## 4 Finite Element Simulation Results
### Study on the Influence of Elevation  
Under the combined effects of fire and wind loads, the elevation of the observation tower affects the stability of the transmission tower-line system. To investigate the impact of changes in the observation tower elevation on the transmission tower, three tower-line models are established with tower base heights of 0 m and 50 m, as shown in Figure 18.  
Under ambient temperature conditions, a comparison is made between the collapse-resisting wind speeds of the transmission tower-line systems with tower base heights of 0 m and 50 m, as shown in Figure 19 and Table 6. From the figures and tables, it can be observed that the increase in tower height reduces the stability of the transmission tower-line system. As the temperature increases, the detrimental effects of a higher observation tower elevation on the transmission tower become more pronounced. Compared to the condition with a tower base height of 0 m, the collapse-resisting wind speed of the observation members significantly decreases when the tower base height is 50 m. This is because as the observation tower height increases, the wind load on the transmission tower also increases, leading to higher stresses in the transmission tower-line system. Moreover, the increased observation tower height also increases the flexibility of the transmission tower-line system, making it more prone to lateral displacement under wind load. When the tower base height is 50 m, compared to the 0 m base height, the collapse-resisting wind speed of the observation members decreases by 18.52%, 23.08%, and 33.33% under ambient temperature, 400 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C, respectively. This is because high temperatures not only reduce the yield strength of the steel but also make the response of the transmission tower-line system more significant under greater wind loads. The collapse process of the transmission tower-line systems with tower base heights of 0 m and 50 m is shown in Figures 8 and 20 and Table 7. The results indicate that an increase in observation tower elevation accelerates the collapse process of the transmission tower-line system, causing it to lose stability in a shorter time. When the tower base height is 0 m, observed member 1 fails first at 3.30 s, losing its load-bearing capacity. Subsequently, between 3.30 s and 3.55 s, observed members 2, 3, and 4 fail in sequence. Observed members 5, 6, 7, and 8 remain intact. The failure of the central and lateral support members leads to a significant decrease in tower rigidity, ultimately resulting in overall collapse. When the tower base height is 50 m, the failure time of observed member 1 is advanced to 2.75 s, and within the subsequent 0.10 s, observed members 2, 3, and 4 fail in sequence. Observed members 5, 6, 7, and 8 remain intact. The tower loses its load-bearing capacity and collapses. The stress-time history of the observation members in the transmission tower-line systems with tower base heights of 0 m and 50 m under fire-wind load conditions is shown in Figure 21.  
model of the transmission tower-line system was established using ABAQUS software to analyze its response under the combined effects of fire and wind loads. The paper studied the effects of temperature, uneven horizontal spacing distribution, and changes in the elevation of the target tower on the response of the transmission tower-line system by comparing collapse-resisting wind speeds and collapse processes under various conditions. The findings of the study are as follows:  
1. The uneven horizontal spacing distribution reduces the stability of the transmission tower-line system. Compared to the condition with a horizontal spacing of 300 m (uniform distribution), the collapse-resisting wind speeds of the observed members are significantly reduced when the horizontal spacing is 250 m (uneven distribution). The more uneven the distribution of the horizontal spacing, the lower the stability of the transmission tower-line system.