For about 40 years, studies conducted on strength and behavior of infill walls have not been resulted in standardized consensus methods. However, it is a well-known fact that infill walls have significant impact on the behavior of steel and reinforced concrete frames under horizontal loads. In this study, positive and negative effects of infill walls (considered as non-structural members despite the aforementioned fact) on steel frames subjected to lateral monotonic loads are investigated. Within the study, a two-story, two-span planar steel frame is designed and modeled for seven different infill wall combinations such as with/without infill wall, band windows etc. For infill walls, gas concrete is used due to its light weight and high modulus of elasticity. In planar frames, the infill walls under horizontal loads are modeled with equivalent compression bar model and analyzed for elasto-plastic analysis. Based on certain assumptions, ultimate loads, energy dissipation, ductility, lateral displacements, and ultimate base shear forces of symmetrical and asymmetrical vertical infill walls on the frames are investigated. Moreover, short column issues on band window model are examined as well as soft story effects. Modeling and analyses are carried out in SAP2000 static analysis program.