The seismic response of reinforced concrete buildings can be significantly affected by masonry infill walls and staircases, which are often not explicitly considered in analytical models. This study evaluates the effect of these components on typical buildings in Portoviejo, Ecuador, to identify their influence on structural capacity and seismic performance. Three representative building typologies of 3, 6, and 10 stories were analyzed through nonlinear static pushover analysis, performed in Seism Struct using inelastic force-based frame elements with fiber sections, considering four structural configurations: frame (P), frame with masonry infill walls (P+M), frame with staircases (P+S), and frame with masonry infill walls and staircases (P+M+S). Based on the capacity curves, performance points, and damage distribution, the results indicate that the influence of these components depends on building height. In low-rise buildings, masonry infill walls increased the initial stiffness by approximately 90–110% and the lateral strength relative to the bare frame, although they reduced global ductility. In mid-rise buildings, the most critical response was observed, associated with vertical irregularity, damage concentration in the lower stories, and a tendency toward soft-story mechanisms, consistent with field observations following the 2016 Portoviejo earthquake. In high-rise buildings, the global effect was smaller, although local effects on the distribution of internal forces and damage remained. These findings indicate that neglecting masonry infill walls and staircases may underestimate relevant seismic demands and misrepresent stiffness, deformation patterns, and damage concentration; therefore, their explicit consideration is recommended in the seismic assessment of existing reinforced concrete buildings with similar configurations in Portoviejo.
