Multiple-order vertical vortex-induced vibration mechanism of a typical streamlined closed-box girder

Abstract

Multiple-order vertical vortex-induced vibrations (VIVs) have been frequently observed in prototype bridges as well as tested models in wind tunnels, but VIV excitation and VIV suppression mechanisms need further investigation. In this study, a simplified vortex model (SVM) derived from the spatial distribution characteristics of the wind pressure around the girder surface was developed to elucidate the multiple-order vertical VIV phenomena of a typical streamlined closed-box girder with additional attachments. The results showed that there are three potential lock-in ranges of vertical VIVs. The 1st lock-in range is excited and sustained by the separated vortex formation at the trailing edge, which is referred to as trailing-edge-vortex-dominated VIV. In the 2nd and 3rd lock-in ranges, the contribution of local aerodynamic forces to the general vortex-excited forces (VEFs) over the upper surface varies with the local positions in the form of simple harmonic curves. Therefore, the local aerodynamic force can be simplified by a concentrated force with an approximately constant loading amplitude and drift velocity, which is illustrated as a simplified moving vortex; based on this, an SVM can be established and utilized to associate the vortex-drift pattern with multiple vertical VIVs in terms of Strouhal number of the separated vortex. With the help of the SVM, the 2nd and 3rd lock-in ranges are excited and sustained by the separated vortex formation at the leading edge and then by a regular vortex drift pattern over the upper surface of the bridge girder, referred to as impinging-edge-vortex-dominated VIVs. Spoilers installed on the handrails can effectively suppress the separated vortex formation both at the trailing edge and leading edge; thus, vertical VIVs in all lock-in ranges can be mitigated.