Efficient Analysis For Seismic Nonlinear Soil-structure Interaction With Deep Soil Layers

The seismic soil-structure interaction(SSI)analysis is usually performed by the direct finite element method.At present,there are two problems: on one hand,for the nonlinear SSI analysis used elastic-plastic constitutive model in soil,the free field gain by site response analysis is far more than the scattered field.The accuracy of site response analysis directly determines the accuracy of SSI analysis.1D site analysis software such as SHAKE will lead to free field results which are inconsistent with the elastic-plastic constitutive model,and the software of SSI analysis have low efficiency due to needing a larger computational model.On the other hand,for the nonlinear SSI analysis with a shallow structure in a deep soil layer,the finite element of the whole soil layer leads to the high degree of freedom of the model and the high computational cost,especially for 3D nonlinear problems.In this paper,for above two aspects,a solution is proposed to improve the finite element method of SSI analysis.The specific research contents are as follows:1.For the first problem,an improved seismic SSI analysis method considering nonlinearity of infinite domain is performed by the direct finite element method.A nonlinear site response analysis scheme based on the 2D or 3D analysis of a column of finite elements with the rigid constraint without rotation is presented to guarantee providing the accurate free field.This method can be used in the analysis of SSI with the same finite element software and soil constitutive model.The viscous-spring boundaries is used approximately here to simulate the scattered field due to the free field is far more than the scattered field,especially in the nonlinear analysis.Numerical results show that the accuracy of the improved finite element method is higher than the result of directly forcing free field load and much higher than used linear elastic free field.2.For the second problem,an accurate and efficient scheme is presented to improve the computational efficiency of the nonlinear SSI analysis with a shallow structure in a deep soil layer.For the shallow structure in a deep soil layer site,the computational cost is very high due to the large finite element domain of the whole deep soil layer,especially for nonlinear case.To save the computational cost significantly,the 2D or 3D nonlinear SSI analysis is performed for a reduced finite element model that is obtained by moving the bottom artificial boundary upward from the bedrock surface to a horizontal location sufficiently near the structure.At the bottom artificial boundary,the seismic site response at the same depth is inputted.The bottom artificial boundary can be assumed as rigid or flexible.The site response at the location of the bottom boundary is imposed at the rigid bottom boundary or is transformed as the equivalent load acting on the flexible bottom boundary with the viscous-spring boundary.3.Comparing the accuracy of four earthquake input including site response with viscous-spring boundary,forcing displacement of site response,free lateral boundary and Tied.The influence of different evaluation criteria,damping ratio,structural size and ground motion on the lateral boundary location is discussed.