Observed Alongwind Vibration of a Suspension Bridge Tower and Girder

Abstract

Wind-induced vibrations of an instrumented suspension bridge recorded during six events are presented in this paper. The bridge has the total length of 1380m consisting of 720m center span and two symmetric side spans of 330m each. Bridge instrumentation consists of 27 channels of vibration sensor placed on fourteen locations and two anemometers deployed on the bridge girder and tower. During six measurement events, wide range of wind velocity with the highest wind velocity reached up to 30m/s was recorded. The study focuses on the wind-induced responses of the bridge tower and girder, particularly the relationship between vibration amplitude and wind velocity. It was observed that while the girder vertical and tower out-of-plane (crosswind) vibrations increase proportionally with respect to wind velocity following a buffeting trend, the tower in-plane (alongwind) vibration under moderate wind velocity from 14 to 24m/s is significantly higher than the buffeting trend. In this wind velocity region, the 131m tower vibrates in its strong axis direction and this alongwind motion is characterized by the single-frequency harmonic-like response with self-limited and relatively constant amplitude that resembles the in-line vortex shedding response. The phenomenon occurs on the downstream tower leg when subjected to wind with certain inclination angles and low turbulence intensity. While the tower single-frequency harmonic-like response associated with vortex-induced vibration is not uncommon during a free-standing construction stage, its occurrence on a completed suspension bridge tower especially on its strong axis is very rare. Observation also shows that the tower in-plane alongwind vibration energy is transferred onto girder lateral motion causing an increase in the girder amplitude vibration within a moderate wind velocity range.