This study investigates the adequacy of the design shear force in reinforced concrete walls according to the Turkish Building Earthquake Code 2018 (TBEC-2018). For this purpose, linear and nonlinear analyses are conducted in a 20-story building, of which lateral load resisting system consists of reinforced concrete frames and shear walls with a high ductility level. First, the force-based design of the 20-story building is carried out using response spectrum analysis. Then, nonlinear time history analyses (NLTHA) are performed on the building under eleven ground motions matched to the elastic design spectrum. Lastly, design shear forces from linear analysis and shear demands from nonlinear analyses are compared along the heights of the walls with rectangular and L-shaped cross-sections. The study reveals that the design shear force of the L-shaped shear wall is insufficient to meet the demand value. The shear demands of reinforced concrete walls are also calculated by the modified modal superposition approach (MMS), and the applicability of this approach to TBEC-2018 is tested on the shear walls examined.