A three-dimensional (3D) soil-structure interaction (SSI) analysis of 300？m high reinforced concrete chimneys having piled annular raft and annular raft foundations subjected to along-wind load is carried out in the present study. To understand the significance of SSI, four types of soils were considered based on their flexibility. The effect of stiffness of the raft was evaluated using three different ratios of external diameter to thickness of the annular raft. The along-wind load was computed according to IS:4998 (Part 1)-1992. The integrated chimney-foundation-soil system was analysed by commercial finite element (FE) software ANSYS, based on direct method of SSI assuming linear elastic behaviour. FE analyses were carried out for two cases of SSI (I) chimney with annular raft foundation and (II) chimney with piled raft foundation. The responses in chimney such as tip deflection, bending moments, and base moment and responses in raft such as bending moments and settlements were evaluated for both cases and compared to that obtained from the conventional method of analysis. It is found that the responses in chimney and raft depend on the flexibility of the underlying soil and thickness of the raft. 1. Introduction The height of many industrial chimneys in India is more than 200？m. The tallest chimney in India is Dahanu Thermal Power Station’s Chimney (1995) at Mumbai with a height of 275.3？m, and chimney of GRES-2 Power Station (1987) at Kazakhstan is the tallest chimney in the world with a height of 419.7？m. The need of increasing the height of chimney is very essential as it is directly related to social and economic aspects of any country. Due to the unique geometrical features like tall, slender, and tapering geometry, the analysis of chimney should be considered separately from other forms of tower structure. The wind loads are more predominant forces than the seismic loads for very tall chimney. It is very difficult to analyse the chimneys with transient wind loads precisely by available analytical procedures because of uncertain variability of wind, and therefore a designer is forced to use approximate design techniques, Manohar [1]. Most of the design wind codes for chimney, IS:4998 (Part 1)-1992 [2], CICIND [3], ACI 307-2008 [4], and so forth, simplified the dynamic wind loads by considering two components, namely, a steady wind load component and a fluctuating component. This method is known as gust factor method, which is derived by Davenport [5] and modified by researchers Simiu [6] and Solari [7], and it is mainly used to compute the wind
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