STUDY OF THE DYNAMIC RESPONSE OF COOLING TOWERS

Abstract

Axisymmetric shell structures, generated by rotating a plane curve generator around an axis of rotation to form a circumferentially closed surface are generally used for hyperbolic cooling towers. These structures are thin and due to their curvature, are not only aesthetically pleasing but also have adequate strength. Much research into the behaviour of cooling towers has been carried out since development of finite element method. This previous research mainly concentrated on shell behaviour under static loading, with dynamic loads modeled as equivalent static loads, and hence provided limited knowledge on the dynamic behaviour Dynamic loads of interest include wind loads and seismic actions that are time dependent and asymmetric. These are the primary dynamic loads that affect the civil engineering structures. It has been recognized that wind is a very important load for cooling towers. There is also a general feeling that earthquake is of secondary consideration. The two loading types affect different parts of a structure, while earthquake activates the entire 360° axisymmetric cross section,, the wind load tends to concentrate its influence over only 180°. Past research has proved that ring stiffeners significantly improve the buckling strength and free vibration characteristics (natural frequency, mode shapes). No research or literature is available on the performance of vertical stiffeners in the cooling towers. Even though the construction of vertical stiffeners is a bit difficult compared to ring stiffeners, provision of vertical stiffeners is studied. Therefore, the emphasis has been given to study the effect of vertical stiffeners on the dynamic response of cooling towers. The effect of No. of pairs of columns is also studied and the results are compared. Influence of thickness, column area, Number of column pairs, Number of stiffeners, change in thickness of shell with stiffener thickness kept constant and change in thickness of stiffener with shell thickness constant on the free vibration characteristics is studied. Along with above mentioned loads cooling towers are subjected to thermal loading also. Heat transfer between hot air inside the cooling tower and ambient air establishes a variable temperature field in the wall of tower. This variation in the temperature field gives rise to appreciable thermal stresses. A study has been performed to know the effect temperature field on the thermal stresses developed in the shell.