Reassessing the Comparative Effectiveness of Masonry Infill Retrofit Layouts Under Soil–Structure Interaction in Existing RC Frames

This study investigates how soil–structure interaction (SSI) affects the seismic effectiveness and relative performance of high-strength masonry infill retrofit layouts for a three-story nonductile reinforced concrete building. Three retrofit configurations were evaluated under fixed-base, Site Class C, and Site Class D conditions using nonlinear pushover analysis, component-level criteria, and nonlinear soil foundation macro-elements. All three configurations were compared through deterministic SSI analyses, after which HBW1 and HBW2 were retained for Sa,avg-based Multiple Stripe Analysis; consequently, the probabilistic comparison is limited to these two configurations. Under Site C conditions, foundation deformation remained limited and the fixed-base lateral resistance was generally preserved, whereas under Site D conditions, substantially larger translation, rocking, settlement, and differential settlement demands developed and altered the nonlinear response in a configuration-dependent manner. The deterministic screening showed lower global resistance for HBW3, and it was therefore not retained for the subsequent dynamic assessment. Under Site C, the fitted median IO and LS intensity capacities of HBW2 were approximately 32% and 12% higher, respectively, than those of HBW1. Under Site D, this dynamic advantage diminished, with essentially identical median LS capacities, while HBW1 developed greater rocking demand and HBW2 larger settlement demands. The results show that SSI does not consistently improve or degrade seismic performance; instead, retrofit effectiveness depends on soil condition, wall distribution, performance level, and response measure, highlighting the need for combined superstructure–foundation assessment.