The effect of soil-structure interaction on the dynamic response of reinforced concreternbuildings of regular and symmetrical geometry is considered in this study. The structuresrnare presumed to be generally embedded in a homogenous soil formation underlain by veryrnstiff material or bedrock. The structure-foundationâ€“soil system is excited at the base by anrnearthquake ground motion.rnThe superstructure is idealized as a system with lumped masses concentrated at the floorrnlevels, and coupled with the substructure. The substructure system, which comprises of thernfoundation and soil, is represented and replaced by springs and dashpots.rnFrequency-dependent impedances of the foundation system are incorporated in therndiscrete model in terms of the springs and dashpots coefficients. The excitation applied tornthe model is field ground motions of actual earthquake records.rnModal superposition principle is employed to transform the equations of motion inrngeometrical coordinates to modal coordinates. However, the modal equations remainrncoupled with respect to damping terms due to the difference in damping mechanisms ofrnthe superstructure and the soil. Hence, proportional damping for the coupled structuralrnsystem may not be assumed.rnAn iterative approach developed by Worku [13,14] is adopted and programmed to solvernthe system of coupled equations of motion in modal coordinates to obtain the displacementrnresponses of the system.rnParametric studies for responses of building structures with regular and symmetric plansrnof different structural properties and heights are made for fixed and flexible basernconditions, for different soil conditions encountered in Addis Ababa.rnSoil borehole log data of three representative sites in Addis Ababa were used for therncomputation of the stiffness and damping of the soil.rnivrnThe displacement, base shear and base overturning moments are used in the comparison ofrndifferent types of structures for various foundation embedment depths, site conditions andrnheight of structures. These values are compared against those of fixed base structure.rnThe study shows that the flexible base structures, generally exhibit different responsesrnfrom those structures with fixed base. Basically, the natural circular frequencies, the basernshears and the inter-story displacements for the flexible base are less than those of thernfixed base structures. This trend is particularly evident when the flexible soil has largernthickness. In contrast, the trend becomes less predictable, when the thickness of thernflexible soil decreases. Moreover, in the latter case, the iteration undulates significantlyrnmaking the prediction difficult. This is attributed to the highly jagged nature of thernimpedance functions of frequencies for such formations. In this particular case, it isrndifficult to conclude whether the conventional fixed-base approach yields conservativerndesign forces, as is the case for soil formations of large thickness.