Dynamic Characteristics of Buildings Constructed By Pile Top Seismic Isolation System Considering Nonlinearity of Laminated Rubber Bearing

Abstract eng:
A pile top seismic isolation system is used for constructing base-isolated buildings. In this system, seismic isolators are set on the pile’s top directly, and piles are connected with thin foundation girders or a mat slab. In recent years, many logistics centers in Japan have been constructed using this system because it enables significant cost reductions in underground construction. However, this system does have some problems. For example, the laminated rubber bearing’s bottom part easily undergoes bending rotation because the thin foundation girders have low stiffness. This tendency becomes more pronounced in the case of soft ground. If bending rotation occurs, the laminated rubber bearing’s horizontal stiffness reduces under the influence of the horizontal component of the axial load, and its inflection point moves downward from the center height of the device (usually, the point does not move). This, in turn, significantly affects the structural characteristics of the pile top seismic isolation building. In addition, it has been noted that the horizontal stiffness and rotational stiffness of the laminated rubber bearing show deformation-dependent nonlinearity. In order to evaluate such special behaviors of the pile top seismic isolation building appropriately, it is required to consider the dynamic soil–structure interaction and the nonlinearity of the laminated rubber bearing. In particular, analytical models that can consider the geometric nonlinearity and various nonlinear characteristics caused by the bending rotation of a laminated rubber bearing have been proposed. However, highly specialized knowledge is needed to use these models, and such knowledge is difficult to incorporate into general-purpose design software. Therefore, it’s not considered sufficiently their effects in conventional structural design at present. This paper describes the seismic behaviors of laminated rubber bearings in a pile top seismic isolation buildings through numerical experiments by considering the nonlinearity of the laminated rubber bearing and the dynamic soil– structure interaction. The parametric analytical study is performed by the elasto-plastic earthquake response analysis. The analytical model is of the fishbone type, and it represents one span of the logistics center. It consists of a superstructure, seismic isolated layer, thin foundation girders, a pile, soil-pile springs, and free field. The laminated rubber bearing model is based on Haringx’s theory, and this model is constructed using three matrices: horizontal stiffness matrix, geometric nonlinear matrix, and rotational stiffness matrix. The following conclusions are obtained through numerical experiments on the pile top seismic isolation building.

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