1.1.2 The Sub-Structure Approach
The substructure method is more complex than the direct method in modelling the SSI system. In the substructure method, the soil–structure system is divided into two substructures: a structure, which may include a portion of non-linear soil or soil with an irregular boundary, and the unbounded soil.
Usually a dynamic soil–structure interaction analysis by the substructure method can be performed in three steps as follows:
1. Determination of foundation input motion by solving the kinematic interaction.
2. Determination of the frequency dependent impedance functions describing the stiffness and damping characteristics of the soil-foundation interacting system. This step should account for the geometric and material properties of foundation and soil deposits and is generally computed using equivalent linear elastic properties for soil appropriate for the in-situ dynamic shear strains. This step yields the so called soil springs.
3. Computation of response of the real structure supported on frequency dependent soil springs and subjected at the base of these springs to the foundation input motion.
It should be noted that if the structural foundations were perfectly rigid, the solution by substructure approach would be identical to the solution by the direct method. Generally, the foundation input motion is assumed to be the same as free-field motion, i.e. the effects of kinematic interaction are neglected in SSI analysis for most of the common constructions. Kinematic interaction should invariably be considered if the structure and foundations to be constructed are very massive, rigid and very large.