In this contribution, as part of a more extensive research program on seismic retrofit solutions for reinforced concrete buildings, the effects of bi-directional loading on the assessment and design of the retrofit intervention using FRP composite materials will be discussed. Based on experimental evidences on the performance of exterior 3D (corner) under-designed beam-column joints, the limits and drawbacks of standard assessment methodologies when evaluate the hierarchy of strength and expected sequence of events prior to define the retrofit intervention, will be discussed. In addition, the preliminary results of an on-going experimental campaign to further investigate and address this issue will be presented, based on the quasi-static tests under bi-directional loading on two 3D exterior (corner) joint specimens, comprising of a pre-1970s as-built specimen (benchmark) and a “minimum” retrofitted configuration using GFRP sheets. The feasibility and efficiency of the adopted retrofit strategy and solution, aiming at controlling the hierarchy of strength within the beam-column joint system by protecting the panel zone and relocating the plastic hinge in the beam, will be discussed. Considerations and suggestions on the additional design criteria to account for the actual 3-D response under bi-directional loading will be also given.