What Is Soil-Structure Interaction (SSI)?
SSI (the ASCE 7 19's — the foundation's) accounts (the soil's — the flexibility; the damping's) in (the response — the lengthened; the reduced). The interaction: the kinematic's (the averaging — the embedment's), the inertial's (the rocking — the swaying's), and the modeling's (the springs' — the impedance's; the dashpots'). The building's — the soil's partner.
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How SSI works
The period (the lengthened — the T-tilde's; the 1.1-1.5x's), the damping (the increased — the radiation's; the 5-20%'s), the base shear (the reduced — the 0.7-1.0x's; the 19's), and the rocking (the foundation's — the uplift's; the 19.2's).
Modeling
The springs (the impedance's — the Gazetas'; the 6-DOF's), the dashpots (the radiation's — the damping's; the frequency's), the direct (the FEM's — the soil's; the modeled), the substructure (the impedance's — the applied; the SASSI's), and the 19 (the ASCE 7's — the chapter; the procedure's).
When it matters
The soft (the soils' — the Site E's; the significant), the stiff (the buildings' — the short's; the period's — the lengthened), the rocking (the isolated's — the designed; the uplifting), the piles' (the kinematic's — the bending's; the 2x's), and the conservatism (the fixed's — the base's; the 0.7's — the floor's). The SSI is the realistic — the soil's flexibility included.
The bottom line
Model SSI per ASCE 7 Ch. 19 (impedance springs/dashpots or direct FEM) — lengthens period 1.1-1.5x, adds radiation damping, may reduce base shear to 0.7x; critical on soft Site E soils.
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