The effect of turbulence on the aerodynamic response of a bridge deck

The present investigation considers the effect of variation of the turbulence intensity of the wind on the dynamic response of a bridge deck. The model used in the investigation was a 1/150 scale sectional model, the deck section used being that of the West Gate Bridge over the river Yarra, Melbourne. Tests were carried out in turbulent shear flow models of the natural wind of 15% and 9% turbulence intensity, and in a low turbulence, 0.9%, 'smooth' flow. The vertical and torsional or pitching response of the model was measured in the three flow conditions and a direct comparison made. The measurements were also compared with theoretical predictions of vertical response. It was found that the fluctuating displacements increased significantly as the intensity of turbulence increased; mean displacements were unaffected by turbulence intensity. For the high turbulence intensities the fluctuating deflections increased steadily as the . reduced velocity was increased whereas the low intensity, smooth flow, was characterised by deflection peaks associated with vortex shedding. Energy peaks at vortex shedding frequencies were also apparent in the wake spectra in smooth flow but not in the higher intensity flows. The fluctuating displacements were found to vary inversely with suspended mass in the high turbulence intensity conditions, but in the low intensity, smooth flow, case the effect of mass was much reduced, the reduction of deflection with increased mass being about 1/2 to 1/3 of that in the high intensity cases. Increased structural damping reduced the fluctuating displacement. Theoretical preditions of fluctuating response compared well with measured values provided a suitable aerodynamic admittance function was used and a factor applied to take account of unsteady effects on aerodynamic damping. The theory predicted successfully the effect on response of variations in suspended mass and structural damping.