Extraction of Nonlinear Hysteretic Properties of Seismically Isolated Bridges from Quick-Release Field Tests

Keywords: Bridges. Seismic isolation. System identification. Lead rubber bearings. Time domain methods. Hysteretic properties. Damping. Bilinear models. Ramberg Osgood models. Quick release tests.  

Abstract: A time domain method is used to identify the hysteretic properties of lead-rubber bearings installed in seismically isolated bridge systems. The longitudinal motion of the superstructure is idealized as a single degree of freedom (SDOF) system, where the total damping effects has been divided into two parts. The most significant component of damping, caused by hysteretic behavior, is described directly by the nonlinear models. The viscous damping component assumed to be proportional to the velocity of the mass, id described by the damping ratio. Two theoretical models are used for modeling the force-displacement characteristics of the rubber-lead bearings: the generalized Ramberg-Osgood model and the bilinear model. A closed form solution for the response of a bilinear SDOF oscillator to quick release excitation was derived and a step by step integration method is used for computing the displacement, velocity and acceleration time histories of the nonlinear SDOF system numerically. The displacement and acceleration time histories of the superstructure observed during quick release tests are compared with theoretical ones in order to identify the important characteristics of the lead-rubber bearings from field experiments. It is shown that this is a simple and efficient method to interpret the data from quick-release field tests and that this method captures the essential in-situ hysteretic characteristics of lead-rubber isolation bearings.