BEHAVIOUR OF TALL GUYED TOWERS

Abstract

A guyed tower can be looked upon as a beam on discrete elastic supports subjected to transverse and axial loads. Due to very high axial loads coupled with transverse loads on the tower, it is customary to consider the tower shaft as a beam-column. The cable geometry and its load-displacement response are nonlinear. The analysis for an equivalent static wind load is also nonlinear. The effects of the important parameters on the structural behavior of the guyed tower are necessary to be studied such as initial guy tension, strength of the guy, wind speed, wind direction, moment of inertia of the tower shaft, and loads due to antennae attached on the tower. In this thesis, analysis of guyed towers with triangular section, hinged at base was considered. The method of analysis developed by Odley [1966] was adopted. A computer program, GTAP - Guyed Tower Analysis Program, was developed by Dr. A.K. Jain based on this method, has been selected in order to carry out the analysis. The author has decoded the program, modified the wind loads part corresponding to IS: 875, 1987 — Part III, calculated member forces from the overall moments and shear force in each panel, and a subroutine has added to take into account/the equivalent modulus of elasticity of guy cables. In addition to that, the input and output have been modified to make the program becprs more user-friendly. Three guyed towers with different heights of 60m, 110m, and 425m were examined. The results show that the initial guy tensions should be selected around 12.5% of the breaking strength of the cable. The rate of change of maximum deflections of the tower shaft increase incredibly when the wind speed exceeds 40 m/ sec. For guyed towers with less than 4 guy levels, face wind and parallel wind are necessary to be considered in practical design and corner wind can be neglected. But for towers with greater guy levels, it is necessary to consider all three directions of the wind in designing. When moment of inertia of the tower shaft increases or decreases 20%, differences in maximum deflection of the tower shafts are less than 1.7%, and differences in forces of the leg members are less than 7%. Loads from two high performance antennae applied to the 60m guyed tower at levels 55.5m, and 60m respectively, the maximum deflection of the tower shaft increases significantly (140%), and the forces of the legs at the antenna level also increase by 150%. In all cases, the maximum force of the leg members is quite small as compare to the critical force (up to 35% of the critical force).