Reinforced concrete (RC) framed buildings with masonry infill are widely used in earthquake-prone regions; however, variations in the distribution of masonry infill and the presence of vertical irregularities can significantly alter their seismic behaviour. Although masonry infill walls are generally considered non-structural elements, they contribute substantially to the lateral stiffness, strength, energy dissipation, and overall dynamic response of RC frames. The sudden absence or reduction of infill at a particular storey can produce a soft-storey condition, resulting in concentration of lateral deformation, storey drift, shear forces, and damage at that level. This review examines the influence of vertical irregularities, including variations in storey height, stiffness, strength, column dimensions, and infill distribution, on the seismic response of RC frames. Previous numerical and experimental investigations using pushover analysis, response spectrum analysis, nonlinear dynamic analysis, and other analytical approaches are reviewed with respect to base shear, lateral displacement, inter-storey drift, stiffness, strength, energy dissipation, plastic hinge formation, and collapse vulnerability. The reviewed studies indicate that increasing masonry infill generally enhances lateral strength and stiffness and reduces displacement, whereas discontinuous or improperly distributed infill may generate severe stiffness irregularities and torsional effects. Soft-storey buildings, particularly those with open or weak ground floors, exhibit considerably higher seismic vulnerability than regular infilled frames. Strengthening techniques such as additional infill, bracing, and improved beam-column detailing can significantly enhance lateral resistance, stiffness, deformation capacity, and energy dissipation. Overall, the review emphasizes the need to consider the interaction between masonry infill and vertical irregularities during seismic assessment and design to achieve safer and more reliable RC frame structures.

 Keywords: Reinforced concrete frames; Masonry infill; Vertical irregularity; Soft storey; Seismic response; Infill-frame interaction; Storey drift; Lateral stiffness; Pushover analysis; Seismic vulnerability.