## 3 Results and Discussion
### Tensile Strength  
The tension load-displacement curves of each specimen, as shown in Figure 6, exhibit similar trajectories for specimens with the same type of connection joints. The specimens demonstrate low dispersion, remaining primarily in the elastic phase during the early stages of loading. Moreover, during this stage, as indicated in Figure 6c, it can be observed that even for specimens with different types of connection joints, their tensile stiffness is essentially similar. As the load increases, the specimens enter the yielding stage, and stiffness gradually decreases. For welded joint specimens, this stage begins at around 500 kN of load, similar to the trend observed in the improved bolted joint specimens. However, for normal bolted joint specimens, this stage is initiated at around 460 kN of load. When the stiffness decreases to a certain extent, it begins to stabilize until the steel bars of the specimen fracture. The variation in displacement at fracture among specimens with the same type of connection is attributed to the presence of different defects in the prestressing steel bars during the fabrication of the bar heads. Because the pre-stressing steel pier head is in the steel production is completed after the secondary processing production, so may be left in the pier head processing defects, and the pier head and easy to connect with the end plate uneven force, which is also easy to occur in the actual project where the problem. At the point of failure, the displacement of bolted joint specimens is consistently greater than that of welded joint specimens, indicating better ductility. However, the ultimate load capacity of bolted joint specimens is slightly lower than that of welded joint specimens. In comparison to normal bolted joint specimens, the curves of improved bolted joint specimens show little difference in the early stages of loading. However, after entering the yielding stage, the curves of improved bolted joint specimens surpass those of normal bolted joints.  
Table 3 provides the experimental results (\(N^{\text{t}}\)) for the ultimate tensile bearing capacity of each joint specimen, as well as the design value (\(N\)) calculated according to the empirical formula specified in the regulations [23], the formula is shown in equation 1. The ultimate tensile bearing capacity of both types of joint specimens exceeds the design value for the pile's axial tensile bearing capacity and is generally greater than 30% above it. Additionally, the ultimate tensile bearing capacity of the improved bolted joint specimens exceeds that of the normal bolted joint specimens. Furthermore, throughout the testing process, no failure occurred in the joint connections, indicating that the ultimate tensile bearing capacity of both types of joints meets the design requirements for pile connections.  
\[N\leq Cf_{\text{py}}A_{\text{P}} \tag{1}\]  
where: \(C\) represents the discount coefficient considering the influence of uneven force at the connection between the pier head and end plate of prestressing reinforcement, and 0.85 is taken here; \(f_{\text{py}}\) represents the design value of tensile strength of prestressing  
Figure 6: Load–displacement curve of test specimen.  
reinforcement, which is 1000 MPa; \(A_{\rm p}\) represents the cross-sectional area of all longitudinal prestressing reinforcement.