On the dynamic behavior of nonlinear lightweight isolator for museum artifacts

Abstract

In earthquake prone areas, museum buildings that contain precious artifacts of our cultural heritage ought to be seismically isolated. Alternatively, a lower cost solution is to seismically isolate certain items of great importance exhibited in the open space or in existing museum buildings, which ought to be designed or strengthened to comply with current codes and regulations. This practice is followed in many instances around the world. The proposed system, adequate for light artifacts up to one tone for a slim design, consists of three plates the one on the top of the other. The bottom plate is fixed and levelled to the museum floor, while the two others are sliding on rails in two mutually orthogonal directions, accommodating any planar motion. The restoring force, for the system developed at the J.P. Getty museum, is provided by springs that are positioned perpendicular to the directions of motion, with their tip rolling on a horizontal V ramp. To avoid impact, when passing from the equilibrium point, a modification is proposed herein considering two alternatives. The first introduces a circular arc within the V ramp and the second uses a symmetric parabolic ramp, which introduces nonlinear features to the behavior in both restoring and damping force. The general equations of motion are determined accounting also for the rolling and sliding of the tip rubber wheel on both geometries. The use of a general parabolic curve leads the system dynamics to a Duffing type oscillator. In addition, a viscous damper in the transverse direction and/or viscous or frictional dampers can be attached along the direction of motion, which in the case of no sliding, determine the overall damping behavior of the isolator. For a specific artifact, the parameters of the system are adjusted aiming at reducing the absolute acceleration transmitted to the artifact for an acceptable displacement capacity. An extensive parametric study is performed to reveal the main features of the behavior and the influence of the different parameters. In addition, the behavior of the system for different earthquake excitations is examined and the key results and conclusions are presented.