Experimental and numerical study of RC beam joints to steel-encased RC columns with a transitional part

The use of composite columns has attracted considerable attention due to their enhanced load-carrying capacity and ductility. However, the comparative behavior of reinforced concrete (RC) beam and RC column connections and, RC beam and steel encased in reinforced concrete columns (SRCC) incorporating a transitional part, aimed at improving load capacity, ductility, relocating the plastic hinge formation zone, and preventing shear failure of the RC beam under combined axial and cyclic loading, has not been comprehensively investigated. In this study, five specimens were tested, including RC beam RC column connections and RC beam and steel encased in reinforced concrete columns (SRCC) joints with a transition part employing different longitudinal reinforcement anchorage details. The specimens were subjected to quasi static cyclic loading under a constant axial load ratio of 0.08 of the column compressive capacity. Furthermore, a nonlinear finite element model was developed and validated against the experimental results. The experimental results demonstrated that the use of steel encased in reinforced concrete column enhanced the ultimate load-carrying capacity by an average of 35% compared to the conventional reinforced concrete beam to column specimen. In addition, a significant improvement in ductility and crack propagation control was observed. The incorporation of a transition part in the joint region not only increased the load capacity but also effectively relocated the plastic hinge formation from the joint core to the beam span, thereby reducing the likelihood of shear failure in the reinforced concrete beam. The numerical results showed good agreement with the experimental data, with the prediction error of the ultimate capacity remaining below 10% for all specimens. Based on the findings of this study, the application of steel-encased reinforced concrete columns combined with a transitional part and different anchorage details of beam longitudinal reinforcement can be considered an effective strategy for enhancing the seismic performance of beam–column connections. Furthermore, the proposed nonlinear finite element model provides a reliable tool for further analytical investigations and parametric studies.