Devdas Menon

This paper reviews the prevailing international codal procedures to evaluate the across-wind response of R/C chimneys. The disparities in the codal estimates of across-wind moments as well as the load factor specifications are examined from a reliability viewpoint. Conditions are also identified wherein the across-wind response, rather than the along-wind response, governs the design.

There exist considerable disparities among the prevailing international codai provisions for the wind-resistant design of reinforced concrete (RC) tubular towers - with regard to (1) estimation of peak wind moments, (2) estimation of ultimate moments of resistance, and (3) specification of safety factors. This paper highlights these disparities in terms of structural reliability. It is shown that there exists a very wide range of 'global safety factors' underlying the prevailing codai methods for the range of practical design situations. These global safety factors are translated in terms of 'lifetime probabilities of failure' by reliability analysis, using a 'generalized reliability model', and applying Monte Carlo Simulation and Numerical Integration. The results indicate a considerable lack of uniformity among international standards and the presence of widely ranging reliability levels under different design situations. There is an evident need to resolve the codai disparities and to adopt optimal safety factors which consistently achieve a 'target reliability'. In this paper, it is shown how this can be achieved.

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.