Gap ratio sensitivity of the aerodynamic performance and flow patterns around twin parallel steel box girders.

The escalating demand for road and rail transport has prompted the widespread construction of twin parallel steel box girder bridges to enhance capacity and mitigate limitations arising from aging infrastructure. However, the airflow around these closely spaced thin-walled steel box girders is highly complex, leading to intricate wind-induced effects. This study investigates the sensitivity of the aerodynamic performance and flow patterns of twin parallel steel box girder bridges to the gap ratio at Re=1.64×10⁴. The dimensionless gap ratio (G/D) is defined as the ratio of the gap width (G) between adjacent steel box girders to the deck height (D), for G/D values ranging from 0 to 30. Wind-tunnel experiments were conducted using surface-pressure measurements, hot-wire anemometry, and smoke-wire visualization. The results reveal that pressure distributions, aerodynamic forces, Strouhal numbers (St), and gap flow patterns vary markedly with increasing G/D. At small gap ratios, shear-layer impingement from the windward box girder induces a secondary wake on the leeward box girder, characterized by a lower harmonic frequency. As G/D increases, the dominant frequencies of the twin steel box girders shift toward those of the isolated steel box girder. Aerodynamic interference between adjacent steel box girders progressively diminishes to negligible levels at G/D=30. Four distinct gap flow patterns are identified over various G/D ranges: (1) overshoot flow (0≤G/D≤0.625); (2) shear-layer impingement flow (0.75≤G/D≤3.125); (3) alternating vortex impingement flow (3.25≤G/D≤30); and (4) flow similar to the isolated flow (G/D→∞).