              !"# "$   %&’ NONLINEAR SOIL–FOUNDATION INTERACTION: NUMERICAL ANALYSIS Ioannis ANASTASOPOULOS 1 , Marianna LOLI 2 , Fani Gelagoti 3 , Rallis KOURKOULIS 4 , George GAZETAS 5 ABSTRACT Modern theoretical studies and experimental investigations of the dynamic response of soil–footing– structure systems have revealed the fallacy behind the prohibition of nonlinear foundation response, which is currently one of the cornerstones of aseismic design. Shallow foundations have been found to unavoidably respond nonlinearly, experiencing uplifting and/or bearing capacity failure mechanisms when subjected to seismic episodes of significant magnitude. What is more, such nonlinear behavior appears to have a beneficial role in the performance of the supported structure. Yet, before allowing foundation nonlinearity in engineering practice, it is essential to develop valid and comprehensible tools for modeling the nonlinear rocking behavior and predicting the associated foundation permanent displacements with sufficient accuracy. To this end, a numerical methodology has been formulated — which makes use of a simplified but fairly comprehensive constitutive soil model — and implemented within the ABAQUS FE code. The methodology is rigorously validated through the reproduction of a variety of physical model tests conducted on different soils (sand and clay) and at different modeling scales (making use of both large scale and reduced scale experiments). The paper presents the results of this validation procedure, showing that the numerical method is capable of reliably reproducing the details (ultimate capacity, stiffness degradation with increasing rotation, hysteretic response, settlement–uplifting behavior in relation to the rotation amplitude and the number of loading cycles) of cyclic foundation response. Keywords: Foundation rocking; nonlinear behavior; numerical analysis, constitutive modeling INTRODUCTION Figure 1a illustrates the problem under consideration: a shallow foundation carries the combined N–Q–M loading arising from the gravitational and inertial distress of the superstructure when submitted to dynamic excitation. If the load transmitted to the foundation is significant, strongly nonlinear response may take place at the soil–foundation interface, expressed through loss of contact between the foundation and the supporting soil (uplift) and/or transient mobilization of bearing capacity failure mechanisms (soil yielding). 1 Assistant Professor, Laboratory of Soil Mechanics, National Technical University of Athens, email: ianast@civil.ntua.gr 2 PhD Student, Laboratory of Soil Mechanics, National Technical University of Athens. 3 PostDoctoral Researcher, Laboratory of Soil Mechanics, National Technical University of Athens. 4 PostDoctoral Researcher, Laboratory of Soil Mechanics, National Technical University of Athens. 5 Professor, Laboratory of Soil Mechanics, National Technical University of Athens.