Interference effect on vortex-induced vibration in a parallel twin cable-stayed bridge

A vortex-induced vibration (VIV), amplified by an interference effect caused by two parallel decks, was observed in the upstream deck of a twin cable stayed-bridge. This represents the first case of such an observation in an actual long-span cable-supported bridge. The observed VIV was successfully reproduced in a wind tunnel. Both decks were equipped with vanes and the single deck alone showed an allowable performance, in terms of VIV, based on wind tunnel tests that were carried out. It therefore appears that the observed vibration was affected by the parallel arrangement of the decks. A particle image velocimetry technique was successfully applied to investigate the complicated flow field between the upstream and downstream decks. Alternating eddies were formed in phase with the upstream deck motion and were transmitted to the downstream deck introducing “Ω” and “℧” shaped flow fields as a result. Following the alternating eddies, upward and downward wind streams were in turn fed into the gap and these flows amplified the vibration in the upstream deck of the bridge. Several modifications of aerodynamic additives were not effective in reducing this VIV. However, an increase in structural damping effectively mitigated the vibration.