LONGITUDINAL DAMPING LEVEL OF GIRDER MOTION OF LONG-SPAN CABLE STAYED BRIDGE WITH LARGE LONGITUDINAL FLUID VISCOUS DAMPERS

A typical application in seismic design of energy dissipation is the installation of longitudinal large fluid viscous dampers (FVDs) between pylons and girders. The determination of parameters of FVDs is based on the optimal effects of longitudinal displacements of girders and seismic forces of the bridge key components. The fundamental period of girder longitudinal sway motion is long and far away from the site predominant period because the long-period compositions in seismic wave are scare. Therefore, the longitudinal seismic girder displacement is insensitive to damping increase and the determination of the FVDs’ parameters might lead to much too high damping level of girder longitudinal movement. Based on present researches, the estimation model of single degree of freedom (SDOF) and corresponding estimation parameter are put forward. Taking a long-span cable stayed bridge as example, the reliability of the SDOF estimation model is verified through a comparison with the whole bridge model. Further, the too much high damping level of girder longitudinal movement caused by FVDs is explained by the calculated using the estimation model and free vibration results of the whole bridge model. The damping level of girder longitudinal movement is so high that it exceeds the damping limitation of conventional use of FVDs and changes the dynamic boundary of the girder.