## 2 Experimental Dataset Construction
### Input Selection and Dataset Compilation  
In addition to the output variable \(N_{u}\) through multiple tests, the model input variables need to be determined for experimental dataset construction. Seven input variables (\(D\),\(t\),\(L\)\(f\)\(y\)\(E\)\(s\)\(f\)\(c\)\(c\)\(c\)) were identified according to different expressions from current codes of GJB 4142-2000 (see Equation (1)), AIJ 1997 (see Equation (2)), AISC-LRFD 1999 (see Equation (3)), and EC4 2004 (see Equation (4)) [37], which are marked in Figure 1 and described in Table 1. Specifically, the first five variables (\(D\),\(t\),\(L\)\(f\)\(y\)\(f\)\(c\)) were selected based on Equations (1) and (2). \(E_{s}\) and \(E_{c}\) were then selected as input variables since both \(\lambda_{c}\) in Equation (3) and \(\chi\) in Equation (4) are related to them. These seven selected variables include geometric, steel, and concrete properties, which can represent most features of CFST specimens of a certain specification. A total of 180 groups of data were ultimately compiled into a complete experimental dataset, and the descriptive statistics of model input and output variables are shown in Table 1.  
\[N_{u}=(1.18+0.85\frac{f_{y}A_{s}}{f_{c}A_{c}})f_{c}A_{sc}, \tag{1}\]  
\[N_{u}=0.85f_{c}A_{c}+f_{y}A_{s}, \tag{2}\]  
\[N_{u}=\left\{\begin{array}{ll}0.658^{\lambda_{c}^{2}}F_{my}A_{s},\;\lambda_{ c}\leq 1.5\\ \frac{0.877F_{my}A_{s}}{\lambda_{c}^{2}},\;\lambda_{c}>1.5\end{array}\right., \tag{3}\]  
\[N_{u}=\chi(f_{c}A_{c}+f_{y}A_{s}), \tag{4}\]  
where \(f_{y}\) is the yield strength of steel, \(f_{c}\) is the compressive strength of the core concrete,\(A_{s}\),\(A_{c}\),\(A_{sc}\) are respectively the cross section area of the steel tubular, core concrete, and CFST, \(\lambda_{c}\) is the relative slenderness ratio, \(F_{my}\) is the modified yield strength of steel, and \(\chi\) is the axial stability factor.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Direction** & **Category** & **Symbol** & **Description** & **Minimum** & **Maximum** & **Mean** \\ \hline \multirow{6}{*}{Input} & Geometric & \(D\) (mm) & Side length of the square section & 100.00 & 323.00 & 152.88 \\  & properties & \(t\) (mm) & Thickness of the steel tubular & 1.44 & 7.47 & 4.13 \\  & & \(L\) (mm) & Length of the specimen & 300.00 & 969.00 & 466.53 \\  & Steel & \(f_{y}\) (MPa) & Yield strength of steel & 198.00 & 835.00 & 340.83 \\  & properties & \(\bar{E}_{c}\) (MPa) & Elasticity modulus of the steel & 180,518.00 & 214,000.00 & 202.273.54 \\  & Concrete & \(f_{c}\) (MPa) & Compressive strength of the core concrete & 10.65 & 91.10 & 45.79 \\  & properties & \(\bar{E}_{c}\) (MPa) & Elasticity modulus of the core concrete & 23,528.00 & 42,600.00 & 30.928.75 \\ \hline \multirow{2}{*}{Output} & Dependent & \multirow{2}{*}{\(N_{u}\) (kN)} & Ultimate bearing capacity of the specimen & \multirow{2}{*}{507.00} & \multirow{2}{*}{5873.00} & \multirow{2}{*}{1978.91} \\  & variable & & under axial compression & & & \\ \hline \end{tabular}
\end{table}
Table 1: Statistics of model input and output variables.  
Figure 1: Schematic diagram of the axial compression test device for square concrete-filled steel tubular (CFST) short columns: (**a**) front view; (**b**) section view of the specimen.