Multi-Axial Hysteretic Models of Laminated Rubber Bearings

Base-Isolation system using laminated rubber bearings is considered to be an efficient technology of providing a mitigation of seismic damage for structures and equipments, and has proven to be reliable and cost-effective. There are many structures in various countries, which concern not only new constructions but also existing structures, especially after the Loma Prieta (1989), Northridge (1994) and Kobe (1995) earthquakes. Since seismic response of base-isolated structures greatly depends on the mechanical properties of the bearings, deep understanding of the characteristics of the bearings under the conditions of interest is essential toward a rational design of the base-isolation. Considering that the base isolation system in the two horizontal directions has already been used in the bridges and buildings, behaviors of the bearings under the multi-axial loading should be studied experimentally. In the actual design of multi-axial base-isolated structures, bilinear models have often been used independently in the two horizontal directions, and the coupling effects of hysteretic loops are not considered (AASHATO 1999; JRA 1998). Mokha et al. (1990) investigated the behaviors of Teflon friction bearings under tri-axial loading, and proposed a model for this type of bearings (Constantinou et al. 1990; Mokha et al. 1993). Multi-axial behaviors of a HDR and a steel damper have been investigated by Yasaka et al. (1988), and they empirically developed a multi-spring model, in which several bilinear springs or Ramberg-Osgood springs are circularly arranged with regular angle intervals. Huang et al. (2000) investigated the characteristics of tri-axial behaviors of a LRB and proposed a multi-axial model by considering a circular force interaction function of the bilinear model. However, systematic studies have not been conducted on the experimental investigation of several types of laminated rubber bearings. In addition, previously proposed models for laminated rubber bearings do not have definite theoretical backgrounds in the extension into multi-axial case, and they also do not consider the hardening behaviors in the large deformation range. In this paper, multi-axial restoring forces of various types of laminated rubber bearings, which have widely been used in the base-isolated bridges, are investigated experimentally. Then, a one-dimensional and two-dimensional models of the bearings are proposed on the basis of the experimental results.