## 2 Beam-Column Joints of Composite Frames Under Seismic Loads
### Beam-Column Joints of Composite Frames Under Seismic Loads: State-of-the-Art
The model underwent validation through comparison with experimental data, successfully depicting both global and local behaviors, stress distributions, and failure mechanisms. This was achieved by incorporating the Gurson model for steel, accounting for concrete behavior and damage, and ensuring an accurate representation of connections. The validated model can be used for extensive parametric studies and can lead to suggested improvements to Eurocode 8 design provisions. Similarly, [PERSON] et al. (2017) [14] investigated the performance of special-shaped CFT columns--steel beam connections. Four specimens were tested with different shapes and joint stiffeners. Different failure modes were observed, and it was found that exterior diaphragms performed better than vertical ribs in transferring forces. The study also developed finite element models and proposed design formulas based on internal force transfer mechanisms. The findings suggest that exterior diaphragm connections are suitable for engineering applications, while vertical rib connections require improvement. [PERSON] et al. (2021) [15] investigated concrete-encased CFST columns/steel beam joints using experimental tests. An RC slab was installed for each joint, and an external diaphragm design was used to connect the beam flange and the CFST tube. The joints were subjected to constant axial load and reverse cyclic loading to provide test data for composite joints, evaluate their seismic performance, and analyze the effects of key parameters, specifically the joint type. Additionally, [PERSON] et al. (2020) [16] designed three specimens of joints using an integrated combination scheme and tested them with low-cycle reciprocating loading. The results analyzed the strength degradation, stiffness degradation, failure modes, energy dissipation, and ductility, clarifying the effect of the beam-to-column stiffness ratio on seismic performance of modular composite joints. [PERSON] et al. (2016) [17] executed an experimental program on two blind-bolted composite joints to CFTST columns in full-scale under short cyclic loading to understand their seismic performance and failure modes. The results were used to assess various aspects, like hysteresis, strength degradation, ductility, and energy dissipation capacity, with implications for enhancing the design of composite joints in practice. Furthermore, [PERSON] et al. (2018) [4] introduced new connections, of a ring-bar-reinforced type, for connecting reinforced concrete slabs, beams, and T-shaped CFST columns. They conducted tests on 34 columns and derived a formula to determine the axial compression bearing capacity. Additionally, an improved composite column was presented, which is more cost-effective and exhibits better seismic behavior. The connection between the T-shaped CFST column and the RC beams is strengthened by the new ring-bar-reinforced connection, enhancing the stiffness and bearing capacity. The concept and benefits of an innovative prefabricated composite column-beam joint, combining concrete-filled steel tubular (CFST) columns with reinforced concrete (RC) beams, were first presented by [PERSON] and [PERSON] (2021) [18]. To evaluate the earthquake resistance of these connections, which employ bolts and non-adherent post-stressed cables, the researchers conducted alternating load experiments on eight reduced-scale prototypes (one-third of full size). The study examined how various factors, including the vertical load ratio, bolt size, steel tube thickness, and overall structural configuration, influenced the seismic response characteristics of these joints. The resulting data were thoroughly analyzedand interpreted to draw meaningful conclusions about the connections' performance under simulated earthquake conditions. A novel joining technique for linking a steel-concrete composite column (concrete-encased CFST) with a metallic beam was introduced by [PERSON] et al. (2020) [19]. This method utilizes concealed fasteners.