Stability of elastomeric and lead-rubber seismic isolation bearings

Elastomeric and lead-rubber bearings are two commonly used types of seismic isolation devices. During seismic events, some of the bearings in an isolation system will be subjected to large axial compressive loads, caused by gravity plus overturning forces, accompanied by simultaneous large lateral displacements. However, the critical load capacity of elastomeric bearings has been shown to reduce with increasing lateral displacement. The design of isolation systems composed of these types of bearings therefore requires that stability at the maximum displacement be demonstrated. The current procedure to assess the stability uses a ratio of areas, referred to as the overlapping area method, to determine the critical load capacity at a given lateral displacement that must be greater than a combination of axial forces imposed on the bearing. Although the overlapping area method provides a simple means of calculating the critical load at a given lateral displacement, it lacks a rigorous theoretical basis and has not been experimentally verified for bearings with shape factors representative of those used for seismic isolation (i.e.,10-30) or for lead-rubber bearings. Experimental testing and detailed nonlinear finite element analysis were employed to investigate the critical load capacities of an elastomeric bearing and a lead-rubber bearing with shape factors of 10 and 12, respectively, at large lateral displacements. The results of these investigations showed the lead core has a negligible effect on the critical load over a range of lateral displacements corresponding to 150-280% shear strain in comparison with the elastomeric bearing. The overlapping area method is shown to conservatively estimate the critical load capacity of this bearing in comparison with the experimental results.