WIND RESPONSE OF SELF SUPPORTING TALL TOWERS

Abstract

Dynamic analysis of circular cylindrical tapered concrete, latticed steel and composite towers to wind has been carried out analytically as well as experimentally. The along-wind and across-wind dynamic response of towers has been obtained analytically using Davenport's approach [29] and Vickery's procedure [117] with certain modifications respectively. The reliability of the mathematical formulations has been tested by comparing the results with the field measurements available for some existing structures. Next, aeroelastic models of a tapered concrete, latticed steel and composite towers have been tested in a closed circuit subsonic wind-tunnel in shear as well as uniform flow fields. Concrete and latticed steel towers are modelled using brass and aluminium angles respectively. Composite towers are the combinations of these two in different length ratio with the latticed part fixed atop the cylindrical portion. The rms values of tip displace ments of models in along-wind as well as across-wind direction have been determined with the help of a transducer,amplifier and recor ding system, for a range of air velocities. These results are used to predict prototype behaviour and have been compared and found in close agreement with the theoretical values. The mathematical formulations are next applied to dynamic analysis of composite towers and the influence of lattice appendage atop concrete portion has been studied and explained. It is found iii that the replacement of a part of concrete tower top with a lattice appendage results an increase in the along-wind response. The across-wind response, however, is considerably reduced due to lattice appendage under certain conditions — a result which could profitably be used in many situations. The effect of adjustment between diameter and thickness in the design of a concrete tower, on its dynamic response, is also investigated. Using the analytical approach due to Wen and Ang [123], dynamic analysis of these towers has been carried out under tornado effect also.