When a structure supported on shallow foundations is subjected to inertial loading due to earthquake ground motion, the foundation may undergo sliding, settling and rocking movements. If the capacity of the foundation is mobilized, the soil-foundation interface will dissipate significant amounts of vibrational energy, resulting in a reduction in structural force demand. This energy dissipation and force demand reduction may enhance the overall performance of the structure, if potential consequences such as excessive tilting, settlement or bearing failure are accounted for. Despite this potential benefit, building codes, particularly for new construction, discourage designs that allow foundation capacity mobilization. This lack of acceptance to embrace soil-foundation-structure interaction (SFSI) as a design inelastic mechanism may stem from the well founded concern that significant uncertainties exist in characterization of soils. More importantly, the lack of well-calibrated modeling tools, coupled with parameter selection protocols cast in a simplistic fashion are lacking.
In this work, a numerical model based on the Beam-on-Nonlinear-Winkler-Foundation (BNWF) concept is developed to capture the above mentioned foundation behavior. The BNWF model is selected due to its relative simplicity, ease of calibration, and acceptance in engineering practice. The soil-foundation interface is assumed to be an assembly of discrete, nonlinear elements composed of springs, dashpots and gap elements. Spring backbone curves typically used for modeling soil-pile response are taken as a baseline and further modified for their usefulness in shallow footing modeling. Evaluation of the model and associated parameter selection protocol is conducted using a suite of centrifuge experiments involving square and strip footings, bridge and building models, static and dynamic loading, sand and clay tests, a range of vertical factors of safety and aspect ratios. It is observed that the model can reasonably predict experimentally measured footing response in terms of moment, shear, settlement and rotational demands. In addition, the general hysteresis shape of the moment-rotation, settlement-rotation and shear-sliding curves is reasonably captured. However, the model consistently under estimates the sliding demand measured in the experiments, perhaps due to the lack of coupling between the vertical and lateral modes of response.
Following the model validation, input parameter sensitivity is investigated using tornado diagram analysis and the First-Order-Second-Moment (FOSM) method. Among the parameters required for the BNWF modeling, the vertical tension capacity and friction angle have the most significant effect on the capability of the model to capture force and displacement demands.
The model is then exercised by studying the response of shearwall-foundation and shearwall-frame-foundation systems. These analyses indicate that if reliably quantified and designed, SFSI has great potential for reducing system level seismic forces and inter-story drift demands.
Finally, the proposed model is implemented within the framework of OpenSees (an open source finite element software package developed by the Pacific Earthquake Engineering Research center) to encourage its use within engineering community.