Current force-based design procedure adopted by most seismic design codes allows the seismic design of building structures to be based on static or dynamic analyses of elastic models of the structure using elastic design spectra. The codes anticipate that structures will undergo inelastic deformations under strong seismic events; therefore, such inelastic behaviour is usually incorporated into the design by dividing the elastic spectra by a factor, R, that reduces the spectrum from its original elastic demand level to a design level. For a structure supporting on flexible foundation, as SSI extends the elastic period and also increases damping of the structure-foundation elastic system, the structural ductility could be affected by frequency dependent foundation-soil compliances. For inelastic systems supporting on flexible foundations, the inelastic spectra ordinates are greater than for elastic systems when presented in terms of flexible-base structure’s period. This implies that the reduction factors, which are currently not affected by SSI effect, could be altered; therefore, the objective of this research is to evaluate the significance of foundation flexibility on force reduction factors of R/C frame structures. In this research, by developing some generic R/C frame models supporting on flexible foundations, effects of stiffness and strength of the structure on force reduction factors are evaluated for different relative stiffnesses between the structure and the supporting soil. The generic frames are idealized as 2D frames using beam and column elements. The non-linear behaviour of beam elements is modelled by moment-curvature approach; while the multi-spring model, so called as “fiber element model” is employed to represent the non-linear behaviour of columns. Implementing the concept of foundation impedance functions, the effect of SSI is taken into account using substructure method. Using a set of artificial earthquake records, repeated linear and non-linear analyses were performed by gradually increasing the intensity of acceleration time histories to a level, where first yielding of steel in linear analysis and a level in which collapse of the structure in non-linear analysis is observed. The difference between inelastic and elastic resistance in terms of displacement ductility capacity factors has been quantified. The results indicated that the foundation flexibility could significantly change the ductility of the system and neglecting this phenomenon may lead to erroneous conclusions in the prediction of seismic performance of flexibly-supported R/C frame structures