Safety analysis of steel building frames under dynamic wind loading
Das, Nirmal K
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Absolute safety of a structure cannot be achieved because of uncertainties in loading and resistance. Conventional safety factors only qualitatively account for these uncertainties. Thus, the actual risk of failure of a structure designed in accordance with building code provisions remains unknown. Conversely, the probability of attaining a suitably specified limit state is recognized as a quantitative measure of safety. Since such information is needed in determining acceptable risk levels and in decision-making for structural improvements, a method for the evaluation of risk of failure is needed. Traditionally, wind-excited structures have been designed based on equivalent static forces in place of actual dynamic loads. For flexible buildings this may not always provide a reasonable approximation in the response analysis. The objective of this study is to evaluate the lifetime risk of failure of steel building frames subjected to dynamic wind loading. Risk is evaluated in terms of probability of excessive interfloor deflection. The method consists of the following studies: (1) estimation of the probabilities of exceedance of significant wind speeds at a specified site (wind hazard), (2) evaluation of the structural response statistics for a given storm intensity, using multiple time-histories of wind speed; and (3) convolution of the wind hazard with the response exceedance probabilities for each loading intensity considered. The wind field model used is based on the Power Law profile and Davenport's gust spectrum. Wind speeds are generated by a simulation technique. Three steel buildings ranging from three to twenty stories are investigated using shear beam model. An ideal elasto-plastic force-deformation relationship is assumed, neglecting strain-hardening effects. A step-by-step integration method is employed for solving the governing differential equations. This study is restricted to low-rise/moderately tall (up to 20 stories) rectangular steel buildings having equally spaced, laterally unbraced frames. Only along-wind responses are considered, neglecting wind directional effects.