PERFORMANCE CHARACTERIZATION OF FLUID VISCOUS DAMPERS G. Mosqueda 1 , A. S. Whittaker 2 , G. L. Fenves 3 , and D. Mellon 4 ABSTRACT An experimental study on the use of viscous dampers in seismically isolated bridges has been completed at the University of California, Berkeley. This paper discusses the performance characterization of two fluid viscous dampers used in the earthquake simulator tests of an isolated bridge model. Prior to earthquake testing, the two dampers were subjected to uniaxial compression-tension cycles consisting of sinusoidal tests, constant velocity tests and low-velocity friction tests. Thermocouples were used to monitor the temperature of the damper casing. From these cyclic tests, it was found that the dampers exhibited linear viscous behavior at moderate to high velocities and nonlinear behavior at low velocities due to friction in the damper seals. The damping constant did not change for the range of temperatures developed in the dampers but the seal friction force increased considerably with increasing temperature. Based on these uniaxial tests and the subsequent earthquake simulator tests of the damped isolated bridge model, recommendations regarding prototype testing and acceptance criteria for fluid viscous dampers are presented. Introduction The use of seismic isolation will reduce accelerations in a bridge superstructure and the force transmitted to a bridge substructures, but will increase the relative displacement response of the superstructure. This (substantial) increase in relative displacement may place onerous demands on expansion joints that generally have limited displacement capacity. To reduce the superstructure displacements to levels that can be tolerated by expansion joints, bridge engineers have turned to supplemental energy dissipation in seismic isolation systems that can either be integrated directly into a seismic isolator (e.g., as a lead core in a lead-rubber bearing) or added externally in the form of discrete damping devices. To date, the fluid viscous damper is the only supplemental damping device that has been used in seismically isolated bridges in California. Fluid viscous dampers have been shown to be effective in reducing both superstructure displacements and forces transmitted to bridge substructures (Constantinou et al. 1993). To extend this knowledge and to address other issues related to the analysis and design of seismically isolated bridges, the California Department of Transportation (Caltrans) funded the PROSYS research project at the University of California, Berkeley. Two damper-related objectives of the PROSYS project were: (1) to investigate the effectiveness of supplemental damping in isolated bridges subjected to bi-directional excitation, and (2) study the effect of 1 Graduate Research Assistant, Dept. of Civil Engineering, University of California, Berkeley, CA 94720 2 Associate Professor, Dept. of Civil Engineering, State University of New York, Buffalo, NY 14260 3 Professor, Dept. of Civil Engineering, University of California, Berkeley, CA 94720 4 Associate Bridge Engineer, California Department of Transportation, Sacramento, CA