Seismic performance of post-fire reinforced concrete beam-column joints strengthened with steel haunch system

Abstract

This paper proposes an innovative and practical strengthening method for fire-damaged RC joints using steel haunches system. A total of seven specimens consisting of beam-column sub-assemblages were designed including one control specimen without fire damage, one control fire-damaged specimen, and five fire-damaged specimens retrofitted with steel haunches system. The specimens were first exposed to the ISO834 temperature curve in a furnace chamber. Then, two sizes of steel haunches were used to strengthen the joint panel zone, and bolted side plates (BSP) or carbon fiber reinforced polymers (CFRP) were used to repair the beam. Finally, the control and retrofitted specimens were subjected to low-cycle reversed loading until failure. The effects of fire damage and strengthening method on the hysteresis behavior, bearing capacity, stiffness degradation, ductility factor, and energy dissipation capacity of the joints are reported. Results for the control specimens showed that the failure mode changed from beam-end plastic hinge failure at room temperature to shear failure in the joint zone after exposure to fire, accompanied with a reduction in load bearing capacity. Yet, the steel haunches retrofitting relocated the plastic hinge to the beam-end zone. The combination of steel haunches and BSP significantly enhanced the seismic performance in terms of bearing capacity, stiffness, ductility, and energy dissipation. Locating the BSP close to the beam bottom improved the positive load capacity of the upgraded joints. The specimen strengthened with the combination of steel haunches and CFRP also exhibited acceptable seismic performance. The proposed repair systems allow retrofitting fire-damaged existing concrete joints to meet current seismic design requirements.