In-situ Monitoring of the Force-output of Fluid Dampers: Experimental Investigation

This paper presents results from a comprehensive experimental program on fluid dampers in an effort to extract their force output during cyclic loading by simply measuring the strain on the damper housing and the end-spacer of the damper. The paper first discusses the path of stresses within the damper and subsequently via the use of elasticity theory shows that the experimental data obtained with commercially available strain gauges yield a force output of the damper that is in good agreement with the force output from the load cell. The experimental data show that the proposed arrangement is promising for monitoring in-situ the force output of fluid dampers and detect possible loss of their energy dissipation function. INTRODUCTION The rapid success of fluid dampers as seismic protection devices, in association with the increasing need for safe bridges has accelerated the implementation of large-capacity damping devices in bridges. For instance, the Vincent Thomas suspension bridge, the Coronado bridge and the 91/5 highway overcrossing (Delis et al. 1996, Makris and Zhang 2004) all three in southern California, the Bay Bridge and the Richmond-San Rafael bridge in northern California, as well as the Rion-Antirion cable-stayed bridge (Papanikolas 2002) in western Greece are all examples of bridges that have been equipped with fluid dampers. The main challenge with fluid dampers is whether they will maintain their long-term integrity when placed in such large structures which are subjected to a variety of loads, appreciable dynamic displacements and long-term deformation patterns. Whereas large displacements and velocities are expected during earthquake loading, a prolonged wind loading would increase substantially the temperature of the damper. Similarly, traffic loading which induces vibrations of small amplitude but very long duration may fatigue the damper and eventually be detrimental in the event that installation imperfections are present as was experienced with the fluid dampers installed in the Vincent Thomas Bridge in southern California. In this paper, we present an experimental investigation which examines the feasibility to measure the force-output of fluid dampers by only reading strains from strain gauges that are connected on the damper casing and the steel spacer placed between the double-ended damper and its attachment. 1 Dept. of Civil and Environmental Engineering, University of California, Berkeley, CA 2 Dept. of Civil Engineering, University of Patras, Greece 3 Dept. of Civil and Environmental Engineering, University of California, Berkeley, CA 2 PATH OF STRESSES DURING CYCLIC LOADING OF A FLUID DAMPER Figure 1 shows a photograph of a medium size fluid damper (Force output = 250 kips at piston velocity = 42 in/sec) that is mounted on the U. C. Berkeley damper testing machine located at the Richmond Field Station. One end of the damper is connected to the actuator that is imposing the Motion, while the other end of the damper is stationary (see Figure 1). The damper shown in Figure 1 is identical to the eight dampers installed for the seismic protection of the 91/5 overcrossing located in Orange County, California (Makris and Zhang 2004). It is a doubleended damper in which the piston rod extends in both chambers of the damper on shown schematically in Figure 2 in order to achieve a symmetric mechanical behavior. Figure 2 also shows schematically the stressing of the damper casing during tension and compression of the piston rod. When the piston rod is in tension (case 1) the damper casing is subjected to longitudinal tension; while, when the piston rod is in compression the compressive force is transferred directly at the end of the damper (spacer) via the pressurized fluid at the back chamber; therefore, only tangential (hoop) stresses develop during the bursting of the damper. Fig. 1. View of the fluid damper mounted on the damper testing machine at the Richmond Field Station.