Gap effects on the aerodynamic characteristics and flow patterns of a long-span rail-cum-road bridge girder with three separated boxes

Separated box girder configurations enhance flutter stability in long-span bridges but introduce more complex flow interactions within the gaps, potentially inducing vortex-induced vibrations (VIV) at lower wind velocities. This study investigates the effects of gap ratios (G/H) on the aerodynamic characteristics and flow patterns of a long-span rail-cum-road bridge girder with three separated boxes. Stationary wind tunnel tests were conducted for G/H ranging from 0.000 to 7.752. Combined time–frequency analysis, pressure measurements, and smoke–wire visualizations revealed distinct flow characteristics for various gap ratio cases. As G/H increased, the upstream and middle box girders successively influenced the flow through vortex impingement on the windward sides of the downstream box girders. When G/H exceeded a critical threshold, the dominant frequency of the wake flow behind the downstream box girder became pronounced, indicating a general reduction in the influence of the upstream wake. The surface pressure distributions corroborated the observed vortex dynamics. Four distinct gap flow patterns were categorized: Pattern A exhibited overshoot flows in both upstream and downstream gaps for G/H=0.000 to 0.678; Pattern B featured shear layer impingement in the upstream gap combined with vortex impingement in the downstream gap for G/H=0.775 to 1.260; Pattern C displayed vortex impingement in the upstream gap and a secondary vortex street in the downstream gap for G/H=1.357 to 1.841; Pattern D demonstrated alternate vortex shedding in both upstream and downstream gaps for G/H=1.938 to 7.752. The phase lag between the fluctuating lift coefficients of adjacent box girders and the spanwise coherence coefficient elucidated the aerodynamic characteristics of the three-box girder under different gap flow patterns.