Impact of frequency ratio on vortex-induced vibration performance of twin parallel bridges

Owing to the different natural frequencies of the two main girders in twin parallel bridges with parallel asymmetric configurations, the frequency ratio (FR) becomes a critical parameter influencing vortex-induced vibrations (VIV). Segmental model wind tunnel tests were conducted to investigate the influence of FR on the VIV performance of twin parallel bridges. The study focused on the vibration responses of single-girder bridges, streamlined twin parallel bridges with FR ranging from 0.88 to 1.12, and PK (flat-box streamlined girder with central partition)–streamlined twin parallel bridges. Two incoming wind directions were considered for the PK–streamlined bridge. At α = 0°, FR values from 0.88 to 1.53 were tested, while at α = 180°, FR = 0.65 and FR = 1.00 were tested. The surface pressure distributions and vortex shedding patterns of the PK–streamlined twin parallel bridges were also analyzed and discussed. The results showed that neither the single-girder bridge nor the streamlined twin-girder configuration experienced VIV. However, the PK–streamlined twin parallel bridge exhibited VIV at α = 0°, with the vibration response increasing as FR approached 1. The upper surface of the upstream PK-type girder was identified as the critical region contributing to the VIV. The vortex shedding patterns that triggered VIV in the upstream girder varied depending on the FR, with distinct differences observed at FR = 0.88, FR ≈ 1.00, and FR = 1.53. Whether the downstream girder experienced VIV primarily depended on the frequency of the wake vortices from the upstream girder. When the frequencies were close, the VIV of the downstream girder was more likely to be excited.