## 2 A Typical Emulated Connection--Monolithic Precast Joint  
### Force Mechanism  
The mechanical behavior of the monolithic precast joint is closely related to its structural form. The typical form is shown in Figure 4, and its main difference with the RC joint is the existence of the interface between new and old concretes. In the past, people took this adverse factor into consideration, made the upper and lower end faces of the precast column into rough surfaces, and set shear keys on the beam end faces to ensure effective transfer of shear forces between the new and old concretes. However, the existence of the interface is also extremely unfavorable for bending failure, and it has rarely been considered until now. As shown in Figure 4, under a rightward horizontal force, the lower side of the right beam is in tension, and the interface between the new and old concretes (a potential crack) is easily pulled off, causing the stress concentration of the longitudinal reinforcement at this interface. With the increase of deformation, the concrete of column protective layer spalls off, and the strain of the rebar penetrates into the core area. The diagonal compression truss (shaded area) formed by the compression zone of the beam and the column is subjected to increased vertical tension, and diagonal cracks are more easily formed. When the load is reversed (reciprocating seismic load), the lower side of the right beam is under compression again. At this time, due to the cracking and local spalling of the concrete, the restraint at the beam end is weakened, which is more likely to cause concentrated damage. In addition, the existence of the beam seating length reduces the effective width of the core area, thus reducing its shear strength [28].  
Some testing phenomena well confirm the aforementioned force mechanism. Figure 5 shows the general description of the beam rebar strain of joints which conform to the \"strong column weak beam\" principle. During the elastic stage, the strain distribution of the rebar is basically consistent with the  
Figure 4: Force mechanism of the monolithic precast joint.  
bending moment distribution, which reaches the maximum at the beam end and decreases rapidly in the core area. During the plastic stage, the strain distribution of monolithic precast joint is significantly different from that of RC joint. The yield range of the RC joint rebar is generally located at the beam end, and the maximum strain is located slightly away from column surface (the concrete spalling area). The yield range of the monolithic precast joint rebar is located at the beam end and in the core area, and the length is relatively short, while the maximum strain of the rebar is located at the column surface or even inside the core area. For precast joints, it is impossible for the bottom longitudinal bar to pass through the core area, and in order to avoid congestion of the rebars, headed bars are often used. However, due to the existence of beam end seating length and the allowable construction error, the anchoring length of the rebar is limited; coupled with the preference for large-diameter and high-strength rebars in the projects in recent years, it is more difficult to ensure the effective bonding of reinforcements.  
Figure 6 gives the general description of the joint crack distribution. For RC joints, there are fewer diagonal cracks in the core area, the damage is concentrated in the plastic hinge region at the beam end and bending cracks are scattered in a longer range of the beam (generally, there are more reinforcements in the upper part of the beam, so the lower part of the beam suffers more severe damage). For monolithic precast joints, there are more diagonal cracks in the core area, the damage is concentrated in the tension-compression zone at the beam-column junction and the distribution range of the bending cracks is shorter. Therefore, the energy dissipation capacity of monolithic precast joints is usually weaker than that of RC joints.