## 2 Beam-Column Joints of Composite Frames Under Seismic Loads
### Beam-Column Joints of Composite Frames Under Seismic Loads: State-of-the-Art
The composite column, which consists of a concrete-filled steel tube further encased in concrete, is manufactured off-site in a controlled environment, with bolt-holes drilled in the steel tube and concrete poured around it. Blind bolts are then installed, and the core concrete is poured on site. Numerical models and experimental results were used to analyze the connection, considering material models and contacts. Component models for stiffness and moment resistance were developed, considering the load transfer mechanism. The seismic behavior of vertical rib connections for the joint of an H-shaped steel beam to a special-shaped CFT column was tested by [PERSON] et al. (2021) [20]. Frames were tested under vertical and horizontal loads to examine stiffness degradation, failure mode, energy dissipation, strength-bearing capacity, and ductility. Finite element analysis was used to investigate the plastic hinge formation and failure mode. Furthermore, [PERSON] et al. (2021) [21] explored the seismic and shear behavior of joints connecting U-shaped steel-concrete composite beams to T-shaped concrete-filled steel tubular columns. A new joint design with C-shaped slots allowing double-C channels to pass through the joint panel zone was proposed and tested. The seismic performance of composite steel-concrete frame systems using the URSP (uplift-restricted/slip-permitted) connector was studied by [PERSON] et al. (2022) [22]. A numerical model was used to analyze the dynamic behavior of structures with URSP connectors under seismic loads. The results were compared to traditional structures, revealing that the arrangement of full-span URSP connectors had a significant impact on the system's behavior, increasing inter-story displacement and causing adverse effects. It was also found that the partial arrangement of URSP connectors had little influence and still maintained good seismic performance. On the other hand, [PERSON] et al. (2022) [23] analyzed the dynamic response of the beam to CFDST column joints for this connection, using established analytical models and pseudo-dynamic tests. The tests explored the effects of end plate type, column hollow ratio, and concrete slab on dynamic responses and failure mechanisms. Seismic behavior, hysteresis curves, viscous damping coefficients, and energy dissipation capacities were also examined, and analytical models of the joints were developed using OpenSees software ([[https://opensees.berkeley.edu/](https://opensees.berkeley.edu/)]([https://opensees.berkeley.edu/](https://opensees.berkeley.edu/))). [PERSON] et al. (2022) [24] conducted experiments on 3D beam to L-shaped steel-concrete column joints subjected to dynamic loading, considering variables, such as encased steel configuration, axial compression ratio, loading angle, and beam type, to provide test data, evaluate seismic behavior, and analyze the effects of key parameters. Additionally, [PERSON] et al. (2023) [25] introduced a new approach to designing PC joints with distinct advantages, including an efficient and eco-friendly assembly method, simple structure, concentrated failure range, and satisfactory seismic performance. The tests were conducted on full-scale joint specimens, considering various factors, like steel tube thickness and internal structure, and analyzed using an ABAQUS ([https://www.3 ds.com/products/simulia/abaqus](https://www.3 ds.com/products/simulia/abaqus)) finite element model. [PERSON] et al. (2019) [26] developed a new joint system for RC beams and square-reinforced concrete-filled steel tube (RCFST) columns. The system utilizes openings on the square hollow section (SHS) for continuous beam reinforcement and interior diaphragms welded to the cold-formed SHS. These diaphragms confine the steel tube and concrete and can be used to position column reinforcement. The system eliminates the need for internal diaphragms to transfer forces from the RC beam, resulting in improved seismic performance. This joint system is shown in Figure 2.  
Additionally, [PERSON] et al.