Investigation of torsional vortex-induced vibration in twin-box girders: Effects of gap width and flow mechanisms

To investigate the underlying mechanisms of torsional vortex-induced vibration (VIV) of twin-box girders and the influence of gap width on VIV performance, this study conducted sectional model tests with synchronized pressure and vibration measurements across five distinct gap widths. The experimental results reveal that torsional VIV amplitude diminishes as gap width increases. While the mean pressure coefficient remains largely unaffected by variations in gap width, the fluctuating pressure coefficient exhibits pronounced enhancement, particularly on the top plate and windward sloping web of the downstream girder, identifying these regions as primary contributors to torsional VIV excitation. Flow field and Proper Orthogonal Decomposition analysis under stationary and oscillating conditions clarifies the flow mechanism: wider gap widths modify vortex shedding patterns, inducing periodic pressure fluctuations through vortex impingement on critical surfaces. This flow–structure interaction fundamentally drives the onset and development of torsional VIV.