Full-Scale Study of Critical Wind-Direction Screening and Threshold-Based Operational Assessment for Double-Decker Buses on the Queensferry Crossing Bridge

High-sided vehicle safety on exposed long-span bridges depends on local wind direction as well as wind speed. Using a previously validated full-scale computational fluid dynamics model of the Queensferry Crossing Bridge, this study presents a two-stage site-specific assessment for double-decker buses. First, 50-year return period hourly means and one-second gust wind conditions from twelve directions are used to screen for the most adverse aerodynamic response. Within the present modelling framework, directions of 210°, 240°, 270° and 300° produce the largest changes in side force, lift force and rolling moment and are selected for further analysis. Second, a threshold-based operational assessment is carried out for these directions using the existing 60 mph gust closure threshold for double-decker buses and a moving-bus URANS (Unsteady Reynolds-Averaged Navier–Stokes) model. Safety factors for sideslip and overturning are evaluated to assess whether direction-dependent local wind effects create different levels of vulnerability under the same threshold. Additional simulations varying gust speed and bus speed examine the trade-off between local wind effects and vehicle speed in the most adverse direction. The framework links directional screening with operational safety assessment and shows that the same 60 mph gust threshold can correspond to materially different risk levels and mechanisms depending on wind direction.