Direct method for dynamic soil-structure interaction based on seismic inertia forces

The seismic response of dams is significantly influenced by dynamic Soil-Structure Interaction (SSI) phenomena. Most of the dedicated methods to perform SSI analyses may be classified within two main approaches, namely, the so called Direct Method and the Substructure Method. The present discussion focuses on a specific formulation of the Direct Method, valid for uniform free-field motion, and less-often adopted in the main-stream practical applications, by which the seismic action is converted into inertial loads directly applied to the structure. A Substructure Method implementation is also considered, in canonical form. First, the equivalence of the Direct Method and Substructure Method approaches is theoretically discussed; then, the seismic responses of the Pine Flat Dam gathered from both methods are compared to confirm the analytical outcomes. This study demonstrates that the adopted Direct Method approach provides wholly equivalent dynamic response to that from the Substructure Method, with the advantages of being robust enough and rather efficient for self-implementations employing commercial FEM computer programs.