Study On Structural Optimum Design For Transmission Towers Based On Wind-Induced Damage Modes

Statistical analysis of transmission tower collapses under strong wind loading shows that the current structure design of various types of transmission towers is not adequate from the perspective of wind resistance performance. According to existing research, under strong winds, some members of the transmission tower are vulnerable to damage, some parts or positions of the tower are weak and the failure modes of transmission tower under strong winds are distinctive. Therefore, how to considerably improve the wind resistance performance of transmission tower structures with as little increase of the materials possible has become extremely a valuable research topic. In other words, research on optimization of wind-resistance design for transmission towers is of important engineering significance, long-term social impacts and wide applications in practice.For the optimization design of transmission towers under wind loads, calculation of wind-induced response is the prerequisite. However, theoretical models of wind loads for transmission tower-line system have not yet been established. Presently, even though wind-induced responses of transmission towers can be measured with wind tunnel tests, only limited information can be obtained. Also several inversion methods have been proposed to obtain wind loads but there are still some shortcomings in these methods. For example, the assumptions of wind load distribution do not represent the actual situations of wind load. Besides, optimization methods of transmission tower structures still stay at component level presently, and the optimization from the overall perspective cannot take into consideration of the uncertainty of design variables in its application to actual projects.To fill these gaps mentioned above, and on the basis of previous research, optimization of wind-resistance design for transmission towers is studied based on calculation of equivalent static wind loads and the failure modes, where uncertainties of the engineering applications are considered. Specifically, the major works completed in this thesis are summarized as follows:(1) The wind-induced responses of each component of the transmission tower are obtained by fitting the displacement responses of the selected positions of the tower measured from the wind tunnel tests. Firstly, the shape coefficient of the transmission tower is calculated by the average value of the displacement measured on the single tower, and the shape coefficient of the transmission line is calculated through the difference between the average value of displacement measured on the tower-line system and the single tower. Secondly, along-wind and across-wind displacement (co)variances are calculated through the time series of the displacement response measured on the tower-line system. Based on the (co)variances, generalized displacements dominated by the first order modes in along-wind and across-wind directions respectively are obtained. Finally, the mean wind-induced responses are obtained using finite element model of the tower subjected to the mean wind load, and the RMS wind-induced responses are obtained by superposition of the along-wind RMS responses and the across-wind RMS responses. By combining the mean responses with RMS responses and multiplying by a peak factor, the wind-induced responses of each component can be obtained.(2) The failure modes of transmission tower under strong winds and the corresponding strengthening methods are studied. Firstly, by considering the dead weight and wind-induced responses calculated by the above mentioned method, the key failure modes and corresponding ultimate wind loads can be obtained through failure mode identification, the results of which are compared with the practical engineering cases. Secondly, two strengthening methods are developed to improve wind-resistance capacity of the tower:strengthen the weak parts and optimize the design parameters of the key elements. Finally, the tower under wind load with various wind speeds is strengthened numerically through computer programs developed in this research.(3) The optimization design method based on ultimate bearing capacity of transmission tower is proposed. Firstly, the key failure modes and corresponding key elements are calculated by some identification methods which consider the combination factors of dead weight and wind-induced responses based on Limit States Design Method (LSDM). Furthermore, the failure modes with insufficient bearing capacity and the corresponding key members are identified. Finally, from the failure modes chosen, the key members of insufficient bearing capacity can be optimized. With the developed computer program, the tower under several wind speeds can be optimized automatically. As mentioned, the Limit States Design Method (LSDM) is applied because it is a method with widespread application that can ensure structural reliability and also has been used by many design codes in the world, including the Chinese one. This optimization method eliminates the tedious reliability calculation and yet meets the reliability requirements.(4) An optimization design method based on reliability is proposed. Such method can be applied to practical engineering designs and only requires relatively small amount of calculation. To use this method, firstly, the reliability index for each single key failure mode needs to be calculated separately. Then the failure modes with unacceptable reliability index are identified which will be optimized by strengthening the corresponding key elements until they all meet the requirements. Furthermore, the reliability index of the system is calculated by comprehensive analysis of the reliability index of each key failure mode. Finally, if the reliability index of the system does not meet the requirements, it should be optimized through increasing the reliability index for the failure mode with smallest value of the index. Similarly, the tower under the wind load with various wind speeds is optimized automatically through the developed computer programs.