Abstract

Special concentric braced frames (SCBFs) are widely used in seismic design due to their resilience, stiffness and energy dissipation; however, their performance under long-duration subduction earthquakes remains uncertain. Conventional fragility estimation methods, such as Cloud Analysis and FEMA P695, have limitations in capturing parameter variability when collapse data are limited. This research is focused on the seismic fragility evaluation of SCBF integrating the Simultaneous Perturbation Stochastic Approach (SPSA), improving the robustness of collapse probability of SCBF systems subjected to subduction earthquakes in Chile. Twenty SCBF models, with variations in height, bracing configuration, and soil conditions, were analyzed in OpenSees using nonlinear time history simulations with scaled subduction records. Fragility curves were developed using Cloud Analysis with SPSA optimization to iteratively refine the regression parameters. Monte Carlo resampling was used to approximate parameter variability, and the results were compared with FEMA P695 and traditional regression methods. The SPSA-based approach yielded collapse probabilities ranging from 12?% to 17?% under MCE-level earthquakes, compared to approximately 20?% reported by FEMA P695 for similar configurations. Chevron bracing configurations showed lower fragility than two-story configurations, particularly in low-rise frames on soft soils, emphasizing the influence of design typology and site conditions. These findings suggest that stochastic optimization offers a complementary estimation strategy in contexts with limited data. This approach can provide valuable information about the performance-based seismic design and prioritizing the use of the resilient SCBF system in subduction zones. The study demonstrates the feasibility of SPSA for fragility estimation, expanding established performance-based seismic engineering framework. Furthermore, beyond the statistical framework, this study emphasizes structural insights: the influence of bracing configuration, number of stories, and soil conditions on SCBF fragility under subduction earthquakes which are critical parameters to seismic resilience. © 2025 Institution of Structural Engineers. Published by Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.