Aspect ratio effects on the vortex-induced vibration of π-shaped girders: At positive and negative angles of attack

Vortex-induced vibration (VIV) poses significant challenges to the serviceability of long-span bridges employing π-shaped twin-box composite girders. This study investigates the pivotal yet inadequately understood role of cross-sectional width B-to-height H ratio (B/H) in VIV responses. The aerodynamic performance of sectional models with B/H ratios of 10.2, 11.0, and 12.0 was studied using an integrated approach of wind tunnel tests and numerical simulations, over an angle of attack (AOA) range of −5° to +5°. A key finding reveals a contrasting trend in VIV amplitude with increasing B/H: a slight decrease at positive AOAs, but a marked increase at negative AOAs. Notably, at −3° AOA, the amplitude increases with B/H, which becomes the most critical condition for the girder with the highest aspect ratio. This finding underscores the necessity of evaluating VIV performance across all potential AOAs, with particular attention to negative ones. Parametric studies reveal that Railing II is the dominant factor, contributing approximately 50% to the VIV response, while the girder cross section itself accounts for about 30%. Flow field analyses indicate that the increased deck space at larger B/H ratios alters vortex evolution and pressure distributions, explaining the observed vibrational behavior. These quantitative insights establish a physics-based framework to guide the aerodynamic design and the VIV mitigation of π-shaped twin-box girders.