Comparison Between Two Approaches for Non-linear FEM Modelling of the Seismic Behaviour of a Coupled Soil–Structure System

One basic factor influencing the seismic design of new structures, as well as the retrofitting and/or improvement of existing ones, is the dynamic interaction between the foundation soil and the structure. An accurate investigation of the structure and surrounding soil is the first fundamental step in a realistic evaluation of the seismic performance of the coupled soil–structure system. The present paper deals with the dynamic behaviour of a coupled soil–structure system, i.e. a school building in Catania, characterized by a high seismic hazard. The soil properties were carefully defined by means of in situ and laboratory tests. Different 2D numerical analyses were performed, considering both free-field conditions and the soil–structure interaction (SSI), in order to evaluate quantitatively the known differences between the two types of condition. Seven accelerograms scaled at the same PHA, regarding the estimated seismicity of Catania, were adopted. Two different approaches were used to study soil-nonlinearity, which is extremely important in soil mechanics: firstly, adopting constant degraded shear modula G and increased soil damping ratios D, in line with EC8—Part 5 (2003); secondly, choosing G and D according to the effective strain levels obtained for each different input. The main goals of the paper are: (1) to highlight the importance of considering and not considering the dynamic SSI in terms of: acceleration profiles and soil filtering effect; (2) to evaluate the influence of different modelling of soil non-linearity on the dynamic response of the system; (3) to compare the response spectra obtained with that given by the Italian technical code (NTC in New technical standards for buildings, 2008).