This study aims to investigate the effects of vertically irregular stiffness on the strength reduction coefficient of analytical reinforced concrete models. To demonstrate a combination of vertical resistant systems, a set of moment frames and concrete shear walls that are regular in the plans of the entire stories are provided. The building models were 3D reinforced concrete frames. The models were designed with 8, 12, 16 and 20 stories, which is an acceptable range for the height of moment frame buildings with shear walls. Possible situations were incorporated into the development of the models. Hence, a parameter was employed to combine height resistant systems. Based on regulations, the parameter was defined as the ratio of the main period of the structure to the main period of the top part. After the frames were designed, analytical models were developed in Perform-3D while considering the nonlinear behavior of the components. Moreover, the fiber element method was adopted for reinforced concrete beams and columns, in which the nonlinear behavior distribution was made ideal along the components. The shear wall was modeled using panel elements. In the fiber element model, multiple axial springs that function based on the materials’ main stress-strain curve were employed to consider the interaction between two-directional axial and bending loads. In the models, the strength reduction coefficient was defined as the ratio of the basic elastic shear to the basic yield shear of the structure. To this end, reviewing the calculation methods of strength reduction coefficient, incremental dynamic analysis (IDA) was selected and applied. To apply IDA, capacity curve idealization methods were used to obtain the total yield displacement and story yield displacements. The results indicated that combining lateral resistant systems in model heights could reduce the total strength reduction coefficient by up to 30%. Moreover, it was observed that the structural ductility distributions of irregular models were considerably different. Applying irregularity to top stories dramatically raised the ductility of top stories. However, the ductility rise could vary, depending on the main period, lateral load strength, and earthquake excitation frequency content. Keywords : Vertical Combined Resistant Systems, Reinforced Concrete Structures, Coefficient of Strength Reduction, Incremental Dynamic Analysis, Capacity Curve Idealization Methods