Parametric study of masonry-infilled RC frames with openings: subassembly contributions to shear resistance and the role of vertical confinement

The seismic performance of reinforced concrete (RC) frames with masonry infill walls is strongly affected by the presence of openings, which are unavoidable for architectural reasons. While previous research has often neglected or simplified the role of openings, this study presents a systematic parametric investigation of infilled RC subassemblies with both window- and door-type openings. The analysis explicitly considers opening size (Ao/Ai ratio), position (central vs. eccentric), and the presence of vertical confining elements (ties/serklaži). Using a calibrated micromodelling framework, the study quantifies component-wise shear resistance contributions of the RC frame, masonry infill, and confinement across different drift levels associated with EMS-98 damage grades. Results show that opening size and eccentricity substantially influence stiffness degradation and load transfer, with eccentric door openings imposing shear demands on the RC frame that often exceed the bare-frame design capacity. Vertical confinement proves effective at early damage stages (DG1–DG2), delaying infill degradation and enhancing out-of-plane stability, but its relative contribution remains below 20% and diminishes at higher drifts. Comparison with previous unconfined benchmarks highlights that vertical ties improve both shear resistance and safety against expulsion, establishing infills as integral parts of the seismic load-resisting system. Practical trends and interpolation ranges are proposed to inform design and to support future refinement of the contemporary building code provisions and guidelines.