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Base isolation (BI) is a well-known technique with many applications all over the world. This study aims at considering 3D numerical simulations of a soil-structure system applied to a base isolated ordinary building. In particular, the soil has been performed with nonlinear hysteretic materials and advanced plasticity models, implemented in Opensees. The proposed approach enables to drive the assessment of isolation technique with evaluation of soil non-linear response into a unique twist. In this regard, the paper aims at assessing the cases where BI becomes detrimental. In particular, the model of the structure allows to assess the structural performance, by considering accelerations and displacements at various heights. This study overcomes the common simplifications in modelling the soil foundation. In this regard, it can be considered one of the few attempts to propose new design considerations for engineers and consultants. 1155 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 1155-1167 © 2017 The Authors. Published by Eccomas Proceedia. Peer-review under responsibility of the organizing committee of COMPDYN 2017. doi: 10.7712/120117.5482.16931 A. Canini, D. Forcellini 1 BACKGROUND Base isolation (BI) is one of the most convincing solutions in order to protect structures from the destroying effects of earthquakes. This technique allows to decouple the structure from the ground by intentionally concentrating seismic energy dissipation in a unique element with low horizontal stiffness between the foundation and the structure. Many researches have been carried on this issue but few contributions have been focused on the effects of the soil structure interaction (SSI). Extensive research has been conducted in the past 30 years regarding the effects of soil–structure interaction (SSI) on the seismic response of civil engineering structures. Even if many codes suggest to neglect SSI [1], [2] there are some researches where SSI is shown to be non-conservative for safety and cost reduction. In such cases, it is fundamental to take into account SSI in design procedures in order to predict its effects as detailed as possible [3], [4]. The majority of publications study SSI simply by introducing springs, dashpots and artificial masses in the interface between the structure and the soil. This approach is generally accepted since modeling SSI is a challenging problem for numerical simulations. However, it could be insufficiently detailed in order to model the complexity of the problem. In particular, system response is directly connected with the mutual dynamic characteristics (natural frequencies) and thus with structural mass and stiffness, soil shear velocity and layer depth [5]. For this reason, the beneficial effects of BI can be strongly modified by soil deformability and energy dissipation in the ground [6], [8], [9].This study aims at overcoming all previous simplifications considering a 3D numerical simulations of a soil-structure system applied to a shear type building isolated at the base. In particular, the soil has been performed with nonlinear hysteretic materials and advanced plasticity models. The proposed approach enables to drive the assessment of BI technique with evaluation of soil non-linear response into a unique twist. In this regard, the paper aims at assessing the cases where BI becomes detrimental. In particular, the study shows how considering the structure fixed at the base is non conservative and underestimates dynamic effects. 2 CASE STUDY This paper aims at reproducing the seismic response of a benchmark concrete structure on different deformable soils and isolated at the base. The soil has been performed with nonlinear hysteretic materials and advanced plasticity models. This approach enables to reproduce soil hysteretic elasto-plastic shear response (including permanent deformations) and damping foundation impedances by applying the open-source computational interface OpenSeesPL [10], implemented within the FE code OpenSees [11]. In particular, this platform is able to capture the effects of amplification and consequent accumulation of deformation in the ground. At the same time, the model of the structure allows to assess the structural performance, by considering accelerations and displacements at various heights. OpenSeesPL consists of a framework for saturated soil response as a two-phase material following the u-p (where u is displacement of the soil skeleton and p is pore pressure) formulation. This interface, had been originally calibrated for pile analysis. Here it has been modified in order to consider the presence of the system structure – foundation. Based on previous studies [12][13], the soil has been considered a one-layer homogenous cohesive material with a 20 m depth, that consists in a 3D FE mesh (68.4m x 74.4m x 20.5m) composed of 2992 brickUP linear isoparametric 8-nodes elements with 4025 nodes. Horizontally has been discretized in six layers, from center to the outward, while vertically in four layers; 0.5 m thickness for the first and 6.7 m for the others. The model base boundaries were been set at 20.5 m depth from ground surface (Figure 1). OpenSees is able to simulate real wave propagation adopting peri-

3D NUMERICAL SIMULATIONS OF A BASE-ISOLATED RESIDENTIAL BUILDING WITH SOIL STRUCTURE INTERACTION