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This paper describes an along-wind vibration of frame-type pylons of Hakucho Suspension Bridge found from longterm vibration monitoring data. It is shown that the vibration is very harmonic although its amplitude is small and it occurs under certain ranges of wind direction (10~30 degree from the bridge perpendicular axis) and of wind velocity (13~25 m/s). The dominant frequency of the vibration is either 0.6 Hz or 0.8 Hz depending on the wind velocity and these correspond to the natural frequencies of the in-plane pylon dominant modes of the suspension bridge. A series of the wind tunnel experiments using a spring-supported scaled model are conducted under uniform flow and the alongwind pylon vibration was observed and its vibration characteristics is found to be consistent to those observed in Hakucho Bridge. It is also found that the vibration is very sensitive to the incident angle of wind and to the distance between the two columns.

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