Dual Mitigation of Wind-Induced and Seismic Responses in Long-Span Bridges Achieved by Damped Outriggers with Nonlinear Viscous Dampers

Abstract

Long-span bridges are highly susceptible to excessive vibrations in multiple directions induced by wind and earthquakes. Conventionally, vibration control systems are designed separately to addressing wind-induced and seismic responses. This study explores the dual mitigation of these dynamic responses in long-span bridges using damped outriggers (DOs) with nonlinear viscous dampers (NVDs). The DO system consists of outriggers installed on the bridge girder and longitudinal dampers connecting the lower end of the outrigger and a bridge tower or pier. This configuration provides rotational damping to mitigate vertical vibrations and longitudinal damping to suppress the longitudinal response of the girder simultaneously. A comprehensive study has been conducted to design the parameters for DOs of long-span bridges, including the evaluation of external excitations [vortex-induced forces (VIFs) and seismic loads], a corrected modal truncation method for dynamic modeling, and a multiobjective genetic algorithm (MOGA) for optimal parameter design. The framework is validated through numerical application to the Xihoumen Bridge, serving as a representative case. The results demonstrate that DOs installed on the two towers can completely mitigate VIVs across seven modes. In addition, single DO arrangement with independently designed parameters can reduce the longitudinal displacement of the bridge under earthquakes from 18 to 5 cm. Furthermore, MOGA is adopted for optimal design of DOs for dual mitigation, resulting in an optimal system comprising three DOs located at the towers and the north end of the girder. The NVDs in the DOs are designed with an exponent of 0.7 and a viscous coefficient of approximately 10,000    kN · ( s / m ) 0.7 , while the outrigger length is kept no larger than 18 m. The performance of the optimized DOs is found to be comparable to that of systems designed separately for wind-induced or seismic responses. The proposed methodology provides a generalized and transferable design framework for integrated wind and seismic vibration mitigation in long-span suspension or cable-stayed bridges.