This study investigates the contribution of infill walls to the capacity and behavior of reinforced concrete structures against lateral loads. A 27.2 m high 8-storey building with a footprint of 20m x 40m is modelled using ETABS. Cases with bare frame, fully infilled frames, and partially infilled with openings (25%, 50%, 75%) are considered. The study considered infill thicknesses of 90, 140, 200 and 230 mm. Single equivalent nonlinear strut is used to model the infill and non-linear pushover analysis is used to investigate the model behavior. The consideration of infill walls had a significant effect on the natural period that dropped from 1.45s to 0.77s due to infill. It is found that the presence of infill wall increases the lateral stiffness of the structure. The increase in stiffness is directly proportional to the infill thickness. The initial stiffness is 4.4 times the bare frame for the case with 230 mm thick infill walls. Presence of openings significantly reduced the contribution of infill walls. For cases of infill walls with 50 % opening, the additional stiffness is reduced to just 10 % while for 75% opening it is less than 1 %. Therefore, the presence infill with openings of 50% or higher will not play a significant role in the analysis and design process.