Classical and non-classical time history and spectrum analysis of soil-structure interaction systems

The problem of non-classical dynamic analysis of structures resting on flexible bases is studied in this paper. Because of presence of the underlying soil in the dynamic model of structure that acts like an energy sink, the damping matrix is not proportional to structural mass and stiffness and theoretically a non-classical approach should be followed in modal analysis. Considering one to twenty-story buildings, two types of soils, and several suits of ground motions each containing ten earthquake records specifically selected for each building, the seismic responses are calculated using a time history modal analysis in this paper. Three cases are considered: fixed-base buildings with classical analysis, flexible-base buildings with classical and non-classical analysis. It is shown that the code-based soil-structure interaction (SSI) analysis for the fundamental mode is not always safe. Also, on each soil type, instances of importance of accounting for the non-classical nature of the SSI system are clarified. Cases for which the base flexibility should be considered for the higher modes too are distinguished. Finally, simple correction factors are derived for converting the fixed-base responses of moment frames, resting on surface foundations on medium and soft soils, to the responses including soil-structure interaction effects.