Seismic response assessment of RC frame buildings with masonry infill openings and soft story irregularity under dynamic soil–structure interaction

Masonry infill walls are widely used in reinforced concrete (RC) buildings, but their seismic behavior is often studied using fixed-base models without fully considering soil–structure interaction. This study investigates the seismic response of a ten-story, one-bay RC frame with 0%, 30%, and 50% infill openings, with and without a ground soft story, supported on four soil classes: soft soil, stiff soil, soft rock, and hard rock. The frame and infill walls were modeled in SAP2000 using equivalent diagonal compression struts, with reduced strut widths for openings. Soil–structure interaction was represented using a three-dimensional linear-elastic soil model directly connected to the isolated footings. Linear time-history analysis was carried out using the 2001 Bhuj earthquake record, scaled to a PGA of 0.1 g. The results show that soil conditions strongly affect the seismic response. The highest roof acceleration, about 2.7 g, occurred for regular infilled frames on stiff soil, while the largest roof displacement, about 241 mm, occurred for the fully infilled frame on soft soil. Soft-story frames showed higher ground-story drift, reaching about 1.2% on soft soil. Overall, the critical response varied with soil condition, indicating that fixed-base analysis may not always identify the governing case.