## 6 Conclusions  
Soil structure interaction (SSI) presents an important aspect in the complex numerical modelling of buildings under seismic actions. Buildings founded on compliant soils can incline rock or slide which result in seismic energy dissipation, while on other hand the increase of the period of oscillation could potentially lead to higher seismic forces. Therefore, one must be careful when determining beneficial or detrimental soil effects on the building. The goal of this article was to investigate more closely the simplistic SSI modelling parameters and approaches for buildings founded on soft soils represented by sand. Therefore, an existing experimental campaign was studied to learn more about SSI effects and numerical soil modelling. A well-documented experimentally tested model was used to validate the numerical model. The physical model was experimentally tested within the TRISEE project. The physical model comprised of a relatively simple superstructure and a foundation plate. It was founded on a loose Ticino sand embedded in a large rigid tank  
Figure 14: Performance levels in terms of PGA expressed as a function of \(\mathrm{T_{R}}\). and subjected to horizontal cyclic loading. Loose sand case of this experiment was not studied or numerically modelled in detail by other authors up to this day. The soil-structure system was modelled adopting simplified approaches. Simplified numerical models are less time consuming and easily adopted by engineering practice. The soil was modelled using vertical multilinear link elements and the Takeda hysteretic model, while the structure and foundation were modelled using elastic frame and shell elements, respectively. This research shows that for the case of SSI simple numerical models can describe the overall behavior of experimentally obtained data with a satisfactory level of accuracy in the light of rocking of the foundation and energy dissipation in the soil. In contrary, settlement of the foundation soil and behavior of the model in early stages--before plasticity of the soil is achieved--contains larger discrepancies due to imperfections of the experimental setup that were not considered in the numerical model. After the occurrence of pronounced plasticity, under the action of the maximum horizontal force, the numerical model well describes the behavior of the experimentally tested model. In spite of all above, the hysteretic cycles in the moment-rocking angle, as well as rocking time history and the vertical settlement describe the behavior of the experimentally tested model to satisfactory accuracy. Based on the results from the experimental campaign, a case study on a steel building with SSI effects was conducted. A realistic regular five-story steel building designed according to Eurocode regulations found in literature was used. The building was designed as fixed at the base and within this research a single foundation under every column was added. A local river sand from the Drava river was taken as the foundation soil. The building comprised of regularly spaced columns in both directions oriented to form a moment resisting frame in one direction and a braced frame in perpendicular direction. Frames from both directions separately, were studied within this research. Modal and uniform lateral load distributions were used for both of the numerical models in the fixed-base setup and on the soil. Changes in the story drift and target displacement were observed for different levels of peak ground acceleration. Pushover curves were compared for each load case and the corresponding return periods for the models were calculated. It was concluded that the lateral force distribution is important in terms of SSI effects. Models on the soil showed unfavorable mechanisms. However, an increase of the capacity is noticeable for particular lateral load systems. Furthermore, the incremental N2 method showed higher target displacements for the coupled systems as in the fixed-base systems.