Inelastic Displacement Ratios for Structures Considering Nonlinear Soil-Pile-Structure Interactions

Nonlinear behavior of soil and pile is critical in seismic performance-based design of structures, while it is often neglected in seismic soil-pile-structure interaction (SPSI). To quantify the effects of soil-pile nonlinearity, a new nondimensional parameter called Pile Strength index, is proposed. This index determines the level of influence of soil-pile-structure interaction on the response of systems. The Pile Strength index is defined such that as it increases, piles tend to behave linearly. Whereas, small values of the index imply highly nonlinear piles, in which soilpile-structure interaction is of paramount importance. As the focus of this paper is on the lateral behavior of pile foundations, a simplified nonlinear sway model is developed to evaluate the seismic response of structures on pile foundations. An extensive parametric study of the model is conducted to reveal the interplay of the proposed Pile Strength index and displacement ratios. For this purpose, a large set of nonlinear seismic time history analyses are carried out. The results are also compared to the displacement ratios proposed by ASCE 41-17. The comparison shows that in highly nonlinear soil-pile systems, inelastic displacement ratios are significantly larger than the recommended displacement ratios by the code.