This study concerns refined numerical modeling of dapped-end beams subjected to corrosion and the assessment of the performance of an external steel retrofitting system. Widely used in infrastructure heritage, dapped-end beams face several challenges due to the complex stress flows in the discontinuous zones caused by the abrupt change in section depth. To gain a better insight into the impact of local stress transfer and overall performance, extensive numerical investigations are essential. In this study refined 3D solid models are built to complement previous experimental tests on a steel retrofitting system for corrosion-damaged half-joints using the STKO software platform for OpenSees. The results from the experimental campaign are replicated numerically following a process of calibration of material parameters. Then, additional numerical simulations are carried out to examine the structural response of the retrofitted half-joint in different configurations not previously assessed in the reference experimental campaign. An improvement of the steel retrofitting system is finally proposed and numerically simulated. The results aim to provide insights into the efficiency and performance of retrofitted damaged half-joints, highlighting the capability of numerical models to complement experimental tests and improve the understanding of structural behavior, and strengthening strategies.