## 6 Behavior of Composite Beam-Columns Under Cyclic Loads  
### Behavior of Composite Beam-Columns Under Cyclic Loads: State-of-the-Art  
Composite beam-columns are widely used in construction due to their strength, stiffness, and ductility. They typically consist of steel sections encased in or filled with concrete. During seismic events, they exhibit complex behavior that is crucial to understand for earthquake-resistant building design. Important features include load transfer mechanisms, ductility, stiffness degradation, strength deterioration, bond-slip behavior, local buckling, and concrete confinement. Understanding these behaviors is crucial for accurately modeling and designing steel-concrete composite buildings to withstand seismic loads. Researchers and engineers use this knowledge to develop design guidelines, analytical models, and performance-based seismic design approaches for composite structures. The current practices and future trends for the behavior of composite beam-columns under cyclic loads are examined in the following section. [PERSON] et al. (2021) [77] investigated the damage behavior and bearing capacity of circular high-strength CFT columns using quasi-static experimental tests. The moment-axial force relationship was analyzed, and the effects of key parameters were studied to propose a design method. Furthermore, they established a fracture criterion for high-strength steel and a restoring force model. The results showed that steel yield strength, concrete strength, and D/t ratio affected the moment-axial force curves. The proposed model aligned with tested bearing capacity, and the HS steel tube exhibited ductile fracture. Moreover, [PERSON] al. (2016) [78] discussed the analytical behavior of concrete-encased concrete-filled steel tubular columns during cyclic lateral loading. A finite element analysis model accurately predicted failure modes, load-displacement relationships, and ultimate strength. The study compared the behavior of these composite columns with conventional concrete-filled steel tubular columns and reinforced concrete columns, and a simplified hysteretic model for the M-\(\varphi\) relationship of the examined composite columns was proposed. [PERSON] et al. (2017) [79] conducted low cyclic loading tests on CFT columns filled with normal concrete and recycled aggregate concrete (RAC) to analyze their seismic performance. Findings showed that RAC-filled columns had a similar seismic performance to normal concrete-filled columns. However, RAC-filled columns exhibited better lateral bearing capacity, improved ductility, and slightly lower energy dissipation ability. Similarly, [PERSON] et al. (2022) [80] conducted cyclic loading tests on eleven composite columns filled with RAC to investigate their seismic performance. Factors, such as the diameter-to-thickness ratio of circular steel tube, profile steel ratio, axial compression ratio, replacement percentage of recycled coarse aggregate, and section form of profile steel, were considered. The results showed that increasing the recycled coarse aggregate replacement percentage led to a decrease in ductility and energy dissipation capacity but an initial reduction and subsequent increase in bearing capacity and stiffness. Increasing the wall thickness of the circular steel tube and profile steel ratio improved seismic performance, while increasing the axial compression ratio had a negative effect. Modified formulas for the horizontal bearing capacity were proposed and verified with the test results. In the same way, cyclic load tests were performed on nine steel-reinforced RAC-filled square steel tube composite columns by [PERSON] et al. (2024) [81]. Factors, such as steel ratio, width thickness ratio, recycled coarse aggregate replacement rate, and axial compression ratio were studied. The results showed that columns exhibited a compression-bending failure mode and good energy dissipation capacity.