Modeling and seismic collapse assessment of partially encased composite frames

This study develops an OpenSees-based framework for seismic fragility and collapse assessment of partially encased composite (PEC) column–steel beam frames. PEC columns are modeled using fiber-based beam-column elements, with a modified CFSTsteel material assigned to the flange fibers to represent local-buckling-induced post-peak deterioration. A mechanics-based panel-zone shear spring combining the contributions of the steel web and concrete diagonal strut is developed for major-axis PEC joints and incorporated with beam-end IMK springs. Comparisons with available cyclic tests show reasonable agreement in stiffness, peak resistance, hysteretic response, and deterioration. Three prototype frames with 3, 6, and 12 stories designed using an elastic inter-story drift-ratio limit of 1/400 are assessed through incremental dynamic analysis using 44 ground-motion records. Their median collapse-oriented intensities are 4.860, 2.735, and 1.270 g, with corresponding collapse margin ratios (CMRs) of 8.909, 8.332, and 6.152, respectively. Increasing frame height is accompanied by lower fragility capacities and a broader heightwise distribution of drift demand. To quantify the influence of drift-controlled member sizing, the 6- and 12-story frames are redesigned using a 1/250 limit. Their median collapse intensities decrease to 1.036 and 0.722 g, while the CMRs decrease to 4.752 and 3.700, respectively. Sensitivity analyses show that CMR depends on the adopted upper drift limit, whereas the height-dependent and design-limit trends remain unchanged. The results demonstrate that the large collapse margins of the 1/400 designs are strongly associated with drift-controlled stiffness and strength reserve.