Understanding the behavior of reinforced concrete (RC) buildings under seismic excitation is crucial for the safety of the building users, especially in seismic-prone areas. The RC frame may present higher structural responses especially when it has mass irregularity. In this study, regular 8-story RC frame buildings with 3- m floor height, 5 bays in x direction, 3 bays in y direction for meeting facilities and office buildings were inspected against seismic loadings. The seismic performance was evaluated based on the inter-story drift response from non-linear dynamic analyses, refers to Indonesian RC building code and seismic code. Nonlinear time history analyses were conducted using open-licensed software, STERA_3D. The story masses at levels 2 and 3 were doubled compared to other levels due to their function for meeting areas and to represent the irregularity mass distribution. As anticipated, floors 2 and 3 exhibited larger inter-story drifts, nearly 2% of their story heights. This result exceeded the allowable inter-story drift of 1.5% stipulated in the Indonesian building code for their designated functions. Consequently, the building with mass irregularity was enhanced by selecting and installing 4 buckling-restrained braces for levels 2 and 3, respectively. Following the same analysis procedures, an improvement in inter-story drift was achieved, with values falling below allowable drift limits. This improvement results could deliver insights and alternatives to building engineers and stakeholders who face the challenges on accommodating irregularity in seismically active regions.