Seismic behavior of RC beam-column joints reinforced with deformed or smooth bars

Modern seismic design of reinforced concrete (RC) buildings is based on the capacity design principles, which provide for the dissipation of seismic energy through the development of ductile mechanism in the structural elements. Beam-column joints are critical elements, since they are governed by shear and bond behavior, with low ductility and scarce energy dissipation. In the absence of modern seismic codes prescriptions, many existing RC buildings designed before mid-1970 present structural deficiencies, like no joint shear reinforcement and the use of plain bars. These deficiencies may lead to develop brittle failure mechanisms and to the sudden collapse of the structure, under earthquake actions. In this thesis, the experimental seismic behavior of an exterior beam-column joint, designed according to the Italian Building Code for high ductility class, but built without the required horizontal ties in the joint core, to simulate a construction error is investigated. This construction error is plausible because, according to the design prescriptions, the joint panel is very crowded with reinforcement bars and the concrete casting and compaction is objectively difficult. The thesis explains in detail the occurred failure mechanisms due to the construction error. The correct evaluation of beam-column joints shear strength is of fundamental importance to respect the strength hierarchy and allow the ductile mechanisms development. This thesis focuses on shear strength of interior beam-column joints. In particular, a direct formula which accounts for the contributions of three inclined concrete struts and joint reinforcements, the column horizontal stirrups and intermediate vertical bars, is derived. The coefficients of the contributions are calibrated on the basis of experimental results, present in the literature, and the shear strength proposed expression is validated through the comparison with other existing formulae. Finally, an overview on seismic behavior of beam-column joints reinforced with plain bars, collected in the literature, is presented. The tests considered concern both interior and exterior joints and take into account the main features influencing joint behavior, including horizontal reinforcement amount, column axial load and anchorage arrangement solutions. A critical discussion of the damage and failure mechanisms developed in the joints is presented and the possible relationships between the structural inadequacies and the final failure modes are highlighted. Besides, the resisting mechanism contributions to shear strength for joints reinforced with plain bars are IV assessed. This thesis is presented as a useful tool for future aware design of beam-column joints reinforced with deformed bars in new RC buildings, and a comprehensive understanding of behavior of joints with plain bars, to lead to effective retrofit solutions for existing buildings.