PERFORMANCE CHARACTERIZATION OF FLUID ...

PERFORMANCE CHARACTERIZATION OF FLUID VISCOUS DAMPERS G. Mosqueda1, A. S. Whittaker2, G. L. Fenves3, and D. Mellon4 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 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 2

different damper configurations on bridge response. These objectives were achieved through coordinated analytical and experimental studies of a small-scale bridge model subjected to multiple components of earthquake excitation. In the experimental studies, two fluid viscous dampers were installed in symmetric and asymmetric configurations on the bridge model. Prior to the installation of these dampers in the model, each damper was carefully characterized using displacement-controlled testing in a uniaxial damper test machine. The results of these damper characterization studies, related information from the earthquake simulator testing of the model, and implications for practice are the focus of this paper. The reader is referred to Mahin et al. (2001) for a more detailed description of the scope and results of the overall research program. Experimental Testing Program The research program described in this paper made use of two 10-kip fluid viscous dampers fabricated by Taylor Devices of North Tonawanda, New York, to specifications developed by the authors. A cross-section through a double-ended damper similar to that used for this research program is presented in Fig. 1. The total stroke of the dampers used in the research program was 12 in. and the pin-to-pin length at mid-stroke was 50 in. Substantial internal pressures are introduced into Taylor seismic dampers under static conditions to facilitate seal function. Forces are developed in the damper by differential pressure on the two sides of the piston head. Fluid flows from one side of the piston head to the other around the perimeter of the piston head and through orifices in the piston head. Different velocity exponents can be achieved by selecting different orifice geometries and piston head-casing clearances.

Figure 1. Cross-section through a double-ended fluid viscous damper (from Taylor Devices). Damper Test Machine The two viscous dampers (Damper 1 and Damper 2) were characterized by uniaxial testing in a Damper Test Machine in the laboratories of the Earthquake Engineering Research Center at the University of California, Berkeley. The Damper Test Machine, shown in Fig. 2a, was originally designed to characterize uniaxial damping devices of the type proposed for the retrofit of the Golden Gate Bridge (Aiken and Kelly 1996). For the study reported in this paper, a damper was installed in the Machine with thermocouples at the bottom of the damper casing. Thermocouples were also used between tests to monitor the temperature of the damper casing near the mid-stroke position of the damper. These thermocouples were removed during the cyclic tests because their installed location interfered with the damper sleeve. A calibrated load

cell installed in-line and beneath the damper measured the axial force history in the damper.

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(a) Damper Test Machine

(b) isolated rigid block model with dampers

Figure 2. Damper Test Machine and rigid block model The fluid viscous damper testing program included three types of signals, labeled as A, B, and C. Test type A was a low-velocity (