In recent years, many logistics centers have been constructed using the pile top seismic isolation system because it enables significant cost reductions in underground construction. However, in this system, the laminated rubber bearing's bottom part easily undergoes bending rotation because the thin foundation girders have low stiffness. If bending rotation occurs, the horizontal stiffness of the laminated rubber bearing 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). Those phenomena significantly affect the structural characteristics of the pile top seismic isolation building.
The structural design of the piles in the base isolation buildings is generally performed by the seismic deformation method based on the static analysis, and this method is true for the pile top seismic isolation buildings. In order to evaluate the bending rotational deformation of the laminated rubber bearing and the stress distribution accompanying it, the bending rotational stiffness of the laminated rubber bearing is often modeled. However, it is often modeled simply based on designer judgment. Because the bending rotation stiffness of the laminated rubber bearing has various nonlinearities, the method of its modeling is not established in current circumstances.
In this paper, the bending moments of the piles in pile top seismic isolation buildings are compared with earthquake response analyses and the conventional seismic deformation method based on static analyses, and some problems are revealed in the conventional method. As a result of examining the causes of problems, we propose the static analytical models and an application method of seismic deformation method considering the problems and bending moment distribution ratio αM 2). In addition, the consistency and safety factors of pile's stress in the proposal seismic deformation method are quantitatively evaluated.
The following conclusions are obtained through numerical experiments on the pile top seismic isolation building.
1. As a result of comparing with the conventional seismic deformation method and earthquake response analyses for the pile's bending moments, we clarify that the static analytical model of the conventional seismic deformation method can't accurately evaluate the influence of nonlinearity of the laminated rubber bearings.
2. In the pile top seismic isolation buildings, the bending moment of the pile top may be maximized when the bending moment due to the P-Δ effect becomes 0.
3. We propose an application method of the seismic deformation method considering the phenomenon described in 2 and the bending moment distribution ratio αM, and we clarify that the proposed method can reasonably evaluate the pile's stress in the pile top seismic isolation buildings.
4. As a result of quantitatively evaluating the consistency and safety factors of proposal seismic deformation method to earthquake response analyses, we clarify that the proposed method is most reasonable. In addition, regarding the stress of the pile top area, we found that there is a tendency that more proper evaluation is obtained by using simple sum rather than SRSS of the seismic deformation method.
