## 2 Experimental Programs  
### Test Specim  
Based on a super high-rise tower building in China, six large-scale circular CFST column specimens are prepared. The scale of the specimen is 1/8. The specimens are named CFST-N (no constructional measures), CFST-HS (horizontal stiffeners), CFST-VS (vertical stiffeners), CFST-SR (stiffeners and rebar), CFST-DT (double-skin tubes), and CFST-MC (multiple cavities). The steel tube parameters are as follows: The exterior diameter is D = 508 mm; the wall thickness is t = 9 mm; D/t = 56.4; and the column clear height is 1270 mm. The base is built of a cube composed of six welded, square steel plates filled with concrete. The base is 600 mm high. An opening (diameter 511 mm) is drilled at the center of the upper steel plate. The steel tube penetrates through the opening and is welded to the floor. One vertical stiffener plate is deployed at each side of the base loading direction. The stiffener plate is welded to the steel pipe, at the floor and roof. The test loading method is as follows: The top of column is fixed, and the lower end is pushed at the base by a horizontal actuator to perform the quasi-static tests. Therefore, to facilitate fixation of the column top, four square steel plates (thickness of 20 mm) are welded around the column top. The gap between the upper and lower surfaces undergoes lofting and is welded with a 20 mm-thick steel plate with a corresponding shape. Consider specimen CFST-MC for example: The geometrical dimensions and structure are shown in Figure 2. Figure 2a is a top view of the specimen; and Figure 2b is a three-dimensional diagram of the specimen cavity structure. Deployment of a cavity welded steel plate between the internal circular steel tube and external circular steel tube solves the hydration heat problem associated with large-volume concrete. A rectangular opening at the cavity steel plate connects the concrete in each cavity and improves the integrity of the structure. Figure 2e shows a three-dimensional diagram of the entire specimen. Figure 3 shows the section structure of each specimen. Specim CFST-N is a steel tube concrete section without any structure. Horizontal stiffeners are welded to the interior wall of the external steel tube in specimen CFST-HS. The rib plate width is b = 40 mm and the thickness is t = 4 mm. The first horizontal stiffener is deployed at 100 mm above the base of the top surface. Then, more are deployed upward every 200 mm, with six total. Horizontal stiffeners and vertical stiffeners are welded on the interior wall of the external steel tube of specimen CFST-VS. The vertical stiffener width is b = 37 mm,  
Figure 2: Dimensions and details of CFST-MC. (**a**) top view of specimen; (**b**) stereogram of cavity structure; (**c**) section 1-1; (**d**) section 2-2; and (**e**) stereogram of whole model. and the thickness is t = 4 mm, which is deployed uniformly at six locations around the perimeter. Specim CFST-SR is a scaled-down section of a prototype in an actual project. Horizontal and vertical stiffeners are deployed, and three layers of rebar cages are deployed in the steel tube. The inner ring longitudinal reinforcement is 6 Deight, the mid-ring longitudinal reinforcement is 14 Deight, and the outer-ring longitudinal reinforcement is 20 Deight. The distances between the longitudinal reinforcement central lines and the section origin are 65 mm, 123 mm, and 180 mm, respectively. The reinforcement ratio is 1.07%, the stirrup is Bsix@120, and the volume stirrup ratio is 0.52%. Specim CFST-DT is a double-skin circular CFST column specimen designed using the same amount of steel for its internal structure as specimen CFST-SR (including horizontal and vertical stiffeners, longitudinal rebar and stirrup). The exterior diameter of the internal circular steel tube is D = 245 mm, and the wall thickness is t = 6 mm. In specimen CFST-MC, a cavity welded steel plate is deployed between the internal steel tube and the external steel tube.