## 4 Analysis of Influential Factors
### Crack Load  
The variations in the ratio (\(k_{1}(T)\)) of the crack load to the peak load of the composite columns subjected to different fire exposure are shown in Figure 7.  
It can be seen from Figure 7a that the crack load of specimens decreased with the increase in the heating temperature, which declined more slowly within the range of 200 \({}^{\circ}\)C \(\leq T\leq\) 400 \({}^{\circ}\)C and then fell rapidly. Compared with the ambient temperature, the crack loads of the specimens at \(T\) = 700 \({}^{\circ}\)C and 800 \({}^{\circ}\)C decreased by 73.43% and 79.33%, respectively. The \(k_{1}(T)\)-value of the specimen at ambient temperature was 82%, while that of the specimen at \(T\) = 800 \({}^{\circ}\)C was only 33%. It can be seen that the \(k_{1}(T)\)-value was more reduced by the increase in heating temperature, indicating that the higher the heating temperature was, the earlier the surface crack of the specimen appeared.  
Figure 7b reflects that the diameter of the steel tube had no significant influence on the \(k_{1}(T)\)-value at ambient temperature, which was stable at about 82%. At high temperature (\(T\) = 600 \({}^{\circ}\)C), the \(k_{1}(T)\)-value uniformly increased with increases in steel tube diameter and concrete cover thickness, but the increase was small with the increase in steel tube diameter.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Number** & **Parameter** & \(d\times\)**\(\delta\)/mm** & \(a\)/mm** & \(T^{\prime\circ}\)C & \(N_{\text{cr}}\)**(T)/kN** & \(N_{\text{u}}\)**(T)/kN** & \(N_{\text{cr}}\)**(T)/\(N_{\text{u}}\)**(T)** & \(EA_{0}\)**(T)/MN** & \(\mu\)**(T)** \\ \hline ZA-1 & \(T\) & \(114\times 3.5\) & 20 & 20 & 2010.0 & 2451.0 & 0.820 & 720.65 & 2.21 \\ ZA-1A & \(T\) and \(\kappa\) & \(89\times 3.8\) & 20 & 20 & 1920.5 & 2341.5 & 0.820 & 692.52 & 2.08 \\ ZA-1B & \(T\) and \(\kappa\) & \(138\times 3.0\) & 20 & 20 & 2077.0 & 2524.5 & 0.823 & 739.82 & 2.34 \\ ZA-2 & \(T\) & \(114\times 3.5\) & 20 & 200 & 1522.0 & 2056.5 & 0.740 & 675.85 & 2.12 \\ ZA-3 & \(T\) & \(114\times 3.5\) & 20 & 300 & 1454.5 & 2049.0 & 0.710 & 632.22 & 1.84 \\ ZA-4 & \(T\) & \(114\times 3.5\) & 20 & 400 & 1385.5 & 2027.5 & 0.683 & 570.47 & 1.55 \\ ZA-5 & \(T\) & \(114\times 3.5\) & 20 & 500 & 1136.0 & 1775.5 & 0.640 & 515.94 & 1.92 \\ ZA-6 & \(T\) & \(114\times 3.5\) & 20 & 600 & 876.0 & 1653.0 & 0.530 & 439.56 & 2.13 \\ ZA-6A & \(T\) and \(\kappa\) & \(89\times 3.8\) & 20 & 600 & 855.0 & 1628.0 & 0.525 & 421.38 & 1.73 \\ ZA-6B & \(T\) and \(\kappa\) & \(138\times 3.0\) & 20 & 600 & 958.5 & 1682.0 & 0.570 & 453.29 & 2.21 \\ ZA-6-10 & \(T\) and \(a\) & \(114\times 3.5\) & 10 & 600 & 841.0 & 1790.0 & 0.470 & 414.00 & 1.58 \\ ZA-6-30 & \(T\) and \(a\) & \(114\times 3.5\) & 30 & 600 & 946.5 & 1578.5 & 0.600 & 442.81 & 2.98 \\ ZA-7 & \(T\) & \(114\times 3.5\) & 20 & 700 & 534.0 & 1403.5 & 0.380 & 345.91 & 2.49 \\ ZA-8 & \(T\) & \(114\times 3.5\) & 20 & 800 & 415.5 & 1256.0 & 0.331 & 298.35 & 2.61 \\ \hline \end{tabular}
\end{table}
Table 5: Measured characteristic point parameters of specimens.  
Figure 7: The variations in the ratio of the crack load to the peak load of specimens. (**a**) Influence of \(T\). (**b**) Influence of \(d\) and \(a\).