en Optimal design Optimal design پژوهشي Research <span style="font-size:11.5pt"><span new="" roman="" style="font-family:" times="">The seismic resilience of existing reinforced concrete (RC) buildings can be improved by optimizing both energy dissipation and post-earthquake recovery. This study proposes a practical framework for upgrading RC moment-resisting frames using nonlinear fluid viscous dampers (NFVDs). Two typical frames, a four-story and an eight-story structure, were modeled and analyzed in OpenSees. Nonlinear time-history analyses with seven earthquake records were carried out to estimate the Park&ndash;Ang damage index, while incremental dynamic analyses (IDA) with 22 far-field records from FEMA P695 were used to evaluate fragility and collapse performance. The NFVDs were represented through a velocity-dependent Maxwell model, and the optimal damper parameters and locations were determined through a cost-based single-objective optimization scheme under predefined damage limits. The results show that the optimized damper configurations effectively reduced structural damage and improved post-event functionality recovery under seismic hazard levels corresponding to 10% and 2% probabilities of exceedance in 50 years. Overall, the proposed approach provides an efficient and economical solution for improving the seismic performance and resilience of existing RC frame buildings.</span></span> Seismic resilience, Nonlinear viscous damper, Park–Ang damage index, Fragility curve, Optimization, Reinforced concrete frame 643 677 http://ijoce.iust.ac.ir/browse.php?a_code=A-10-6463-43&slc_lang=en&sid=1 M. Arjmand m.arjmand@buqaen.ac.ir 180031947532846002883 180031947532846002883 Yes Department of Civil Engineering, Bozorgmehr University of Qaenat, Iran H. Naderpour 180031947532846002884 180031947532846002884 No Faculty of Civil Engineering, Semnan University, Semnan, Iran A. Kheyroddin 180031947532846002885 180031947532846002885 No Faculty of Civil Engineering, Semnan University, Semnan, Iran