# Fragility Analysis of Transmission Towers Subjected to Downburst Winds
## 4 Summary and Conclusions  
A novel framework for the fragility analysis of the transmission tower under the downburst is proposed. The framework will improve the wind-resistance design and wind-disaster prediction of the transmission tower. The wind-induced fragility of three types of transmission towers is analyzed under the effects of a downburst. Fragility analysis results of the transmission tower will vary with tower height and tower type. The main contents of this paper are as follows. The fragility curves of the transmission tower under the downburst and the ABL wind are compared considering the uncertainty  
Figure 28: Failure probability of transmission tower under full wind speed. Figure 29: Average fragility curve of transmission tower under different basic wind speeds. of resistance and the randomness of wind load effect, and the uncertain collapse of the transmission tower under the downburst is carried out. The position and corresponding probabilities of the initial buckling members of the transmission tower under the downburst are determined. Combined with the probability of wind speed and direction data of a region, the failure probability of the downburst attacking the transmission line causing the collapse of the transmission tower at different directions is calculated, and the optimal layout of transmission towers under a given wind field is studied. The innovation points and specific contributions of the current study are given as follows: (1) The most unfavorable radial distance from the center of the downburst to the tower, as well as the most unfavorable wind attack angles, is analyzed by the parameter analysis of the fragility curves of the tower. It can be concluded that the location where the maximum response occurs is \(R=1.6D\), and the range of radial distance for slight damage status is wider than that for severe damage in all direction angles--the range far greater than the radial distance of collapse--because during the attacking process, with the increase in the radial distance, the time-varying mean wind speed of the downburst increases first and then decreases. (2) Because almost half of the wind load on the tower comes from large-span transmission lines, the wind load effect on the tower varies significantly in different wind directions. On the other hand, the ultimate wind resistance capacity of the tower is also different depending on wind attack angles. Both of the reasons lead to the significant variation in the fragility curves at different angles. (3) Uncertain collapse of the transmission tower under the downburst is carried out by nonlinear buckling analysis to indicate the failure mode and the most vulnerable main members. It provides some guidance for the maintenance and retrofit of the transmission tower under downburst wind load. It can be found that most of the initial buckling members are located near the first section of the transmission tower. The initial failure member is the main member, indicating that the main failure mode of the transmission tower belongs to bending failure. The main member is a compression bending component, which is the main component of the wind resistance of the transmission tower, while the diagonal member is an axial compression component, which is an auxiliary component of the wind resistance of the transmission tower. The instability of the diagonal member will not cause the collapse of the tower structure but will significantly decrease the stability of the main member. (4) The fragility curves of the tower under the downburst are more dangerous than those under the ABL wind with the attack angle increasing from 0\({}^{\circ}\) to 90\({}^{\circ}\). Compared with the ABL wind, the downburst, characterized by localization and high intensity, can often greatly exceed the ABL wind and cause the collapse of the transmission tower. In addition, the maximum average wind speed of the downburst first increases along the height, then decreases rapidly, and the maximum value occurs within the height range of the transmission tower structure.