NONLINEAR DYNAMIC SOIL-STRUCTURE INTERACTION ANALYSIS USING HYBRID METHODS

Abstract

In the present research work, a new hybrid method for nonlinear dynamic analysis of soilstructure systems is developed, validated and demonstrated. By combining the frequency domain and time domain methods for 3D problems, an attractive method is formulated. This formulation facilitates the modeling ofboth frequency dependent radiation damping and time varying nonlinearity. Using this hybrid method of dynamic soil-structure interaction analysis, the dynamics of a special type of pile foundations, namely contact pile foundations, are investigated. Also, inelastic analysis of a single pile and pile groups with free-standing pile caps is presented under both external load and seismic conditions. The new hybrid method adopts the substructure based formulation, wherein the total system is divided into two subsystems, namely a linear far-field and a nonlinear near-field. This method uses the frequency dependent consistent boundary formulation for modeling the radiation damping in the unbounded far-field. For this, the similarity-based Consistent Infinitesimal Finite Element Cell Method (CIFECM), alias, the Scaled Boundary Finite Element Method (SBFEM) is used. The resultant dynamic stiffness matrices are assembled along with the dynamic stiffness matrices ofthe bounded near-field. The assembled equations ofmotion are solved in frequency domain to obtain an elastic response to the applied loading. Subsequently, the elastic response is iteratively corrected in time domain for nonlinear effects. For this, the plasticity based advanced nonlinear model, namely the Hierarchical Single Surface (HiSS) model, is used. The resultant inelastic nonlinearity is expressed in the form ofpseudo-force loading, which when applied to the elastic system gives the true inelastic response. This framework ofanalysis is referred as the Hybrid Frequency Time Domain (HFTD) method, in the literature. In the present work, an algorithm is developed to couple the consistent boundaries and the plasticity-based nonlinearity into the framework ofHFTD analysis. The algorithms developed are sufficiently validated under static, dynamic and nonlinear conditions. The implementation of consistent boundaries in frequency domain is validated through the frequency variation of dynamic stiffness coefficients of rigid rectangular prism and floating pile groups, under various excitation conditions. For validating the implementation of HiSS soil models, triaxial test results are used, as reported by pioneers in the field. Also, the overall implementation ofHFTD method is verified by solving benchmark problems of simplified systems representing the inelastic soil-structure interactions. The linear elastic analysis ofpile foundations is performed, both, as a prelude to the inelastic analysis and as a tool to investigate the effect of a ground contacting cap on the soil-pileinteractions. The importance ofmodeling the cap-soil-pile interaction is depicted through the changes in the frequency variation of dynamic stiffness components. The effect of embedment of pile cap is studied along with parametric study. The influence of pile spacing, thickness of pile cap, slenderness ratio of piles and the relative rigidity of pile material, on the dynamic stiffness ofthe soil-pile system is investigated with respect to depth ofembedment. Through the inelastic analysis of the floating pile group models, a clear understanding of the pile distress under seismic conditions can be obtained. In the present work, the change in the dynamic stiffness of 2 x 2 pile group models under inelastic conditions is presented for various degrees of loading and pile-to-pile spacing conditions. Also, the effect of inelasticity on the frequency variation of kinematic interaction factors is shown. Finally, seismic analysis ofinelastic pile groups is analyzed inthe light ofthe frequency response. In the present work, it was observed that the horizontal dynamic stiffness for the pile groups with embedded caps may be less than that for pile groups with free-standing pile caps. Further, the effect of embedment is very much dependent on spacing of piles but not much affected by thickness of pile cap. Also, the effect is more prominent for softer soils. It is concluded that the cap-soil-pile interactions and the plasticity of the supporting soil can have significant influence on the design ofpile foundations.