Existing Reinforced Concrete (RC) buildings worldwide are often characterized by the total absence of a proper design of Beam-Column Joints (BCJs) to sustain seismic loading. Joints geometric dimensions often result not sufficient and stirrups in joints completely missing even in high-seismicity areas. Therefore, a significant portion of existing RC buildings can experience heavy damage in BCJ due to earthquakes, as observed in the recent seismic events.

The strengthening of BCJ is, thus, a paramount issue in seismic prone areas. Different techniques have been investigated and implemented in last decades, by using FRP materials, fiber reinforced concrete to jacket the joint, or steel jacketing, among others. Pre-stressed steel strips can be also used to provide a (active) confinement and a transverse reinforcement to unreinforced joints.

The present study aims at the experimental assessment of the effectiveness of pre-stressed steel strips for the strengthening of RC BCJ. An experimental campaign on full-scale exterior BCJ subassemblies have been designed and realized, including two as-built specimens and three retrofitted ones. Two different beam longitudinal reinforcement ratios and two columns axial load ratio values have been considered. Setup and instrumentation are described, and the main results analyzed and compared. As-built specimens exhibited a joint failure without (J) or after (BJ) the beam yielding. Retrofitted specimens always showed a significantly better performance with respect to the corresponding as-built specimen in terms of strength or ductility. The selected retrofitting technique allowed the beam to reach yielding before joint failure for the test that had a J-failure mode in its as-built condition. Additionally, a fully ductile behavior of the retrofitted sub-assemblage was observed when the as-built specimen had a BJ- failure mode, irrespective of the column axial load ratio. The results confirm the effectiveness of such strengthening technique for unreinforced BCJ with a minimum building disruption and allow to quantify it by means of practice-oriented predictive formulations.