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Title:Performance-based Engineering Framework and Ductility Capacity Models for Buckling-Restrained Braces
Author(s):Andrews, Blake M.; Fahnestock, Larry; Song, Junho
Subject(s):Performance-Based Design
Buckling-Restrained Brace
First-Order Reliability Method
Low-Cycle Fatigue
Abstract:Buckling-restrained braces (BRBs) have recently become popular in the United States for use as primary members of seismic lateral-force-resisting systems. A BRB is a steel brace that does not buckle in compression but instead yields in both tension and compression. Concentrically-braced frames incorporating BRBs are known as buckling-restrained braced frames (BRBFs). Although design guidelines for BRB application have been developed, procedures for assessing performance and quantifying reliability are needed. This report proposes a performance-based engineering framework (PBEF) for a BRBF subjected to seismic loads. The proposed framework quantifies the risk of BRB failure due to low-cycle fatigue fracture of the BRB core. The components of the PBEF include: stochastic modeling of seismic loads; dynamic analyses of BRBFs; cumulative plastic ductility (CPD) (i.e. fatigue) models for buckling-restrained braces; structural reliability analyses; parametric studies on how BRB and BRBF properties affect performance; and fragility modeling. In addition to the report, appendix files are attached which provide detailed information on the research program. For stochastic modeling of seismic loadings, input ground acceleration records were randomly generated from power spectrum models and modulated with envelope functions (to account for non-stationarity). The generated time records were used as input excitations to single-degree-of-freedom lumped-mass system models that represented the BRBFs. The BRB hysteretic behavior was modeled using a Bouc-Wen model. Non-linear dynamic time-history analyses were performed to obtain BRB core deformation time history records. In this study, significant effort was made to develop models that predict BRB CPD capacity. The result was BRB remaining capacity (RC) models, which, given the BRB core deformation history as an input, predict the remaining CPD capacity of the brace, where values less than zero indicate failure. Given BRB demand (i.e. core deformation histories generated from the dynamic analyses) and supply (i.e. remaining capacity predicted by the RC models), reliability analyses were performed to evaluate the probability of brace failure. The analyses were conducted using the first order reliability method. In the reliability analyses, the epistemic uncertainty in the fatigue capacity predictions was accounted for explicitly, and, as a result, the probabilities of brace failure were calculated in terms of mean probability, 90% confidence level probability, and 95% confidence level probability. Using the tools described above, a parametric study was conducted to explore the effects of the seismic loading, BRB, and BRBF characteristics on the probability of brace failure. For given seismic loadings, surfaces of reliability indices were constructed in order to determine the probability of brace failure directly from BRB and BRBF properties, without the need to perform individual reliability analyses each time. Also, for a given set of BRB and BRBF properties, fragility curves were created that provide conditional probability of brace failure given ground shaking intensity parameters. Though this report describes the specific application of a PBEF to the BRB fatigue problem, the components of the PBEF may be interchanged independently, leading to great overall flexibility and the potential for application of the framework to many other problems.
Issue Date:2008-07 (o
2008-08 (u
Publisher:Newmark Structural Engineering Laboratory. University of Illinois at Urbana-Champaign.
Series/Report:Newmark Structural Engineering Laboratory Report Series 012
Genre:Technical Report
Publication Status:published or submitted for publication
Rights Information:Copyright held by the authors. All rights reserved.
Date Available in IDEALS:2008-07-27

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