Infill walls are treated as nonstructural elements but it has been noticed that they have significant effect in enhancing the lateral stiffness of complete structure. The performance of such structures during earthquakes and lateral loading in general has proved to be superior in comparison to the bare frames, both for new and for existing buildings. Based on the understanding of the actual response, code design provisions need to be developed. The purpose of this research is to study analytically the effect of infill panels on the base-shear capacity of RC frames with different heights. A finite element computer program package (SeismoStruct) of an advanced double-strut nonlinear cyclic model for masonry panels was used under static pushover loading. The case study is a 3-bay RC frame analyzed with different number of stories, 4, 10 and 15 story. The frames were designed according to Egyptian code under gravity loads only and did not meet the modern code seismic design requirements. The results obtained in this study indicate that the base-shear capacity and the initial stiffness of the buildings increase considerably in the presence of masonry infill where the increase percentage in stiffness and strength inversely proportional to the building height.