NUMERICAL SIMULATION AND FIELD MEASUREMENT OF DYNAMIC RESPONSES OF BRIDGES WITH TWIN CIRCULAR-CYLINDER PIERS SUBJECTED TO WATER FLOW

The Dalin Bridge in Tibet is a 7-span concrete beam bridge with a continuous bridge deck, supported with several twin-circular cylinder piers. In July 2018, pier and deck of Dalin Bridge suffer from significant vibration along the bridge direction under the action of water flow. This paper described the field measurements and numerical simulations of dynamic response of the bridge system subject to water flow. The field measurement showed that: the longitudinal vibration of bridge deck is a beat vibration dominated by its fundamental mode while the lateral vibration is random vibration; the longitudinal maximum acceleration is about 0.08 m/s², maximum displacement is about 1.56 mm. Based on the first-order longitudinal fundamental mode, the bridge is simplified as a SDOF system, two-dimensional numerical simulation of twin-circular cylinders is carried out for flow velocity 2 m/s~10 m/s (reduced flow velocity U_r=1.69~8.45, Reynolds number Re=2.6×10⁶~1.3×10⁷), and the lift and drag forces, and the dynamic responses of piers under different damping are obtained. After incorporating a correction accounting for the pier vibration mode and velocity profile of water flow, the vortex-induced vibration amplitude at pier top is derived. The results indicate that the interference effect of upstream cylinder will increase the lift force of downstream cylinder. Vortex-induced vibration (VIV) is observed from flow velocity 3 m/s~6 m/s. The three-dimensional effect has a significant impact on the displacement of pier top in two-dimensional numerical simulation and the numerical simulation result matches well with the measurement when damping ratio ζ=0.01. The maximum VIV amplitude will decrease with the increase of structural damping, but the velocity regime of VIV remains unchanged.