Observed along-wind vibration of a suspension bridge tower

Single-frequency along-wind oscillation under moderate wind velocity was observed on a leeward leg of an H-shaped 132 m suspension bridge steel tower. The oscillation occurred at particular wind velocity range (13–24 m/s) and angle of attack. Two dominant single-frequency oscillations, namely, 0.6 Hz and 0.8 Hz were observed and they had characteristics that resembled the vortex-induced vibration. The single-frequency oscillations 0.6 Hz and 0.8 Hz were related to the tower local in-phase and out-of-phase mode, respectively. Outside the wind velocity range of 13–24 m/s or when wind direction is perpendicular to the tower, the tower responses were mainly characterized by random responses with buffeting trend. The single-frequency tower oscillation influenced girder lateral vibration in that the coupling tower in-plane and girder lateral mode substantially increased the girder lateral vibrations. Wind tunnel experiment on tower model was carried out to investigate the phenomenon under various wind angles of attack and velocities. The results show that under certain angle of attack the bluff body of the windward tower leg created vortex shedding as indicated by the presence of single-frequency oscillation of the wind in front of leeward tower. The vortex shedding generated a periodic force towards the leeward leg that coincides with the tower natural frequency 0.6 Hz and 0.8 Hz in the wind velocity range of 13–17 m/s and 17–24 m/s, respectively, causing in-plane resonance.