Seismic displacement ratios for soil-pile-structure systems allowed to uplift

Abstract

In this paper, the soil-pile-structure interaction effects on seismic displacement demands are investigated using various dimensionless parameters. The code prescribed procedures for estimating seismic displacement demands are built upon fixed-base structure assumption and neglect the effect of foundation uplift. Tensile index, the ratio of the summation of the tensile strength of all piles to the weight of the structure, is introduced as a novel dimensionless ratio for the seismic assessments. The seismic behavior of the structures with a small tensile index is close to the structures with uplift-allowed shallow foundations. Structures with a large tensile index behave like tied foundation structures. The tensile index paves the path to cover the interaction effects from the soil-structure to soil-pile-structure interaction. The considered simplified structure in this study is a single degree of freedom model with a rigid foundation mounted on a group of piles simulated using the Winkler springs. The piles are simulated using nonlinear uniaxial springs with asymmetrical behavior under tensile and compressive forces. The accuracy of the model is verified by calculating the displacement ratios for four cases; The first three cases representing high tensile index (tied foundation), are verified with the code results, and Case 4, which represents low tensile index (uplift-allowed foundation), is compared to the literature. More than 150,000 nonlinear response history dynamic analyses are conducted for several dimensionless parameters and 20 ground motion records with various intensities and for each analysis, the displacement of the structure, linear and nonlinear hinge plastic rotation, and the foundation rotation and uplift are collected. The results, including elastic and inelastic displacement ratios, are finally compared with those of ASCE41 for shallow fixed-base foundations. It is concluded that for the cases with no pile slippage, the code prediction is accurate enough. However, in cases with high soil-pile-structure interaction effects and low tensile index, the displacement ratios are up to twice the code predictions. • Tensile index is developed to assess the soil-pile-structure interaction effects. • Simplified uplift-allowed SDOF model with piles simulated using Winkler springs. • Elastic and inelastic displacement ratios are compared with the code predictions. • Foundation uplift, tensile index and aspect and periodic ratios are investigated.