Dynamic Response And Collapse Analyses Of Transmission Tower- Line System Subjected To Wind And Rain Loads

In the southeast coastal areas of China, typhoons land frequently, and rain intensities are often very large during typhoons. However, the forming mechanism of rain load is still not clear. Recently many transmission tower-line systems have collapsed during severe gales and thunderstorms, and most researchers have attributed these collapse cases to wind loads, with few considering rainfall effect. Therefore, it has become necessary to study rain load to understand both the mechanism acting on these structures and its contribution to tower collapse. Both theoretical and experimental methods are utilized to conduct the systematic research of wind and rain loads for transmission tower-line system, and then the dynamic response and collapse-resistant performance are discussed. The main research contents and conclusions are listed as below:(1) The formulae of raindrop impinging force are derived by theoretical analysis, and a piezoelectric transducer is developed to measure the curves of raindrop impinging force vs. time. Meanwhile the raindrop shape revolution during the impinging process is elaborated. The experimental results show that the transducer surface condition has great impact on the measured results and the impinging mechanism can be changed when the transducer surface is wet.(2) The motion trajectory of wind-driven rain has been simulated through numerical method, and the phenomenon of that the horizontal velocity of raindrop is larger than the corresponding wind speed is certified and explained. Then the formula of velocity ratio is fitted based on the simulation results. The specific catch ratio under uniform flow is derived and then extended to the atmospheric boundary layer wind field.(3) The calculating method of rain load is proposed based on the experiment of single raindrop impinging force and the conservation of momentum, and then applied to the response analysis of transmission tower. After that the wind tunnel test is performed to validate the presented rain load model. In addition, the influence of raindrop spectrum is discussed, and the acting mechanism of rain load is revealed. Both the simulation and experimental results illustrate that rain load has a great impact on the tower response and should be paid enough attention.(4) A simplified method is presented to simplify the calculating process of wind and rain loads, and then the concept of equivalent basic wind speed is used to conduct the fragility analysis subjected to wind and rain loads which avoid the choice of different combinations of basic wind speed and rain intensity. In addition, the concept of critical collapse curve is given to evaluate the collapse status for structures subjected to wind and rain loads, and the critical collapse curve presents a method to compare different combinations of basic wind speed and rain intensity, and judge the collapse status.(5) The mechanism of rain load acting on conductor is preliminarily revealed based on the theory of flow around a circular cylinder, and then the calculating method of wind and rain loads for conductor is presented with the key parameters determined by wind tunnel test. On this basis, the dynamic response and collapse performace of transmission tower-line system subjected to wind and rain loads are simulated, of which the results show that rain load has great impact on the dynamic response and collapse-resistant performance. If ignoring rainfall effect, the tower bearing capacity will be overestimated, and meanwhile rainfall has no effect on the collapse mode and failure path.