## 5 Conclusions  
Quasi-static tests are performed for each specimen under identical axial forces. Based on the test results, the failure mode, hysteresis characteristics, bearing capacity, stiffness degradation, ductility, and energy dissipation of each specimen are analyzed. Additionally, a finite element model is created to verify the test results. The conclusions of the study are as follows:  
(1) When the lateral drift reaches 1/25, the bearing capacity of specimen CFST-N declines rapidly. At the end of loading, the bearing capacity decreases to 62.8% of the peak load. The column root has the most severe buckling deformation. Specim CFST-MC has the fullest hysteresis curve, in which the skeleton curve of the declining section is gentle and the seismic performance is stable. This is mainly  
Figure 19: Min principal stress contour plot of concrete. (**a**) CFST-N; (**b**) CFST-HS; (**c**) CFST-VS; (**d**) CFST-SR; (**e**) CFST-DT; and (**f**) CFST-MC. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimens No.** & \(F_{test}\) **(kN)** & \(F_{simlate}\) **(kN)** & \(F_{simlate}\)/\(F_{test}\) \\ \hline
**CFST-N** & 979.9 & 985.5 & 1.01 \\
**CFST-HS** & 1049.0 & 1068.6 & 1.02 \\
**CFST-VS** & 1151.7 & 1161.9 & 1.01 \\
**CFST-SR** & 1153.0 & 1214.6 & 1.05 \\
**CFST-DT** & 1303.4 & 1312.4 & 1.01 \\
**CFST-MC** & 1422.5 & 1447.6 & 1.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Comparison between tested and simulated values of bearing capacity. because with the increase of section steel ratio, the constraint effect on core concrete is continuously improved, and the stability of members is significantly improved. (2) The horizontal stiffener has a significant constraining effect on the steel tube column surface external deformation, and it enhances the confining effect on concrete. The vertical stiffener improves the section bending resistance capacity and initial stiffness. The core structure rebar has a small impact on the bearing capacity, but it effectively improves the structure ductility. The embedded circular steel tube has a substantial constraining effect on concrete in the core area, and it significantly improves the bearing capacity, ductility coefficient, and accumulated energy dissipation of the structure. Specim CFST-DT has the same steel consumption as specimen CFST-SR; however, compared to specimen CFST-SR, the bearing capacity and accumulated energy dissipation at peak load improve by 13.13% and 37%, respectively. This is because the double-skin steel tube increases the contact area between steel tube and concrete. The members can dissipate more energy through bond slip, so the accumulated energy dissipation is significantly increased. Compared to specimen CFST-N, specimen CFST-MC has the following improvements: The section steel ratio increases by 4.42%, and the yield load and peak load improve by 47.25% and 42.72%, respectively. (3) The ductility coefficient of specimen CFST-MC was 24.2% higher than that of specimen CFST-N. When the section steel ratio increases, the ductility coefficient increases, and the stiffness degradation slows significantly. Various constructional measures significantly improve the accumulated energy dissipation when the structure reaches peak load. (4) A finite element model is created to perform a numerical simulation analysis for each specimen. The simulation results and test results match well. It can be seen from the finite element simulation results that the stiffeners can significantly reduce the buckling at the bottom of the column. The bearing capacity of the concrete inside the steel tube is increased greatly due to the constraint effect of the steel tube. [PERSON] performed the experiments and analyzed the data and wrote the paper. [PERSON] proposed the topic of this study. [PERSON], [PERSON], [PERSON], and [PERSON] participated in the manuscript preparation. This work was supported by National Key Research and Development Plan of China (2017 YFC 0703304) and National Natural Science Foundation of China grant funded by the Chinese government. (No.