Seismic strengthening of external beam-column joints with plain bars using minimally invasive FRP layouts

Recent seismic events have underscored the role of seismic detailing in earthquake-prone regions, emphasizing the high vulnerability of existing reinforced concrete (RC) structures. Beam-column joints (BCJs) are among the most critical members influencing the overall response of buildings during seismic events. BCJs designed under obsolete regulations often lack adequate transverse reinforcement, making them susceptible to brittle failure, while the use of plain bars is associated with bond degradation and lower steel yield stress. Externally bonded FRPs are proven effective in enhancing the strength of BCJs, preventing shear failure, thereby reducing the overall seismic risk of buildings. However, many of the proposed strengthening layouts disrupt building usability, limiting the effectiveness of seismic risk mitigation at a regional scale. To address this, FRP-based minimally invasive (MinInv) strengthening layouts have been recently proposed.This study reports and discusses the results of an experimental program on four BCJs with plain internal steel bars typical of RC buildings in the Mediterranean built before the 70’s. They are tested in both the as-built and FRP-strengthened configuration, to validate the performance of MinInv-layouts. Results demonstrated improved strength, a shift from brittle to ductile failure, and significantly increased energy dissipation. A comparison with available capacity models is proposed.