Understanding the nonlinear behavior of shallow building foundations under large-amplitude loading is an important aspect of performance-based earthquake engineering (PBEE). Soil yielding beneath foundations can be an effective energy-dissipation mechanism; however, this yielding may lead to excessive permanent deformations. The objective of this report is to develop and test procedures to account for foundation nonlinearity and uplift in PBEE
In order to accurately represent PBEE in current design, a Winkler-type finite element mesh generator is written to simulate a shallow strip footing, and nonlinear springs are used to capture permanent displacements. The beam-on-nonlinear-Winkler foundation (BNWF) is chosen due to the familiarity of current engineering practice with the simplified Winkler approach. The model parameters, including material models and varying pressure and stiffness distributions, are calibrated against a suite of model centrifuge and one-g tests with a broad range of design vertical safety factors and by considering both clay and sand soil mediums. Numerical results show that a reasonable comparison between the nonlinear Winkler-based approach and the experimental data (moment, settlement, lateral displacement) can be obtained, given appropriate selection of soil properties.
Simulations using the Winkler model are extended to current code simplified design methods. Current methods account for increased di splacement of an equivalent SDOF system relative to the reduced design strength, but not explicitly for the case of foundation uplift. Suggestions are made to account for foundation uplift in simplified design, as well as for incorporating Housner’s rocking block model and empirical test data to estimate settlement.
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