The Effect of Soil Structure Interaction on the Behavior of Base Isolated Structures

Abstract This master thesis investigates the effect of soil structure interaction on the behavior of base isolated structures. A linear soil model coupled with a nonlinear model of a base-isolated structure is assembled as a nonlinear system in state-space. Transient responses to earthquake ground motions are computed by integrating the nonlinear equations with a fixed-time step.Earthquake ground motions representative of near fault and far-field conditions are generated and tested for this system. The effects of soil compliance and friction in the base isolation system are then evaluated. An eigenvalue analysis of the dynamics matrix of the couple system gives the evidence that the soil structure interaction increases the periods of the structure positively. It is shown that the base isolation system with large hysteretic frictional force is less effective in achieving the goals of seismic isolation; large isolation-level friction couples the structure above it too tightly, which results in less deformation in base isolator and larger roof acceleration than intended for a seismically-isolated structure. In addition, the base isolation system with large hysteretic frictional force produces larger residual displacements for both base isolator and the above floors. The impact factor of shear wave velocity to the stiffness of soil is more difficult to assess. Therefore, nonlinear liquefied soil model is recommended to be calibrated for soil structure interaction evaluation as well as to be compared with empirical testing. To this end, nonlinear models for liquefying soil are investigated.