Devdas Menon

This paper reviews the prevailing international codal recommendations to determine the design along-wind moments in reinforced concrete (RC) chimneys and towers. A comparative study reveals significant disparities among the various codal estimates of the design moments, given the same basic wind speed and terrain condition. These disparities are a matter of concern to practising chimney designers and approving authorities in many countries. The relative accuracy of these differing estimates, which are based on different gust factor methods, have been quantitatively assessed with reference to rigorous stochastic dynamic analyses. This study covers a number of linearly tapered RC chimneys with heights in the range 100-400 m, located in different terrain conditions and subject to the range of wind speeds encountered in practice. It is observed that the estimates of the design moments are generally conservative in varying degrees, and are unconservative only in some cases. In very rough terrains, the lack of appropriate design provisions results in extremely conservative designs. With a view to improving the accuracy in the predictions, modifications to the gust factor method have been proposed in this paper. Copyright © 1996 Elsevier Science Ltd.

Second-order moments of considerable magnitude arise in tall and slender RC chimneys and towers subject to along-wind loading, on account of eccentricities in the distributed self-weight of the tower in the deflected profile. An accurate solution to this problem of geometric nonlinearity is rendered difficult by the uncertainties in estimating the flexural rigidity of the tower, due to variable cracking of concrete and the 'tension stiffening' effect. This paper presents a rigorous procedure for estimating deflections and second-order moments in wind-loaded RC tubular towers. The procedure is essentially based on a generalised formulation of moment-curvature relationships for RC tubular towers, derived from the experimental and theoretical studies reported by Schlaich et al. 1979 and Menon 1994 respectively. The paper also demonstrates the application of the proposed procedure, and highlights those conditions wherein second-order moments become too significant to be overlooked in design.