## 3 Test Results and Analysis
### Bond Load-Slip Curve Analysis
The British code [28] and the Australian code [29] suggest that the bond strength of round steel-tube concrete be taken to be 0.4 MPa, and the U.S. [30] code suggests that the value of the bond strength of round steel-tube concrete be taken to be min (5300 t/D2,1.4 N/mm\({}^{2}\)). From the bond strength of the speci-mens in Table 6. It can be seen that the bond strength of all specimens is much larger than the reference value of the existing specification, which meets the needs of the project. Compared with the calculated values of the bond strength formulae in the existing literature, all of the specimen test values were greater than the calculated values. This shows that the construction measures of internally welded reinforcement rings are conducive to increasing the bond strength of the members, as shown in Table 7.  
As can be seen from the analysis of Figure 5, the longitudinal strain of the specimen's steel tube was negative, indicating that the steel tube was in a pressurized state during the push-out test. At the beginning of loading, the load is small, and the longitudinal strain of the steel tube changes along the length direction in a non-obvious manner. With increasing load, the different positions of the steel tube's wall strain exhibited large differences, and the strain at the measured end was much greater than that at the loading end. The load is transferred to the steel tube through the action of bond stress: the faster the change in the strain value, the faster the interface force is transferred here; the greater the bond stress, the closer the specimen is to the measured end; and the faster the change in the strain value, the greater the interface bond stress at the measured end. The longitudinal strain value of the \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Specimen** & \(P_{s}\) & \(P_{u}\) & \(P_{r}\) & \(S_{s}\) & \(S_{u}\) & \(S_{r}\) & \(\tau_{s}\) & \(\tau_{u}\) & \(\tau_{r}\) \\
**Number** & **kN** & **kN** & **kN** & **mm** & **mm** & **mm** & **MPa** & **MPa** & **MPa** \\ \hline ZU\({}_{0}\)N\({}_{1}\)W & 835.62 & 1373.51 & 1267.58 & 3.5 & 25.5 & 40.0 & 2.41 & 3.96 & 3.65 \\ ZU\({}_{0.25}\)N\({}_{0.75}\)Y & 1577.74 & 2145.77 & 1986.29 & 5.5 & 12.5 & 40.0 & 4.54 & 6.18 & 5.72 \\ ZU\({}_{0.25}\)N\({}_{0.75}\)E & 1963.42 & 2857.48 & 2825.57 & 5.0 & 35.5 & 40.0 & 5.66 & 8.23 & 8.14 \\ ZU\({}_{0.5}\)N\({}_{0.5}\)Y & 2169.07 & 2154.99 & 2102.97 & 7.0 & 7.5 & 40.0 & 6.25 & 6.21 & 6.06 \\ ZU\({}_{0.5}\)N\({}_{0.5}\)E & 2059.42 & 2900.62 & 2887.81 & 6.5 & 22.5 & 40.0 & 5.93 & 8.36 & 8.32 \\ ZU\({}_{1}\)N\({}_{0}\)W & 920.57 & 1665.81 & 1550.22 & 5.0 & 28.5 & 40.0 & 2.65 & 4.80 & 4.47 \\ JU\({}_{1}\)N\({}_{0}\)W & 686.87 & 1586.89 & 1420.18 & 3.5 & 30.5 & 40.0 & 1.98 & 4.57 & 4.09 \\ JU\({}_{0.5}\)N\({}_{0.5}\)E & 1859.32 & 2713.33 & 2687.06 & 6.0 & 36.0 & 40.0 & 5.36 & 7.82 & 7.74 \\ \hline \hline \end{tabular} Note: The average bond \(\tau\) is the ratio of the load \(P\) to the area A of the steel tube–concrete interface; \(\tau_{u}\), \(\tau_{u}\), and \(\tau_{r}\), are the nominal average bond strengths corresponding to \(P_{s}\), \(P_{u}\), and \(P_{r}\), respectively.  
\end{table} 
Table 6: Specim \(P\)-\(S\) curve eigenvalues. steel tube increases with increasing load, and the strain value at each measurement point also increases more uniformly. The overall distribution of the longitudinal strain value of the steel tube shows a triangular shape, with a larger peak at the bottom and a smaller peak at the top, which indicates that the bond effect on the steel tube is uniformly distributed from the loading end to the measured end, and that the synergistic effect between each reinforcement ring is very good.