SUMMARYWhen a structure supported on shallow foundations is subjected to inertial loading due to earthquake motions, the foundation may undergo sliding, settling and rocking movements. Even if the capacity of the foundation is mobilized, the soil-foundation interface may dissipate significant amounts of vibrational energy, resulting in reduced force demands to the superstructure. If the capacity is not mobilized, these movements introduce additional flexibility to the system, which may shift its period away from the potentially hazardous zone of response spectra for most earthquake ground motions. In either case, transient and permanent deformations will need to be accounted for.To consider the aforementioned benefits and consequences in performance-based seismic design, robust and reliable numerical modeling tools are needed. In this article, a Winkler-based modeling framework is proposed to address this issue. The model includes a distributed array of mechanistic nonlinear inelastic springs, dashpots, and gap elements, with backbone curves of the nonlinear springs calibrated against shallow foundation experiments. Model evaluation is conducted by simulating the response of a number of centrifuge experiments. Experiments considered include square and strip footings, bridge and building models, static and dynamic loading, footings on sand and clay, a range of static vertical factors of safety, and a range of aspect ratios. It is observed that the model can reasonably predict 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.

Practical guidelines for characterization of soil-structure interaction (SSI) effects for shallow foundations are typically based on representing foundation-soil interaction in terms of viscoelastic impedance functions that describe stiffness and damping characteristics. Relatively advanced tools can describe nonlinear soil-foundation behavior, including temporary gap formation, foundation settlement and sliding, and hysteretic energy dissipation. We review two tools that describe such effects for shallow foundations and that are implemented in the computational platform OpenSees: a beam-on-nonlinear-Winkler foundation (BNWF) model and a contact interface model (CIM). We review input parameters and recommend parameter selection protocols. Model performance with the recommended protocols is evaluated through model-to-model comparisons for a hypothetical shear wall building resting on clay and model-data comparisons for several centrifuge test specimens on sand. The models describe generally consistent moment-rotation behavior, although shear-sliding and settlement behaviors deviate depending on the degree of foundation uplift. Pronounced uplift couples the moment and shear responses, often resulting in significant shear sliding and settlements. Such effects can be mitigated through the lateral connection of foundation elements with tie beams.

No abstract

No abstract