Document Type : Structural Earthquake Engineering
Isfahan University of Technology
The problem of non-classical dynamic analysis of structures resting on flexible bases is studied in this paper. Because of the 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 10 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. Using the non-classical analysis, it is shown that soil-structure interaction effects cannot be recommended to be taken into account for moment frame buildings with the fundamental fixed-base periods smaller than one second. Cases for which the base flexibility should be considered for the higher modes are distinguished too. Finally, it is made clear that on each soil type, when the actual non-classical nature of the SSI system must be accounted for.