Strengthening of Unreinforced External RC Beam-Column Joints with Prestressed Steel Strips

The response of unreinforced beam-column joints represents a key issue in seismic vulnerability and fragility assessment of existing Reinforced Concrete (RC) buildings. Within this framework, a great attention is focused on the investigation of the experimental response of unreinforced joints strengthened with different technologies, based on traditional or innovative materials, and with different complexity/cost. In this study, an experimental investigation of the response of external RC beam-column joints is presented. Four specimens are tested, namely one as-built, unreinforced (without stirrups in the joint panel region) specimen, one reinforced with a code-compliant amount of transverse reinforcement (stirrups) in the joint panel – for reference and comparison purposes, and two specimens strengthened with the application of prestressed steel strips as external transverse reinforcement, using the CAM® technology with two different layouts characterized by a different complexity of installation. The response of the unreinforced specimen was controlled by joint panel failure following beam yielding, whereas the specimen reinforced with code-compliant joint stirrups showed a ductile behaviour, controlled by beam flexural failure. The response of the strengthened specimens demonstrated the effectiveness of the adopted technology, that led to a ductile response – as for the specimen reinforced with stirrups – or to a joint panel failure following beam yielding – as for the as-built unreinforced specimen, but with a higher global ductility. The experimental results are reported and discussed, in terms of global response of the sub-assemblage and of observed damage evolution, and in terms of local response, such as shear distortion of the joint panel and measured strain of strengthening steel strips. The effectiveness of the tested strengthening solutions is evaluated through a comparison with the response of the reference (as-built unreinforced and reinforced with code-compliant joint stirrups) specimens.