## 3 Test Results and Analysis
### Bearing Capability  
Based on the energy equivalence method [23], the nominal yield load of each specimen \(\rm F_{y}\) is calculated from the skeleton curve. The calculation method is shown in Figure 9. Connecting point O to point C under the condition of area (1) equal to area (2) and ensuring the ordinate value of point C is \(\rm F_{p}\). Point B is on the skeleton curve and has the same displacement value as point C. Hence the ordinate value of point B is the nominal yield load of each specimen. \(\rm F_{p}\) is the peak load; the ultimate load is defined as the load when the horizontal force decreases to 85% of the peak load, represented by the symbol \(\rm F_{u}\). Actual measurement results of the specimens are shown in Table 4 and Figure 10. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimens**} & \multicolumn{3}{c}{**Nominal Yield**} & \multicolumn{3}{c}{**Peak**} & \multicolumn{3}{c}{**Ultimate**} \\ \cline{2-11}  & \(\mathbf{F_{y}}\) & \(\Delta_{y}\) & \(\theta_{y}\) & \(\mathbf{F_{p}}\) & \(\Delta_{p}\) & \(\theta_{p}\) & \(\mathbf{F_{u}}\) & \(\Delta_{u}\) & \(\theta_{u}\) & \(F_{y}/\mathbf{F_{p}}\) & \(\mu\) \\ \multicolumn{1}{c}{**No.**} & **(KN)** & **(mm)** & **(rad)** & **(kN)** & **(mm)** & **(rad)** & **(kN)** & **(mm)** & **(rad)** & & \\ \hline
**CFST-N** & 816.3 & 13.50 & 1/113 & 987.9 & 26.35 & 1/58 & 839.8 & 49.10 & 1/31 & 0.826 & 3.64 \\
**CFST-HS** & 858.1 & 15.79 & 1/96 & 1043.9 & 31.46 & 1/48 & 887.4 & 61.42 & 1/25 & 0.822 & 3.89 \\
**CFST-VS** & 970.8 & 13.74 & 1/111 & 1152.9 & 23.60 & 1/65 & 980.0 & 57.09 & 1/26 & 0.842 & 4.16 \\
**CFST-SR** & 983.8 & 14.49 & 1/105 & 1153.5 & 33.49 & 1/46 & 1002.0 & 63.96 & 1/24 & 0.853 & 4.42 \\
**CFST-DT** & 1097.0 & 14.01 & 1/109 & 1304.9 & 25.39 & 1/60 & 1109.1 & 60.68 & 1/25 & 0.841 & 4.33 \\
**CFST-MC** & 1202.0 & 14.20 & 1/107 & 1409.9 & 30.27 & 1/50 & 1224.8 & 64.23 & 1/23 & 0.853 & 4.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measured load and displacement at characteristic points. Figure 8: Skeleton curves of specimens. Figure 9: Diagram of energy equivalent method. Table 4 and Figure 10 show the following: (1) When the steel ratio increases, the yield load, and peak load of each specimen increase accordingly. Compared to CFST-N, specimen CFST-MC shows the following improvements: The section steel ratio increases by 4.42%, and the yield load and peak load improves by 47.25% and 42.72%, respectively. (2) Compared to CFST-N, specimen CFST-HS has the following improvements: The section steel ratio increases by 0.62%, and the yield load and peak load improve by 5.12% and 5.67%, respectively. While the bearing capacity improvement is marginal, the horizontal stiffener has a notable local stabilizing effect on the steel tube column, which delays buckling deformation of the steel tube. The skeleton curve also shows that when the horizontal stiffener is added, the degradation speed in the declining section slows significantly. (3) Compared to specimen CFST-HS, specimen CFST-VS has the following improvements: The section steel ratio increases by 0.53%, and the yield load and peak load improves by 13.13% and 10.44%, respectively. This means that the vertical stiffener specimen significantly improves the bending resistance capacity. (4) The core structure rebar has a small impact on the specimen bearing capacity. (5) The embedded circular steel tube has a significant constraining effect on the core area concrete, which improves the bearing capacity by 32%. (6) Specim CFST-DT has the same steel ratio as specimen CFST-SR; however, the yield load and peak load improve by 11.51% and 13.13%, respectively. (7) Based on specimen CFST-DT, specimen CFST-MC has a cavity welded steel plate between the internal steel tube and external steel tube to further improve the bearing capacity of the structure. The yield load and peak load improve by 9.57% and 8.05%, respectively.