Comparative Probabilistic Seismic Performance Assessment of Tall CFT and RC Moment-Resisting Frame Systems under Long-Period Ground Motions

High-rise buildings are significantly vulnerable to long-period ground motions due to resonance effects. This study presents a probabilistic assessment of the seismic vulnerability of two 40-story structural systems, a reinforced concrete moment-resisting frame (RC-MRF) and a concrete filled steel tube moment-resisting frame (CFT-MRF), using incremental dynamic analysis to develop seismic fragility curves under both long- and short-period ground motions. Multiple damage states are defined according to HAZUS guidelines to capture nonlinear response progression, while nonlinear analyses with Perform-3D provide the basis for quantifying structural performance. The results indicate that for the CFT system, the median seismic fragility at the complete damage state under long-period excitations is nearly 65% lower than under short-period records, highlighting the severe impact of long-period motions on tall-building safety. Furthermore, under long-period excitations, its median fragility is 24% higher than that of the RC-MRF at the complete damage state. Although both systems demonstrate comparable performance at slight and moderate damage levels, the CFT system exhibits superior seismic resilience and energy dissipation capacity at near-collapse states. This enhanced capability results in larger safety margins and a reduced probability of damage. This probabilistic framework enables a quantitative comparison of structural performance under different seismic demands and underscores the importance of considering long-period effects in the performance-based seismic design of high-rise buildings. The study further demonstrates the advantage of CFT systems as a robust solution for mitigating damage and enhancing safety under long-period ground motions.