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 condi- tions and the soil–structure interaction (SSI), in order to evaluate quantitatively the known differences between the two types of condition. Seven accelero- grams scaled at the same PHA, regarding the estimated seismicity of Catania, were adopted. Two different approaches were used to study soil-nonlin- earity, which is extremely important in soil mechan- ics: 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 ac- cording 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 stan- dards for buildings, 2008).