The vortex-induced vibrations of a triple-box girder model of a long-span rail-cum-road bridge

Triple-box girders, i.e., three separated box girders are arranged in tandem, are gradually used in super-span bridges because of their excellent flutter stability. However, the gaps between triple boxes can lead to complex vortex dynamics around the structures that result in vortex-induced vibrations (VIVs). In the present study, we experimentally investigate VIVs in a long-span bridge with a triple-box girder cross-section using wind tunnel tests. In the static test, in which the test model is fixed, a ‘dual-frequency’ phenomenon that expands the VIV lock-in region is observed in the global flow field. Furthermore, the results of a dynamic test indicate that the existence of gaps is essential to the occurrence of VIVs in triple-box girders. The smoke-wire flow visualization technique is employed to explore the excitation mechanism by examining the surrounding flow dynamics of the triple-box model. Separated motion-induced vortices (MIVs) and von Karman vortices (KVs) are found to be coupled with structural vibrations to sustain the VIVs. Five stages of VIV regions, along with five types of vortices in the surrounding flow field, are identified for the girder model. Finally, the effects of accessory structures of the bridge model on the VIV responses are also studied. This study contributes to our understanding of the unsteady flow dynamics and VIV mechanisms of a typical triple-box girder.