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In the first winter after opening, the Second Severn Crossing cable-stayed bridge exhibited occasional vortex-induced vertical oscillations. Vortex excitation had been detected by the wind tunnel tests on sectional models during design and an aerodynamic solution partly developed but, based on damping and turbulence assumptions from recognised design codes, it had been concluded that the probability of large vibrations occurring was small. Full-scale monitoring of both wind and structural response enabled the causes of the observed oscillations to be investigated. The structural damping and wind turbulence were found to differ significantly from the assumed values. Additional wind tunnel tests were undertaken using the full-scale measured values with a view to replicating the full-scale behaviour and developing a solution. Taking into consideration the revised damping and turbulence and an often-ignored factor to correct for the full-scale mode shape, good agreement between the model and full-scale measurements was obtained. Also, aerodynamic solutions were further explored, as a result of which baffle plates were installed under the bridge. Since their installation, no further signs of vortex-induced oscillations have been observed.

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Hungerford Bridge Millennium Project in London, UK—a complex seven-span multi-cable-stayed structure—was under construction at the time of the oscillations of the nearby Bankside Millennium Footbridge. The design criteria and methods of calculation used to assess oscillation and displacement are described. The results of physical testing of the bridge by mechanically induced vibration and by a walkover test are compared to the calculated values. The calculations predicted performance in service with adequate accuracy. Measurement of structural damping during the physical tests indicated that it was lower than expected.

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