Performance-based wind design for footbridges: Evaluation of pedestrian comfort

The modern design philosophy known as the performance-based design has earned the attention of the scientific community over the last decade, especially in the field of earthquake engineering. The general scheme entails the evaluation of the probability distribution of a decision variable, usually represented by an economic index; in a simplified formulation, performance-based design is accomplished by assessing the probability of exceeding a structural damage state. The approach is quite general and may be extended to wind excited structures. In this paper, the probability of violating some criteria which define the pedestrian comfort is evaluated for a suspended footbridge excited by wind loading. The examined footbridge is an actual structure whose aeroelastic characteristics are known. The structural capacity, in terms of allowable vibrations, is quantified according to the guidelines given by ISO 2631, that establish the user’s perception and acceptance criteria. The buffeting and vortex shedding effects on the footbridge deck are investigated through a 3D finite element nonlinear analysis in time domain. The wind loading due to buffeting is modeled on the basis of a stochastic field representing the wind velocity and a spectral description of the wind turbulence; the model allows to generate the time histories of lift, drag and aerodynamic moments. The vortex shedding action is modeled in frequency domain, reproducing the amplitude and frequency content of the motion that has been observed in an experimental campaign carried out in a wind tunnel. To perform the reliability assessment, two simulation techniques, i.e. Monte Carlo and Subset simulation, are implemented and compared.