PEER has just published Report No. 2017/08 titled “Influence of Kinematic SSI on Foundation Input Motions for Bridges on Deep Foundations.” It was authored by Benjamin J. Turner, Scott J. Brandenberg, Jonathan P. Stewart.
Seismic design of bridges and other pile-supported structures often utilizes a substructure method of dynamic analysis in which the foundation elements are not explicitly modeled but are replaced by springs and dashpots representing the foundation impedance. The ground motion appropriate for input to the free end of the springs, known as the “foundation input motion” (FIM), differs from the free-field motion (FFM) due to the difference in stiffness and deformation characteristics between the pile(s) and soil, which is typically overlooked in practice. Results of a parametric study of the influence of kinematic pile–soil interaction on FIM are presented. One-dimensional nonlinear ground response analyses were used to define free-field motions, which were subsequently imposed on a beam-on-nonlinear-dynamic-Winkler-foundation pile model. The free-field ground surface motion and top-of-pile FIM computed from these results were then used to compute transfer functions and spectral ratios for use with the substructure method of seismic analysis. A total of 1920 parametric combinations of different pile sizes, soil profiles, and ground motions were analyzed.
Results of the study show that significant reductions of the FFM occur for stiff piles in soft soil, which could result in a favorable reduction in design demands for short-period structures. Group effects considering spatially-variable (incoherent) ground motions are found to be minor over the footprint of a typical bridge bent, resulting in an additional reduction of FFM by 10% or less compared to an equivalent single pile.
This study aims to overcome limitations of idealistic assumptions that have been employed in previous studies such as linear-elastic material behavior, drastically simplified stratigraphy, and harmonic oscillations in lieu of real ground motions. In order to capture the important influence of more realistic conditions such as material nonlinearity, subsurface heterogeneity, and variable frequency-content ground motions, a set of models for predicting transfer functions and spectral ratios has been developed through statistical regression of the results from this parametric study. These allow foundation engineers to predict kinematic pile–soil interaction effects without performing dynamic pile analyses.
While previously available elastic analytical models are shown to be capable of predicting the average results of this study, they do not adequately reflect the amount of variability in the results that arises from consideration of more realistic conditions. The new model is also used to re-examine available case history data that could not be explained by existing models.