ANALYSIS AND DESIGN OF TRANSMISSION LINE TOWERS

Abstract

The electrical transmission line serves as a vital link between the generating stations and load centres and also between different generating stations in an inter- connected system. With the rapid growth in generating capa-cities as well as load demand, large blocks of power are required to be transmitted over long distances, which poses a challenging task for the structural engineer to design relatively large transmission structures to support heavy conductor loads, with a high degree of reliability and economy. This requires more efficient and accurate methods of analysis and design for the towers. Transmission line towers are highly redundant, statically indeterminate structures and their analysis by classical structural analysis procedures is very tedious and time con-suming and indeed, Impossible in certain cases. In the con-ventional methods of analysis, the tower is reduced to several statically determinate planar frames together with the resolution of the loads into the planes in which they act, ignoring entirely or approximating the effect of the redundant systems in the distribution of forces. Thus conservative assumptions are made in the conventional methods resulting in costlier structures. iv In this dissertation, the analysis and design of trans. mission line towers has been presented, treating them as pin-jointed space structures, using di zital computer. The analysis has been based on the "stiffness Matrix approach. A general digital computer programme has been developed for analysing and designing single circuit as well as double circuit self supported. towers. The input to the programme consists of the geometrical configuration of the tower, the contactor and groun'9wire sizes, the. design spans, the climatic conditions, and sizes and properties of available steel sections. The programme first determines the maximum working tensions for the conductor and groundwire by carrying out necessary sag-tension calculations. All the design loads are then computed and the tower is analysed for different load combinations corresponding to the normal and brokenwire conditions, and the maximum tensile and compressive member forces are determined. The programme then estimates the minimum cross sectional areas for the members and selects 0 appropriate sizes from a list of available steel sections. Using this programme, a single circuit 132 kV trans-mission line tower has been designed and a comparison is made with the conventional design. It is noted that a saving of 5.8 percent in steel is possible by analysing this tower as a three dimensional space structure . From the results of member forces, it is also seen that the three dimensional analysis through computer gives a more realistic picture V of stress distribution. I~ny design based on this will therefore produce a more economical as well as safer tower in minimum possible time. Further, the programme developed in this work, with the ability of analysing and designing a tower in a single run, entirely within the core memory of the computer is expected to be quite useful.