Abstract

The accurate analysis of the seismic response of isolated structures requires the incorporation of the flexibility of supporting soil. However, it is often customary to idealize the soil as rigid during the analysis of such structures. In the present paper, seismic response time history analyses of base-isolated buildings modelled as single degree-of-freedom (SDOF) and multi degree-of-freedom (MDOF) systems with linear and nonlinear base models considering and ignoring the flexibility of supporting soil are conducted. In the study, a large number of parametric investigations for different earthquake excitations, natural periods, base isolations and soil properties, are considered. The behaviour of the underlying soil is modelled through a lumped parameter model which represents the soil foundation system with swaying and rocking spring-dashpots. Laminated rubber bearings (LRBs) as well as high damping rubber bearings (HDRBs) are used as the isolation systems. Responses of the isolated buildings with and without SSI are compared under different ground motions leading to the following conclusions: (1) soil flexibility may considerably influence the stiff superstructure response and may only slightly influence the response of the flexible structures; (2) the use of HDRBs for the isolation system induces higher structural peak responses with SSI comparing to the case of the system with LRBs; (3) although the peak response is affected by the incorporation of soil flexibility, it appears insensitive to the variation of shear wave velocity in the soil; (4) the response amplifications of the SDOF system get closer to unit with the increase in building natural period, indicating inverse relation between SSI effects and natural periods for all the considered ground motions, base isolations and shear wave velocities; (5) the incorporation of SSI increases the number of significant cycles of large amplitude accelerations for all the storeys, especially for earthquakes with low and moderate PGA levels; (6) buildings with linear LRB base-isolation system exhibit larger differences in displacement and acceleration amplifications, especially at the level of lower storeys.

Authors (3)