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.

In the context of structural reliability analysis at the level 3, it is necessary to formulate full scale probabilistic models of the various random variables related to load and resistance. Often, it is convenient to work with the probabilistic models of the resultant load or resistance variable. Such models of resultant variables are presently developed by means of Monte Carlo simulation techniques. It is possible to develop analytical formulations for these probabilistic models, using classical probability theory. However, this involves the solution of multidimensional improper integrals, which are generally impractical to handle, using even the best of numerical techniques. This paper shows how the analytical formulation may be reduced to a form that is ideally suited for the convenient application of Hermite Integration, provided the basic variables follow normal or lognormal distributions. This situation is frequently encountered in practice in the case of basic resistance variables. Hence, the proposed Hermite Integration solution is well suited for the generation of the probability density function (pdf) of resistance, particularly in complex problems, where a large number of basic variables are involved. It has been shown that the vital key to the efficiency of the Hermite Integration technique lies in a judicious allocation of the various integration orders. This paper demonstrates the application of the proposed method to two practical examples. The results clearly indicate the superiority of the proposed method over the conventional Monte Carlo Simulation method in terms of computational efficiency. Furthermore, the Hermite Integration Method and the Monte Carlo Simulation method together provide useful and necessary validation of the solution developed.

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.

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.