Experimental investigation on the effectiveness of prestressed steel strips for strengthening corner beam-column joints with plain bars

Unreinforced beam-column joints often represent a critical issue for the seismic vulnerability of existing Reinforced Concrete (RC) buildings. This is evident in pre-70 Mediterranean RC buildings, whose structural elements are generally reinforced with plain bars and always totally miss stirrups within joints. Over the last decade, significant attention has been focused worldwide to develop effective strengthening strategies for unreinforced joints, implementing traditional or innovative materials and technologies with various levels of complexity or cost. This work deals with the effectiveness of Pre-Stressed Steel Strips (PSS) in strengthening unreinforced beam-column sub-assemblies with plain bars. Four specimens are tested, reproducing the corner joints of pre-70 Italian RC buildings reinforced with plain bars. Two of them are characterized by a lower amount of beam and column reinforcement; the remaining two specimens have a higher beam/column reinforcement. Two specimens are tested in as-built condition without stirrups, whereas the others are strengthened with PSS, designed to make ductile the sub-assemblage. Quasi-static tests are performed under increasing cyclic displacements. Experimental results are discussed in terms of global response, observed damage evolution, and local response. Strengthened specimens show reduced damage in the joint core, along with higher strength and displacement capacity, thus proving the effectiveness of the adopted technology. Final remarks about shear strength prediction for joints strengthened with PSS are carried out, also based on the second-generation Eurocode model.