This study focuses on the vortex-induced vibrations(VIVs)of narrow streamlined steel box girders of the long-span railway bridge.A railway bridge with a main span of 335 m is used as a case.A 1∶30 scale main girder segment model was prepared for wind tunnel tests,to investigate the VIVs of the main girder segment at different wind attack angles and under different damping ratios.Together with the numerical simulation,the causes of the VIVs were analyzed and vibration mitigation measures were studied.The main girder was not subjected to VIVs at a vertical damping ratio of 0.20％and at the wind attack angles of-5°,-3°and 0°.At wind attack angles of+3°and+5°,vertical VIVs were observed in the main girder,with maximum dimensionless vibration amplitudes of 14.4 and 19.4,respectively.Increasing the vertical damping ratio can effectively suppress the vertical VIVs,but the wind velocity ranges of the dimensionless VIVs showed no change.The plinths of railings were regarded as the main cause of VIVs,the vortex shedding between the railing plinths and the ballast walls as well as behind the railings was closely related to the occurrence of VIVs.After removing the railing plinths,the big vortices were continuously broken into smaller vortices.Sealing up the railings did not show effective mitigation of VIVs.When the rectangular plinths of the railings were removed,and the base of the railings were changed to be catch pits,the vortex shedding effect at the wind fairings on the windward side of the main girder was weakened,resulting in the complete mitigation of vertical VIVs of main girder.
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