The role of uncertainty in Aeolian risk assessment

The design of wind sensitive structures must take into account many sources of uncertainty related to: (a) the basic environmental parameters that describe the wind field at the site, like mean wind velocity, turbulence intensity, wind direction, etc.; (b) the structural and mechanical properties, like stiffness and damping; (c) the parameters that influence the interaction phenomena (aerodynamic coefficients, flutter derivates, Strohual number, etc.). Roughly, three categories of uncertainty can be distinguished: the aleatoric uncertainty, related to the natural variability of the parameters and their unpredictability; the epistemic uncertainty related both to the lack of information and to the errors of experimental measures; the model uncertainty related to the choice of the models of wind action, structural response and interaction phenomena. In this paper, specific reference is made to the optimal design of a long span suspension bridge: the effects of the three types of uncertainty on the Aeolian risk assessment of the bridge are evaluated. The attention is focused on the minimum information needed to characterize satisfactorily the wind field, since the data available today (essentially, turbulence spectra and maps of expected wind speeds) are rather limited. The influence of model uncertainty is investigated with reference to buffeting, by implementing in time domain different formulations of the aeroelastic forces. The relevance of epistemic uncertainty is investigated in evaluating the flutter instability, by a parametric analysis that assumes different values of the aerodynamic coefficients. Finally, the relevance of aleatoric uncertainty that characterize the mean wind velocity is considered in assessing the fatigue life of the secondary suspension cable system under buffeting effects. The final aim of these studies is the development of procedures for Performance-Based Wind Engineering.