Wind induced dynamic response of the Wye bridge

As part of the research effort to improve the understanding of the effect of wind on bridges, the Transport and Road Research Laboratory (TRRL) has measured the wind-induced dynamic response of the 235 m span cable stayed box girder bridge that carries the M4 motorway over the River Wye. Measurements are presented which were made during the winter months of 1977 and 1978, using automatic recording equipment developed at TRRL.

The main wind-induced vibration frequency measured was at 0.46 Hz, which corresponds to the first bending mode. The response occurs mainly when the wind speed is in the range 7–8 m/s and the direction is close to normal to the bridge. This agrees with wind tunnel section tests but the maximum amplitudes predicted in the laboratory have not been attained to date. The strong dependence of vibration amplitude on wind speed, showing a peak in the range 7–8 m/s is characteristic of vortex shedding excitation. The problem of predicting the likelihood of the bridge response from wind speed data is discussed.

Finally, the effect on the fatigue life of the structure is discussed. It is shown that it is most unlikely that wind-induced oscillations will contribute to a significant shortening of the fatigue life of the bridge.