Theoretical Modeling Of Elastomeric Isolation Bearings And Seismic Response Analysis Of Its Superstructure Including Vertical Load Effects

Base isolation is an effective and economic seismic design concept that affords a high level of protection to a structure from the damage caused by earthquakes.This is achieved by introducing flexible bearings at the base level and lengthen the period of the structures away from the predominant period of the earthquake.In recent years,base isolation reveals high performance in the strong earthquakes,i.e.,Wenchuan Earthquake and Lushan Earthquake,and has been wildly used in high-intensity region.A typical elastomeric bearing consists of alternating layers of rubber bonded to intermediate steel shim plates.The bonded rubber sheets can provide large horizontal deformation capacity and simultaneously sustain large vertical loads.The currently adopted design methods for base isolation assume that the horizontal stiffness of the bearings remains constant and neglect the effects of the vertical load.However,this assumption may not quite consistent with the actual mechanical behavior of the bearings:(i)Experimental results show that the horizontal stiffness of the bearings decreases with increasing vertical loads and reduces to zero when the vertical loads approaches the critical load.Moreover,the critical load decrease with increasing horizontal displacement and thus will lead to the reduction in horizontal stiffness with increasing horizontal displacement.In extreme circumstance,the bearings may buckle under compression;(ii)In engineering projects,the bearings may subjected to eccentric vertical load due to the offset between the center of mass and stiffness,or the geometric centroid,in the isolation layer of the superstructure.In view of this,many researchers have conducted experimental studies and developed several mechanical models of elastomeric bearings to account for the effects of vertical load.All these models can reproduce the experimental results to some extent.However,there still lacks systematic evaluation on the modelling ideas and predictive capacity of the existing analytical models.Moreover,the mechanical behavior of elastomeric bearings under eccentric vertical load have not been studied to date.On the other hand,because the base-isolated structures are assembled by isolation bearings and the superstructure in series,the mechanical behavior and failure pattern of the bearings will significantly affect the dynamic response of the superstructure.When subjected to strong earthquake,especially the near-fault pulse-like ground motion.Meanwhile,the combination of the vertical ground motion and the overturning force of the superstructure will lead to the significant variation in the vertical load of the bearings.As a result,the bearings may suffer a large reduction in its horizontal stiffness or occur instability that will lead to the reduction in the isolation effects or even the overturning of the isolation system.Moreover,most of the former researches concerning the dynamic response of the base-isolated structures neglect the interaction between the horizontal and vertical mechanical behavior of the bearings and did not pay much attention on the failure patterns of the bearings.In view of this,the mechanical model for elastomeric isolation bearings that including vertical load effects is developed based on the work of former researchers in this paper.Then the dynamic analysis of base-isolated structures under the combined action of the horizontal and vertical components of the near-fault ground motion is conducted.This research mainly contains the following aspects:(1)The controlling mechanisms for predicting the critical loads of elastomeric isolation bearings in the framework of the Koh-Kelly model are identified as degeneration constitutive rules in the rotational and shear springs through theoretical derivation.Then the predictive capacity of the existing analytical models are evaluated.Based on the evaluation study,the Iizuka model is improved in an effort to gain more accurate predictions;(2)Full scale bearing tests under eccentric vertical loads are designed and conducted to investigate the effects of eccentric vertical loading on the horizontal mechanical behavior of the bearings.Based on the experimental results,an analytical model that can account for the effects of eccentric vertical loading is proposed in the framework of the Koh-Kelly model.Moreover,the effects of different eccentricity in vertical loads on the horizontal and vertical mechanical behavior of the bearings are investigated via the analytical model.(3)On the basis of the pre-proposed vertical load dependent bearing model.A nonlinear analytical model for the LRB base-isolated structures is established in the ABAQUS software.Then the seismic response analysis of the structures under the combined action of the horizontal and vertical components of near-fault pulse-like ground motion is conducted.The effects of different parameters of bearings(including the bearing diameter,the bearing yield force,the eccentricity in vertical load)on the failure pattern of the bearing and nonlinear seismic response of the superstructures are investigated.