Nicolas Blaise

Optimal Processing of Wind Tunnel Measurements in View of Stochastic Structural Design of Large Flexible Structures

Wind loads are decisive for a wide range of structures and must therefore be modeled adequately in a structural design. Some codes and standards provide a general set of design guidelines only for structures with limited dimensions and under the assumption of a dynamic response in the fundamental mode (Eurocode, 1991). As a matter of fact, very large and flexible structures as, bridges and stadiums, do not fall within the context of application of such simplified procedures. One reason is that large flexible structures may evince a serious sensitivity to the random gust loading, although being stiff enough to limit strong aeroelastic phenomena, but flexible enough to allow for a significant dynamic response. The buffeting analysis of civil structures, i.e. subjected to random pressures due to the fluctuations of the oncoming flow and to the weak interaction of that flow with the windward part of the structure, is typically tackled as a stochastic dynamic analysis. In this view, the usual analysis is performed with a probabilistic description of the wind velocities in the atmospheric boundary layer (local statistical properties as well as spatial coherence), as well as aerodynamic admittances. Based on site-specific and structure-specific data, they allow the determination of the probabilistic description of the loading, namely power spectral densities of (and coherence between) forces resulting from the wind loading at various spots of the structure. A traditional stochastic analysis follows (Clough & Penzien, 1993; Preumont, 1994), for which structural engineers are used to cope with. The well-known decomposition into mean, background and resonant contributions of the wind-induced responses (Davenport, 1961; Holmes, 2007) offers an affordable access to stochastic analysis in the everyday practice. As a ultimate outcome of the structural, extreme values of some structural responses, such as displacements, internal forces or stresses, have to be estimated. They are actually expressed with peak factors, for which there exist various analytical expressions, depending on the properties of the considered random process (Floris & Iseppi, 1998; Rice, 1945). The wind tunnel testing of large flexible structures is much more realistic than the aforementioned codified procedure since it allows a precise estimation of the time-space distribution of the pressures and the modeling of a number of phenomena as the aerodynamic instabilities and aerodynamic admittance, that are difficult to estimate. Design codes therefore recommend wind tunnel measurements for large structures, with a need to model carefully the wind flow surrounding the construction site. Wind tunnels have been being developed since the 1960’s with early contributions from Scanlan, Scruton among others, although the 0