What Is a Soil-Structure Interaction (SSI) Analysis?
SSI analysis models the structure and the supporting soil as the coupled system — the soil's flexibility affecting the structure's response (the period lengthening, the damping increasing, the base shear changing), instead of the fixed-base assumption. The analysis: the foundation impedance (the springs and the dashpots), the kinematic vs. the inertial interaction, and the direct or the substructure methods. The seismic design's refinement — the soil's reality in the model.
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Why SSI matters
The period (the flexible soil lengthening — the spectral acceleration changing; the soft soils' — the significant), the damping (the radiation — the energy dissipating into the soil; the effective increasing), and the response (the base shear — the usually reduced; the drifts — the increased; the foundation's — the rocking). The fixed-base's conservatism — the SSI's realism.
Analysis methods
The substructure (the impedance functions — the springs/dashpots; the SASSI — the frequency domain; the practical), the direct (the FEA — the soil and the structure meshed; the nonlinear — the gapping, the sliding; the comprehensive), and the code (the ASCE 7-16 Chapter 19 — the SSI provisions; the reduction — the base shear's; the limits).
When SSI is used
The soft soils (the Site Class E/F — the effects largest), the heavy structures (the nuclear, the LNG — the regulatory), the tall (the period's — the sensitivity), and the performance-based (the PBSD's — the realism). The geotechnical (the shear-wave velocity — the profile; the modulus — the strain-dependent). The SSI is the advanced seismic — the soil and the structure modeled together.
The bottom line
SSI analysis (ASCE 7 Ch. 19) couples soil flexibility into seismic response — period lengthening, radiation damping, usually reduced base shear; use on soft soils and for performance-based design.
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