Wind tunnel modeling for the vortex-induced vibrations of a yawed bridge tower

The design of modern bridge towers tends toward more and more complex-shaped (tapered and/or inclined cantilevers, etc.) and lighter structures. Consequently, the wind effects are often assessed through full-aeroelastic tests, even for small/medium-span flexible bridges. The lightness of these structures, together with a bluff shape, give rise to a certain susceptibility to vortex-induced vibrations (VIVs), which cannot be assessed by using simple sectional models due to the nonnegligible tridimensionality of the main phenomena involved. This paper deals with the VIV of a yawed and tapered tower of a 110-m-span cable-stayed road bridge in the South of Italy. The VIV response in smooth and turbulent flow estimated through a full-aeroelastic model is compared with those of two sectional models referring to two different cross sections along the height of the tower. The comparison shows that the predictions of the two sectional models significantly overestimate the drag force of the entire structure and underestimate the VIV response of the tridimensional model by a factor of two, confirming the previously mentioned need for full-aeroelastic tests. The work also gives some insight into the issue of scaling to prototype the sectional model results through the introduction of a modified modal shape factor. Finally, wake measurements in smooth and turbulent flow, together with flow visualizations by means of woolen yarns attached to the tower, shed some light on the complex flow past yawed bluff bodies.