## 4 Finite Element Simulation Results
### Study on the Influence of Horizontal Spacing  
Under the combined effects of fire and wind loads, the uneven distribution of horizontal spacing between the crossarms affects the stability of the transmission tower-line system. To study the impact of uneven horizontal spacing distribution on the transmission tower, three tower-line models with horizontal spacings of 300 m (uniform distribution), 250 m (uneven distribution), and 200 m (uneven distribution) are established, as shown in Figure 13.  
Under ambient temperature conditions, a comparison of the collapse-resisting wind speeds of the transmission tower-line system with horizontal spacings of 300 m (uniform distribution), 250 m (uneven distribution), and 200 m (uneven distribution) is conducted, as shown in Figure 14 and Table 4. From the figures and tables, it can be observed that the uneven distribution of horizontal spacing reduces the stability of the transmission tower-line  
Figure 11: Collapse process of the transmission tower under 600 \({}^{\circ}\)C conditions. (stress in Pa). Figure 12: Time-history diagram of stress under different temperature conditions. Figure 13: Three-tower four-line model. system. The more uneven the distribution of the horizontal spacing, the lower the stability of the transmission tower-line system. As the temperature increases, the adverse effects of the uneven horizontal spacing distribution on the transmission tower become more significant. Compared to the 300 m (uniform distribution) condition, the collapse-resisting wind speed of the observed members at 250 m (uneven distribution) significantly decreases. This is because the uneven distribution of the spacing causes uneven loading on the tower, further exacerbating stress concentration within the structure and reducing the stability of the transmission tower-line system. Under the 200 m (uneven distribution) condition, the collapse-resisting wind speed of the observed members further decreases. This is because the more uneven the horizontal spacing distribution, the more uneven the load on the transmission tower, which further intensifies the stress concentration. Under the 250 m (uneven distribution) condition, compared to the 300 m (uniform distribution) condition, the collapse-resisting wind speed of the observed members at ambient temperature, 400 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C decreases by 7.40%, 7.69%, and 16.67%, respectively. Under the 200 m (un-even distribution) condition, compared to the 300 m (uniform distribution) condition, the collapse-resisting wind speed of the observed members at ambient temperature, 400 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C decreases by 11.11%, 15.38%, and 25.00%, respectively. This is because high temperatures not only reduce the yield strength of the steel but also exacerbate the stress concentration caused by the uneven horizontal spacing distribution. The collapse processes of the transmission tower-line system with horizontal spacings of 300 m (uniform distribution), 250 m (uneven distribution), and 200 m (uneven distribution) are shown in Figures 8, 15 and 16, and Table 5. The results indicate that the uneven distribution of horizontal spacing accelerates the collapse process of the transmission tower-line system, causing it to lose stability in a shorter period. Moreover, the greater the degree of unevenness in the horizontal spacing, the more pronounced this trend of accelerated instability becomes. When the horizontal spacing is 300 m (uniform distribution), 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 reduction in tower rigidity, ultimately causing overall collapse. When the horizontal spacing is 250 m (uneven distribution), the instability of observed member 1