## 2 Methodologies
### Numerical Validation
#### 2.2.2 Results and Analysis  
As shown in Figure 2, the temperature of the section in Model 1 was a ladder-like distribution according to heating position and sectional form. The temperature isograms distributed more densely when the position was closer to the heating surface, and more sparsely when it was far away from the heating surface. The isotherm behaved like a circular arc protruding toward the heating surface. The temperature of the section decreased along the circular arc. The highest temperature at the top of the model was approximately 311.75 K, whereas the lowest temperature was 294.95 K at the bottom of the model.  
Figure 1: Different heating directions of the model.  
Figure 2: Temperature cloudy map and isopleth in Model 1 (unit: K).  
The temperature field of the solid section in Model 2 is illustrated in Figure 3. Unlike the solid Model 1, there was a sudden temperature change in the interface of the void. The isotherm was no longer like a circular arc. The maximum temperature at the top of the section was as high as 327.75 K, but the temperature at the bottom was only approximately 295.05 K, which was almost identical to that in Model 1. Because of the void's existence, the maximum temperature at the top of the model had significantly increased. The highest temperature difference was 16 K higher than in Model 1, and the lowest temperature rose by nearly 0.1 K.  
The temperature distribution of Model 3 was similar to that of Model 2. Both had a sudden temperature change at the interface of the void. As can be seen in Figure 4, with the increase of the void height, the difference in temperature became more evident on both sides of the interface and the temperature at the top of the model increased even more. The maximum temperature at the top was 334.15 K, and the lowest one at the bottom was 295.25 K. The highest temperature when comparing it with Model 1 and Model 2 increased by 22.4 K and 6.4 K, whereas the lowest temperature increased by 0.3 K and 0.2 K, respectively.  
The temperature distribution under different heating angles is shown in Figures 5-8. For comparison, all models were simulated based on a void height of 25 cm. It was observed that although the heating angle had changed, the high temperature area of the concrete-filled steel tube structure was still concentrated in the void section. The temperature distribution on the surface of the model was no longer symmetrical as the heating direction shifted. The temperature of the left side was gradually higher than the right side. The distribution of isotherms in the void area was dense, but the distribution of isotherms in the solid part was sparse, with stepped distribution characteristics. There was also an obvious temperature change at the interface of the voids. Although the temperature difference of the right side was smaller than that of the initial heating direction, it was still possible to identify. As in the case of the 90\({}^{\circ}\) anticlockwise rotation, the sudden temperature change on the right side was the smallest. Furthermore, the whole temperature distribution of the model was completely different from the complete structure.  
Figure 3: Temperature cloudy map and isopleth in Model 2 (unit: K).  
Figure 4: Temperature cloudy map and isopleth in Model 3 (unit: K).  
Figure 5: Temperature cloudy map and isopleth at 30\({}^{\circ}\) (unit: K).  
Figure 6: Temperature cloudy map and isopleth at 45\({}^{\circ}\) (unit: K).