Criticality of Gust Pressures on the Members of Self Standing Towers

-Tall latticed steel towers are being pursued for the purpose of microwave transmission, T.V. transmission etc. Antennas are fixed at the top of the tower for transmission. The height of the towers may range from 50m to 250m. The structural form consists of latticed steel work, suitable bracing and suitable foundation for the tower legs. Usually bolted connections are provided for the joints. Mainly dead and wind loads are considered for the design of such towers. The I.S. Code [8] has given recommendations for force coefficients to be adopted while estimating the wind force. The force coefficient depends on the solidity ratio. The design wind pressure is multiplied by the effective frontal area and the force coefficient to arrive at the wind force. The code is silent on the method to be followed to determine the design wind pressure. The latticed steel towers which have lesser self weight relative to the height are highly flexible in nature and under the action of the fluctuating wind the towers vibrate and undergo large deflections or deformations. Hence the study of wind-structure dynamic interaction becomes necessary for accurate assessment of wind effects on towers. The gust effectiveness factor takes into account the dynamic properties of the structure, the wind-structure interaction and then determines the wind loads as equivalent static loads. In this paper, steel towers of heights ranging from 50m to 250m are considered for wind load analysis. Wind loads are determined based on static method as well as gust effectiveness factor method. The critical gust loads for design are determined. The variation of wind force with height, geometry and the dynamic properties of the structure are studied. Based on the above, important conclusions are drawn. The conclusions will be useful for safe and rational design of latticed steel towers.