DYNAMIC RESPONSE OF PILE-SUPPORTED CONCRETE CHIMNEYS

Abstract

Tall chimneys of increasing height are being built in India, for chemical and power industries, resting on different soil conditions varying from rock to alluvium. The main dynamic forces to which chimneys will be subjected, are those caused by earthquakes and wind excitation. So far, seismic response of chimneys has essen-tially been studied for the fixed base which is likely to be achieved for rocky. condi-tions. Studies on chimneys resting on flexible base condition are lacking. The seismic response of chimneys has been studied here treating the chimney and the foundation as two substructures. The formulation of the problem has been done using the method of 'Frequency Domain Analysis'. Consistant mass matrix and Rayleigh type damping have been adopted for chimneys. In the first part of the thesis, analysis of the behaviour of 'soil-pile system under static and dynamic loads has been carried out. This has been done using the method of analysis develop-ed for single piles by Novak (23) and Novak and Sharnouby (26). For obtaining the system properties of the pile group the approach of Poulos (33) for interaction between the various piles has been adopted. However a general approach has been worked out to obtain the properties of pile-groups. The pile group is replaced by a system of translational and rotational springs with cross coupling terms and corres-ponding dashpots placed at the base of the pile cap. The response is analysed due to harmonic ground excitation acting at the base of these springs. Responses have been studied for two chimneys of 120m and 180m in height as actually constructed at Patratu and Bokaro respectively, for harmonic excitation in the transverse and vertical directions. Three types of soils giving shear wave velocity of 50m/s, 100m/s and 150m/s have been considered. Earthquake response of these chimneys has also been studied due to transverse component of Koyna earhquake of Dec. 11, 1967. The force distribution has been compared with the distributions as specified in IS: 1893 (14) and ACI: 307 (2). The responses of the chimneys have been studied under fixed base conditions as well as when, the chim-neys are supported on pile, groups and the differences in the behaviour have been discussed. Conclusions have been drawn regarding the effect of base flexibility on the time periods and dynamic response of the chimneys and inadequacy of the distributions given in the two codes in comparison to the force distribution obtained for the actual chimneys subjected to a real earthquake ground motion. a (v)