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Many parallel bridges have been recently designed and built around the world to accommodate the ever-increasing volumes of vehicle and rail traffic. Accordingly, the aerodynamic interference around the parallel girders becomes important to investigate for a more accurate estimation of wind-induced effects on bridges. In this study, a wide range of gap-width ratios (0–15) for parallel box girders are selected to investigate the aerodynamic interference using three-dimensional large eddy simulation at zero wind attack angle. Specific aerodynamic characteristics, including the flow structures, pressure distributions, mean values and standard deviations of the force coefficients, Strouhal number as well as spanwise correlations of the aerodynamic coefficients are examined with various gap-width ratios. The detailed comparison work demonstrates a good agreement with the experimental work. The simulation results reveal that all the aerodynamic characteristics are significantly influenced by the gap-width ratio. The three regions are bounded by critical gap-width ratios of G/B = 0.25 and G/B = 5. There are large changes in behavior at G/B = 0.25, whereas, for G/B > 5, interference effects are not evident. The interference effects on both mean and fluctuating aerodynamic coefficients of parallel box girders are also quantified and summarized. • A wide range of gap-width ratios(0–15) for parallel box girders is selected to investigate the aerodynamic interference using three-dimensional large eddy simulation. • The three regions are bounded by critical gap-width ratios of G/B = 0.25 and G/B = 5. There are large changes in behavior at G/B = 0.25, whereas for G/B > 5, interference effects are not evident.. • The aerodynamic interference factors for root mean square of fluctuating force coefficients are proposed to express the impact of aerodynamic interference effect. • The spanwise correlations of force coefficients suddenly increase at G/B = 0.25. [ABSTRACT FROM AUTHOR]