Seismic Fragility of Informally Constructed Masonry-Infilled RC Frames: A Parametric Study Considering Structural Variability

This study presents a numerical investigation on the seismic performance of non-engineered reinforced concrete (RC) frame houses infilled with unreinforced masonry (URM) walls, typical of informal construction in Colombia. A set of building archetypes was developed to systematically evaluate the influence of key structural parameters, including number of stories (two and three), gravity load levels, RC member sizes, longitudinal reinforcement ratios, and stirrup spacing. Each archetype was modeled in OpenSeesPy using fiber-based beam–column elements for RC members and compression-only diagonal struts for URM walls. Nonlinear static (pushover) and dynamic analyses were performed using ground motions compatible with the Colombian seismic hazard. The results reveal that increasing the number of stories and gravity loads notably reduces the base shear-to-weight ratio (V/W) and drift capacity, while larger cross-sections and closer stirrup spacing enhance ductility and delay strength degradation. Archetypes with wider stirrup spacing (30 cm) exhibited premature bar buckling and earlier lateral strength loss, whereas denser confinement (10 cm spacing) improved post-peak stability. The study highlights the strong sensitivity of structural performance to reinforcement detailing and load level, providing a calibrated set of archetypes suitable for fragility analysis and regional risk assessment of informal housing stock.