RELIABILITY OF CONCRETE CHIMNEYS UNDER WINDS

A reliability analysis of tall reinforced concrete chimneys under wind loads is presented. A first order second moment (FOSM) approach is used for this analysis. The safety criteria, i.e. excessive deflection at the top of the chimney and exceedence of the ultimate moment capacity of the chimney cross-section at any level, are used to evaluate risk in terms of the probability of failure. The ultimate moment capacity is estimated using the ultimate stress method based on a second-order stress strain relationship for concrete. The formulation of wind induced response analysis is based on random vibration theory. The structural response in the alongwind and the acrosswind directions are considered for this analysis. A covariance integration method is used for determining fluctuating overall wind load on chimneys. A Monte Carlo simulation scheme is used to evaluate multi-fold numerical integration involved in the response analysis. The maximum fluctuating response is determined from upcrossing probabilities. Both the structural loading and the resistance parameters are treated as random variables and the FOSM approach is used to examine their uncertainties. The wind loading parameters included here are, extreme distribution, wind velocity profile, wind velocity spectrum, spectral bandwidth, Strouhal number, drag and lift coefficients; and for the structural resistance: material properties, structural dimensions, natural frequency, structural and aerodynamic damping. The Type I, Type II, and Rayleigh distributions are used to model the annual maximum wind speed distribution. The chimney is modeled as a multi-degree-of-freedom system, and the frequency domain approach is used to compute the r.m.s. and the peak values of the response components. Four different approaches, i.e. complete FOSM model, modified FOSM model, predictive model and direct method, are used to evaluate the lifetime probability of failure of chimneys. Bounds on probability of failure are computed by conditioning which gives narrow bounds regardless of the high correlation between failure modes. An example is presented to illustrate the methodology presented in this study. A tall reinforced concrete chimney, designed in accordance with ACI Code, is used in this example. The procedure presented in this study would enable a designer to assess the lifetime probability of failure, i.e. excessive deflection or exceedence of the ultimate moment capacity of chimneys.