## 6 Further Research on Hybrid Connection
### Various Energy Dissipators in the Literature for Hybrid Frame Connections  
Several attempts have been made to improve the energy dissipation of the hybrid connection to reduce the displacement demands. To address the large displacement that may be expected during earthquakes for certain frames with hybrid connections, [PERSON] et al., 2004 [14] developed a friction damper for hybrid connections. They reported that a large amount of energy dissipation was provided by friction dampers while maintaining the self-centering capability of the frame. Dampers can be replaced after an earthquake event. The dampers are visible and may interfere with the architectural appearance of the structure. [PERSON] et al., 2015 [47] performed experiments using hybrid frames (Figure 24) to study the behavior of frames under cyclic loading. A friction device was used instead of mild steel bars. The viscous damping achieved by this system was 11%, whereas the code requirement was 12.5%. The proposed frame confirmed its self-centering capability. Due to the type of dissipators used, the crushing of concrete at the column-beam interface was greatly reduced. This indicated that research must be done on different types of energy dissipators to obtain potential benefits and improvements in the hybrid system. [PERSON] et al., 2018 [48,49] proposed a replaceable energy-dissipating bar placed at the sides of the beam, as shown in Figure 25. This led to no interference with the slab connection. The mild steel bars can be easily replaced after an earthquake event. The main drawback is the performance of the connection under fire load, as the bars are exposed and may require fire protection. Figure 24: Frame drift [47]. [PERSON] et al., 2019 [50] proposed a replaceable energy-dissipating bar placed at the top of the beam (Figure 26). The connection performed well in terms of strength, ductility, and energy dissipation. However, it may not perform well under fire loads since it is exposed, and the connection may interfere with slab connection. [PERSON] et al., 2020 [24] proposed a new hybrid connection with a damper, as shown in Figure 27. The connection performed well with regard to seismic loads. The energy dissipators were placed outside the beam on the sides. Figure 26: Proposed precast post-tensioned beam–column connection [50]. Figure 27: Proposed precast post-tensioned beam–column connection [49]. [PERSON] et al., 2020 [51] tested unbonded post-tensioned precast concrete frames with no mild steel for a sudden column removal scenario. The test indicated that the frame achieved required a load redistribution capacity to mitigate progressive collapse. Column removal duration in the test was 0.008 s and 0.005 s, satisfying the DoD requirement that the column removal duration should be less than 1/10 natural period of vibration. Weights were attached to the beams, which acted as gravity loads. The test setup accurately replicated the sudden column removal scenario due to blast load. However, the flooring system was not considered in the test. [PERSON] et al., 2021 [21] performed a test on a self-centering single-bay two-story hybrid frame. This consisted of an RC column with unbonded PT steel bars and steel concrete hybrid beams with four unbonded PT strands. Self-centering was provided by PT steel, and energy dissipation was provided by web friction devices at beam ends. Two frames with PT steel and one frame without PT steel were examined. The PT frames performed better than the non-PT frame in terms of bearing capacity, ductility ratio, and self-centering capacity, with values 88%, 62%, and 272% higher, respectively. Energy dissipation was better in PT frames as compared to the non-PT frame specimen. The beam column connection in the proposed frame contains too many components, including column plates, steel I sections, friction devices, and beam plates. The connection is complicated compared to the bonded mild steel connection.