In this thesis, the effectiveness of the 100 percent + XX percent seismic response directional combination rule available in codes and standards is evaluated using simulated nonlinear seismic response of two existing multi-story reinforced concrete plan-irregular buildings. The 5-story and 8-story buildings were designed according to the 1975 seismic codes of Turkey and experienced the 1999 Ismit, Kocaeli, earthquake. Nonlinear structural models of the buildings are developed in OpenSees and subjected to 22 sets of two bi-directional components of ground motions that are applied simultaneously in two possible orthogonal orientations. In addition, the building seismic response is also evaluated when ground motion records are applied independently along the two major axes of the building. Comparison of the structural response obtained using the bi-direction and unidirectional inputs allows for a critical evaluation of the XX percent that bounds the drift and acceleration responses of these two buildings. Ultimately, recommendations on the value of XX-percent to be used for seismic response assessment of existing plan-irregular RC buildings are provided.