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 a wholly equivalent dynamic response, to that from the Substructure Method, with the advantages of being robust enough and rather efficient for self-implementations, possibly employing commercial FEM computer programs.