# Fragility Analysis of Transmission Towers Subjected to Downburst Winds
###### Abstract  
A downburst is a typical local highly intensive wind all over the world, which is attributed to be the main cause of wind damage to transmission lines in inland areas worldwide. The collapse accidents of transmission towers under the downburst still occur every year. Therefore, it is of great significance to assess the safety of the transmission towers under downbursts. The motivation of the present study is to propose a fragility assessment method for transmission towers under the action of a downburst considering the uncertainty of wind-resistance capacity and the stochastic wind load effect. First, the downburst wind field of the transmission tower with different wind attack angles and different radial distances is simulated according to the mixed stochastic model. Then, random material characteristic samples are generated by the Latin hypercube sampling technique and applied to establish uncertain finite element models for transmission towers. Next, the static nonlinear buckling analysis is carried out by numerical methods to determine the ultimate capacity under the downburst wind load. The parameter analysis of different wind attack angles and radial distances between the downburst and the tower is conducted to determine the most unfavorable location of the maximum response. The failure mode of the transmission tower and the probabilities of the initial failure main members are summarized. Finally, the fragility curves of the transmission tower under the downburst and the atmospheric boundary layer (ABL) wind are compared. The results show that the maximum response is located at \(R=1.6\)D. Most of the initial buckling members are located close to the first section of the tower. The fragility curves of the tower under the downburst are more dangerous than the ABL wind with the attack angle increasing from \(0^{\circ}\) to \(90^{\circ}\). Furthermore, considering the probability model of intensity and direction of the downburst and based on the previous fragility analysis, the collapse probability of the transmission tower caused by the downburst is obtained. By probability analysis of the parameters, including layout conditions, different directions, and different wind speeds, it is found that the most favorable arrangement is \(157.5^{\circ}\), and the most unfavorable arrangement is \(112.5^{\circ}\). downburst; transmission towers; fragility; nonlinear buckling analysis; atmospheric boundary layer wind +
Footnote †: journal: Applied  
1
Footnote †: journal: Applied  
0000-0002  
Based on the investigation of transmission tower damage accidents in Ontario, Canada, in recent 15 years, almost all accidents have been caused by strong winds, especially downbursts. A downburst is a kind of small-scale local strong wind near the ground, which is very sudden and destructive [1]. The velocity profile of downbursts is evidently different from that of ABL winds [2; 3; 4]. Consequently, the failure characteristics caused by downbursts and ABL winds are obviously different [5]. Transmission towers are prone to damage, even collapse, when subjected to downbursts [6; 7; 8; 9], as shown in Figure 1.  
The fragility analysis of transmission towers under downbursts is of great significance to evaluating the existing transmission towers and optimizing the new ones. It can provide support for the disaster prevention and mitigation strategies of government departments, and it can also provide important data for risk management and transfer in the insurance industry. In addition, it also provides a reference for the design, maintenance, and retrofit of transmission towers. Many published works only discuss the fragility analysis of transmission towers under ABL winds. [PERSON] et al. (2019) presented a simplified model of a transmission line and conducted a fragility analysis considering the uncertainties of structural parameters and wind loads [10]. [PERSON] et al. (2019) described a modeling framework to evaluate the fragility of transmission towers under extreme winds [11]. [PERSON] et al.