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Steel moment-resisting frames (MRFs) are widely used in the United States to resist seismic forces. MRFs have many advantages, including high ductility, architectural versatility, and vetted member and connection detailing requirements. However, MRFs require large members to meet story drift criteria. Moreover, strong-column-weak-beam requirements can result in significant member sizes, and – even in the cases where strong-column-weak-beam requirements are satisfied – MRFs can still be vulnerable to story mechanisms in one or a few stories. Recently, the concept of a strongback has been utilized successfully to delay or prevent story mechanism behavior in braced frames. The strongback is represented by a steel truss or column that is designed to remain essentially elastic, thus allowing the system to transfer inelastic demands across stories. Although systems including strongbacks exhibit more uniform story drift demands with building height and reduced peak drift response, the elastic nature of the strongback can also result in near-elastic higher-mode force demands. This study compares the dynamic response of a baseline MRF to that of a retrofit using a strongback column. Several ground motions are considered to determine which cases produce the largest drift, acceleration, and force demands.
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This content will become publicly available on July 13, 2027
Assessment of MPA as an estimate of NRHA in SBF
Due to their ability to provide more uniform story drifts with building height, strongback braced frames (SBFs) have been proposed as enhanced-performance structural systems. Conventional design approaches, however, tend to underestimate force demands in strongback elements compared to nonlinear response history analysis (NRHA). To address this gap, alternative methods such as modal pushover analysis (MPA) have been suggested to obtain less computationally intensive estimates of seismic demands. This study presents a statistical assessment of MPA as an estimate of NRHA force demands for an 8-story SBF subjected to 44 far-field ground motion records scaled at the risk-adjusted maximum considered earthquake (MCE𝑟 ) intensity level. Unlike prior studies that compare MPA results to the statistics of the NRHA response, this work treats each ground motion as a separate test to characterize how MPA accounts for record-specific spectral characteristics. Accuracy in estimates of the force demands (i.e., how close the MPA estimates are to the NRHA “truth”) is characterized using root mean square error. Additional comparisons are made across the MPA parameters, such as the number of modes employed, as well as the use of initial versus elongated periods. Results provide a comprehensive statistical assessment of MPA, illustrating that the approach can be sensitive to spectral assumptions and is better suited to aggregated estimates from NRHA.
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- Award ID(s):
- 2309829
- PAR ID:
- 10657014
- Publisher / Repository:
- 13th National Conference on Earthquake Engineering (13NCEE)
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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