ANALYSIS OF HYBRID T.V. TOWERS UNDER WIND FORCES

Abstract

T.V. towers are essentially tall structures with heights ranging from 100 m to 400 m. A newly developed concept in their design is that of hybrid towers. A hybrid tower has a steel lattice appendage at the top of'a concrete shaft. In India about 20 such towers are existing and about 80 more towers are to be constructed in the 8th FiVe'Year Plan. These tall towers are very sensitive to vibrations under wind. Wind caused along wind as well as across wind excitation. Along wind excitation is due to gustiness of wind while across wind is mainly due to periodic shedding of vortices under steady winds. Published literature for the analysis of such towers is very meager. The present study is an attempt to analyse these towers under wind forces, For computing along wind response Modal Gust Factor 'Method has been used while across wind analysis has been carried out using Rumman's approach and also comparing some cases with Vickery and ACT methods. Four T.V. towers. having concrete shaft heights 56 mt 106 m, 130,m and 165 m and each having a 70 m high steel lattice appendage for antennas at the top of concrete shaft, have been.analysed for both along and across wind excitations. The towers were assumed to be located in open terrain, which is.the usual site for such towers. One particular tower has beep,analysed for various terrains. One, tower has also been analysed considering shielding. The responses have been computed for fixed base condition. For all 4 towers, it has been found that the along wind excitation causes greater moments, shears and deflections as compared to across wind excitation. The.study shows that under across wind excitation both first and second modes are important. It has been observed that the type of terrain affects the along wind excitation significantly. However, the across wind excita. tion is not directly affected by the nature of terrain so that its effect is small. It has also been observed that with the provision of lattice appendage at the top of concrete shaft the across wind excitation can be reduced. It is found that shielding of the latticed portion by a cylindrical plastic cover increased the moments in the top 100 m height of the 235 m high tower in the across wind direction, while the base moment has decreased. In the along wind direction, however, the bending moments decrease throughout the height.