Seismic performance of RC beam-column-slab joints strengthened with steel haunch system

Function alteration and revision of seismic design codes require the improvement of seismic performance of existing spatial RC joints. An innovative and practical seismic retrofitting method using steel haunch system is recommended for existing spatial RC joints with the floor slab and the transverse beam. Four spatial RC joints were designed including one control specimen without strengthening, three retrofitted specimens strengthened with only steel haunches, combined steel haunches and carbon fibre-reinforced polymer (CFRP) sheets, and combined steel haunches and bolted side plates (BSP) on beam's sides. All specimens were subjected to low-cycle reversed loading. The effects of different retrofit methods on failure modes, crack patterns, hysteretic behavior, ultimate load capacity, stiffness degradation, ductility factor, and energy dissipation capacity were analyzed. Results showed that hysteretic curves of retrofitted specimens were more stable and plump. The plastic hinge of spatial joints was transferred from the beam end to the zone outside steel haunch after retrofit. The ultimate load capacity, stiffness, ductility and cumulative hysteresis energy were significantly enhanced with a maximum proportion of 63.8%, 315%, 30% and 165%, respectively. Steel haunch can be effectively combined with bolted side plates and CFRP in the strengthening of RC beam-column-slab joints to make full use of material properties. The preload degradation of the bolts used to install the steel haunch reduced the retrofit efficiency. Combined steel haunches and BSP exhibited the best cooperative performance with the spatial joint. The strains demonstrated that the steel haunches were still in the elastic stage and subjected to eccentric loading during the entire loading stage. Finite element models of retrofitted joints were established based on ABAQUS. The loaddisplacement curves calculated by ABAQUS agreed well with test results. Numerical analysis shows the stress and strain contribution of each component which is helpful to further optimize this retrofit system.