## 3 Test Results and Analysis
### Ductility and Energy Dissipation  
The displacement ductility coefficient \(\mu\) is calculated using \(\mu=\Delta_{u}/\Delta_{y}\), where \(\Delta_{u}\) is the displacement when the bearing capacity decreases to 85% of the peak load, and \(\Delta_{y}\) is the displacement corresponding to the nominal yield load. The ductility coefficient is based on the average of values in the push and pull directions. When there are two loading cycles, the average of the ductility coefficients in the two cycles is used. The ductility coefficient of each specimen is listed in Table 4. Specim CFST-N has the smallest ductility coefficient; however, it reaches 3.64. The average ductility coefficient of 6 specimens is 4.16, which means that the circular CFST column still has desirable deformation performance under a high axial load ratio. The results show that when the axial load ratio decreases, the ductility coefficient increases. It is also worth noting that when loading stops, the bearing capacities of specimens CFST-SR and CFST-MC are still above 85% of the peak load. Therefore, when calculating \(\Delta_{u}\), the horizontal displacement values of the two specimens during the final loading cycle are used, and the ductility coefficient is actually larger than the calculated value in the table. A comparison of specimen CFST-SR and specimen CFST-VS shows that the deployment of core structure rebar in  
Figure 11: Stiffness degradation. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\(\theta_{d}\) (rad)} & \multicolumn{6}{c}{**K (10\({}^{4}\) kN/m)**} \\ \cline{2-7}  & **CFST-N** & **CFST-HS** & **CFST-VS** & **CFST-SR** & **CFST-DT** & **CFST-MC** \\ \hline
1/250 & 91.38 & 95.66 & 114.34 & 121.68 & 125.66 & 120.74 \\
1/200 & 80.49 & 76.44 & 83.25 & 96.71 & 107.64 & 120.51 \\
1/150 & 63.86 & 64.19 & 69.27 & 79.27 & 89.31 & 95.16 \\
1/100 & 48.86 & 50.12 & 53.61 & 59.40 & 66.06 & 71.55 \\
1/75 & 38.10 & 41.54 & 41.87 & 48.54 & 54.42 & 57.21 \\
1/50 & 26.22 & 29.92 & 28.35 & 32.65 & 38.45 & 43.18 \\
1/33 & 14.77 & 19.52 & 19.23 & 20.19 & 24.72 & 28.01 \\
1/25 & 7.47 & 13.41 & 13.27 & 14.74 & 16.33 & 19.76 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Secant rigidity at each drift level. the steel tube effectively improves the ductility of the specimen. This is because cement hydration heat produces significant thermal stress in concrete and cracks on the external surface of the concrete, which weakens the bonding between the steel tube and the concrete. Additionally, concrete contraction deformation reduces bonding. Previous research [8] has shown that strong bonding between the steel tube and the concrete results in superior structure ductility. The core structure rebar functions as thermal rebar, which enhances bonding between the steel tube and concrete, thus improving the ductility of the structure. Deployment of an internal circular steel tube improves the structure ductility coefficient by 18.96%. Table 4 shows that the elastoplastic lateral drift of each specimen always exceeds 1/31, which means that the structure has excellent plastic deformation capability. Energy dissipation capability is another important index to evaluate the seismic performance of structures. In this paper, accumulated energy dissipation is used as an evaluation index. A CFST structure relies on steel tube plastic deformation, concrete micro-fracture development, and a bonding slip between the steel tube and concrete to dissipate absorbed energy. The method to calculate accumulated energy dissipation is to calculate the sum of areas surrounded by horizontal force-displacement hysteresis curves that correspond to previous grades of lateral drifts when the specimen reaches a certain grade of lateral drift. In the formula, \(\mathrm{E_{P}}\) is the accumulated energy dissipation when each specimen reaches its peak load, and \(\mathrm{E_{t}}\) is the accumulated energy dissipation when the loading on each specimen ends.