What differentiates a 'direct' method from a 'substructure' method in Soil-Structure Interaction (SSI) analysis?

The key difference between a 'direct' method and a 'substructure' method in Soil-Structure Interaction (SSI) analysis lies in how the soil and the structure are modeled and analyzed. In a direct method, the soil and the structure are modeled together in a single finite element model. The entire soil-structure system is analyzed simultaneously, capturing the interaction effects directly. This approach requires a large computational domain to accurately represent the soil's behavior, especially when considering nonlinear soil behavior. In a substructure method, the SSI problem is divided into two or more separate analyses. First, the free-field ground motion at the site is determined without the structure present. Then, the dynamic stiffness and damping characteristics of the soil surrounding the foundation are calculated. Finally, the structure is analyzed separately, with the foundation supported by the soil's dynamic stiffness and damping. This method simplifies the analysis by decoupling the soil and structure, but it requires assumptions about the soil-structure interface and the nature of the free-field ground motion. The direct method is generally considered more accurate, especially for complex soil conditions and nonlinear soil behavior, but it is also more computationally demanding. The substructure method is more efficient but may be less accurate, particularly for cases with significant nonlinear SSI effects. The choice between the two depends on the complexity of the problem, the desired level of accuracy, and the available computational resources.