On October 15, 2013, a magnitude 7.1 earthquake severely damaged buildings on Bohol Island, the Philippines. This paper briefly reports on the typical damage to reinforced concrete (RC) buildings observed in the authors’ post-earthquake investigation. The current study focuses on the seismic performance of an earthquake-damaged building with exterior beam-column joint failure. Cyclic loading tests were conducted to propose a practical seismic strengthening method by installing RC wing walls for substandard moment-resisting frames with brittle beam-column joints. A scaled model representing the earthquake-damaged frame reproduced the damage to the exterior beam-column joint, which could not be evaluated using Japanese seismic evaluation methods because of an overestimation of the joint performance. Another specimen strengthened by the proposed method was successfully upgraded, forming a ductile beam yielding mechanism. The ultimate strength of the upgraded specimen estimated by the Japanese methods agreed well with the experimental results. The strengthening mechanism by wing walls was elucidated, knowledge of which will be useful for future applications to substandard buildings in developing countries.