Assessing the Effects of Masonry Infill Irregularities on the Seismic Behavior of Irregular RC Buildings

The seismic performance of Reinforced Concrete (RC) frame structures is critically dependent on the interaction with masonry infill walls. Although infills significantly enhance stiffness, strength, and energy dissipation, current seismic codes often neglect their contribution, which can lead to nonconservative safety assessments. A prevalent architectural feature – irregular distribution of infills (both vertically and horizontally) – can induce detrimental seismic responses, most notably the soft story mechanism. This paper rigorously assesses the impact of these infill irregularities on the nonlinear seismic behavior of RC buildings. Detailed structural models were subjected to both nonlinear static (pushover) analysis and Nonlinear Time History Analysis (NLTHA). Infill panels were modeled using equivalent strut elements, calibrated to accurately simulate their inelastic behavior, including cracking, crushing, and strength degradation. The results quantify how infill irregularity alters the structural capacity, seismic demand, and fragility. The study demonstrates that vertical irregularity, specifically the soft story effect, leads to a significant increase in seismic vulnerability and displacement demand across various performance limit states. The findings underscore that ignoring the realistic distribution of infills leads to a dangerous underestimation of seismic risk, confirming the necessity of integrating infill irregularity into modern performance-based evaluations.