## 2 Experimental Programs
### Test Device and Loading Procedure  
The quasi-static test is performed with a 4000 t large-scale test device from the Beijing University of Technology. This device has a 4000 t vertical loading capability and a 400 t horizontal cyclic loading capability. The specimen is fixed on a test bed that can move horizontally. The horizontal loading device pushes the base to achieve horizontal cyclic loading. The test loading direction is South-North. The top of the test device is a spherical hinge; the vertical loading system applies a load to the top end of the specimen and restricts the top end of the specimen from horizontal displacement. A diagram of the test loading device is shown in Figure 4, and a field image is shown in Figure 5a. In the test, draw-wire displacement meters one and two are deployed on the ground and test bed to measure the base horizontal displacement and relative slip between the base and test bed. The test results show that the base is steadily fixed and that the relative slip between the specimen and test bed can be ignored.  
When the test starts, axial pressure is applied to the specimen until it reaches a predefined value, which then remains stable during horizontal cyclic loading. Then, the horizontal actuator pushes the base, fixed on a scooter, to apply a horizontal cyclic force to the specimen. The six specimens are under an identical axial pressure of 9180 kN. The axial load ratio n of each specimen is calculated via Formula (1). In this formula, the steel tube constraint effect on the core concrete is considered [21]; N is the specimen vertical axial force; \(A_{\mathit{stiff}}\) and \(A_{\mathit{rebar}}\) are the total areas of the vertical stiffener and longitudinal rebar, respectively; and \(f_{\mathit{stiff}}\) and \(f_{\mathit{rebar}}\) are the yield strengths of the vertical stiffener and longitudinal rebar, respectively. The constraint effect coefficient is \(\theta=A_{\mathit{sfy}}f_{\mathit{y}}/A_{\mathit{cf}}c_{\mathit{m}}\), where \(A_{\mathit{s}}\) and \(A_{\mathit{c}}\) are the areas of the steel tube and core concrete, respectively, and \(f_{\mathit{y}}\) is the steel tube yield strength.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Property Parameters**} & \multicolumn{2}{c}{**Steel Tubes**} & \multicolumn{2}{c}{**Stiffeners**} & \multicolumn{2}{c}{**Rebar**} \\ \cline{2-6}  & **T = 9 mm** & **T = 6 mm** & **T = 4 mm** & **D = 8 mm** & **D = 6 mm** \\ \hline \(f_{y}\) **(MPa)** & 328 & 343 & 358 & 716 & 608 \\ \(f_{u}\) **(MPa)** & 462 & 445 & 476 & 966 & 870 \\ \(\delta\) **(\%)** & 23.4 & 25.3 & 14.6 & 15.9 & 16.7 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mechanical properties of steel.  
Figure 4: Test setup. Note: **1**. Specim; **2**. M48 bolts; **3**. Test pedestal; **4**. Test car; **5**. Rail; **6**. Load transducer; **7**. Spherical hinge; **8**. Beam; **9**. LVDT-1; **10**. LVDT-2; **11**. Reaction frame; **12**. Balance weight; and **13**. Wire rope.  
\[n=N/\left[0.9f_{c,m}A_{c}(1+1.8\theta)+f_{stiff}A_{stiff}+f_{rebar}A_{rebar}\right] \tag{1}\]  
The structure steel ratio is defined as \(\rho=V_{s}/V_{c}\), where \(V_{s}\) is the overall steel volume of the CFST column and \(V_{c}\) is the volume of concrete. The steel ratio and axial load ratio of each specimen are listed in Table 3. The test is based on a displacement loading procedure [22]. When the lateral drifts are 1/1000, 1/500, 1/300, and 1/250, cyclic loading is applied once. When the lateral drifts are 1/200, 1/150, 1/100, 1/75, 1/50, 1/33, 1/25, 1/20, and 1/16.7, cyclic loading is applied twice. The loading procedure is shown in Figure 5b.