Currently, some design requirement documents, such as ASCE-7 Standard, still rely on design criteria based on the determination of forces to establish demands to provide adequate structural stiffness and strength. Acknowledging from past seismic events that structures can actually withstand forces of a greater magnitude than that for which they were designed, today’s standard practice is to reduce those design forces that one would use to keep the structure’s behavior elastic by means of the Response Modification Factor, R. As a result of this reduction, the expected elastic base shear strength, Ve, is reduced to a design base shear strength, V. Although buildings are designed with prescribed base shear strength based on mathematical models, actual structures can develop higher strengths during seismic events. This increase in structural strength beyond the design forces is referred to as structural overstrength, denoted as . Some studies have focused on evaluating these factors (R and ), using models of varying sophistication with and without consideration of soil-structure interaction (SSI) effects to assess their behavior. This study specifically examines the variation of the overstrength factor in reinforced-concrete buildings, considering SSI effects, and proposes an analytical expression to estimate its value. To this end, nonlinear curve-fitting techniques are employed to approximate an adequate ̃, which has been shown to depend primarily on the fixed-base fundamental period of vibration, Tn.