magnitude of the peak force demands from nonlinear dynamic analysis of eight- and four-story strongback-braced frames with buckling-restrained braces, while maintaining the conceptual simplicity of nonlinear static analysis procedures.
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Data Availability Statement
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon request.
Acknowledgments
This study was the result of many years of support and guidance from Professor Stephen Mahin. This work would not have been possible without his insight and advice. Numerous individuals, companies, and organizations donated their time, expertise, and services and were essential to the completion of this work, including Rafael Sabelli (Walter P. Moore and Associates, Inc.), James Malley (Degenkolb Engineers), Leo Panian (Tipping Structural Engineers), and Walterio Lopez (Rutherford + Chekene), among many others. The analytical work reported herein was supported by a research grant titled “Design of Strongback Braced Frames,” funded by the American Institute of Steel Construction (AISC). Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of AISC or other participants in the research program.
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Journal of Structural Engineering
Volume 147 • Issue 12 • December 2021
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© 2021 American Society of Civil Engineers.
History
Received: Jun 5, 2020
Accepted: Jun 29, 2021
Published online: Sep 17, 2021
Published in print: Dec 1, 2021
Discussion open until: Feb 17, 2022
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