Capacity Evaluation of Power Transmission Line Towers under Wind Loading: A Case Study

A. U. Qazi, Muhammad Sameer Azhar, Safeer Abbas, Asif Hameed

Abstract


Pakistan is currently facing massive energy crisis and requiring huge investment into the power transmission line infrastructure. The longest transmission lines of the country are of 220 kV lines stretching up to 7359 km. The contemporary factored design wind load effects for overhead lattice transmission line tower are evaluated based on the applicable wind load factor, gust response factor and design wind speed as recommended by WAPDA/IEC Specifications (1988), ASCE-74 (1991) and ASCE-7 (2005). The current factors and design wind speed were developed considering linear elastic responses. However, information on the non-linear inelastic responses of such towers under dynamic wind loading and on the structural capacity of towers in relation to the design capacities is lacking. The knowledge and assessment of the capacity curve and its relation to the design strength is important to evaluate the integrity and reliability of these towers. Therefore, in this study, analysis was performed on 220 KV river crossing tower of 78.25 high (the highest transmission line tower in Pakistan), using a nonlinear static pushover analysis (NSPA) and incremental dynamic analysis (IDA). Firstly, NSPA was performed on 2D and 3D structural models for aforementioned codes and also for three additional load patterns according to first mode shape, power law and microburst/downburst phenomena. Afterwards, the IDA was performed for which time histories were generated by using NATHAZ Online wind simulator followed by a series of nonlinear direct integration dynamic analysis. Finally, for each loading pattern of NSPA and each time history of IDA, base shear load versus top displacement curves were plotted which are known as capacity curves. Numerical results showed that the structural capacity curves of the tower determined from the NSPA depend on the loading pattern and are similar to those obtained from IDA. The results indicated that the towers in Pakistan are reasonably reliable yet over designed due to standardized design approach across the country.

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