<p>The growing population in urban areas, combined with the increased utilization of available land, has made the incorporation of basement floors in buildings a common practice. Due to the high lateral pressure exerted on these floors, the structural systems of basement levels typically incorporate reinforced concrete perimeter walls, which exhibit relatively high rigidity. Previous research, along with seismic design codes, recommends raising the earthquake base elevation to the top of the perimeter walls when conducting seismic analysis of such buildings. However, studies have shown that shifting the seismic base from the foundation to the top of the perimeter walls does not fully capture the structure's true behavior under seismic loading conditions. This study evaluates the seismic response of two-dimensional rigid steel frames with basements which surrounded by rigid perimeter walls, using the spectral solution method. The impact of perimeter walls' stiffness and the rigidity of basement floors on force distribution, displacement, and seismic performance was investigated by analyzing a two-degree-of-freedom system using MATLAB programming. By applying harmonic loads and precisely solving the spectral equations, the sensitivity of the structure to loading frequencies and the influence of the basement floors on the overall structural behavior were examined. The results were compared to numerical simulations performed through both pseudo-static and dynamic analyses. The findings indicate that an increase in the stiffness of basement floors has a minimal effect on the overall seismic behavior of the structure. Raising the seismic base elevation to the top of the perimeter basement walls leads to a significant reduction in base shear demand. This reduction is consistent with what has been emphasized in various international seismic design codes. The solving spectral equation method proposed in this study can be used as a new design tool to enhance the established standards and for practical purposes.</p>