Earthquakes - Geotechnical Engineering Investigation - page 977

11.4.7 Soil-Structure Interaction Analysis (SSI)

General
The purpose of a SSI analysis is to evaluate a coupled bedrock-soil-structure system
including resonance and feedback effects, for foundations on or below the surface.

Feedback from Structural Oscillations
For relatively light structures founded on rock or strong soils, the influence of the struc-
ture is minimal and the structural model excitation is essentially the same as for the pre-
scribed ground motion. The situation is different for massive structures on strong soils and
conventional structures on deep, weak deposits. Feedback of structural oscillations to the
underlying soils, in these cases, may significantly affect the motion at the soil-structure
interface, which in turn may result in amplification or reduction of the structural response.
Deformation of the soil formation may also be caused by the feedback from the horizon-
tal, vertical, or rotational oscillatory motion of the structure. The problem is complicated
by founding below ground level, the usual procedure for most heavy structures.

Soil Classes in Seismic Loadings
Stable soils undergo plastic and elastic deformations but will dampen seismic motion, will
maintain some characteristic strength level, and are amenable to SSI analysis. Both the
dynamic input to the soil from the excitation of the underlying rock (which basically
applies a shearing stress to the soils), as well as the feedback from structural oscillation,
should be considered in dynamic analysis, for which dynamic soil properties (shear mod-
ulus and damping ratio) should be used (see Section 11.3.2) .
Unstable soils are subject to a sudden and essentially complete strength loss by liquefac-
tion, or sudden compression resulting in subsidence, and are not readily considered in SSI
analysis.

Half-Space Analysis
The Model
An early approach to SSI analysis was the half-space analysis (Seed et al., 1975). The soil
effects on structural response are represented by a series of springs and dashpots (energy
adsorbers) in a theoretical half-space surrounding the structure as shown in Figure 11.46.
The approach has limitations when applied to buried structures and is best used to ana-
lyze surface structures.

Ground Motions
For the problem illustrated, the horizontal earthquake motion was specified at the ground
surface in the free field (a location where interaction between soil and structure is not occur-
ring). It was assumed that the ground surface motions for the relatively thin soil deposit
were the result of vertically propagating shear waves and the maximum accelerations and
corresponding time histories were found for other depths down to the bedrock surface. A
maximum acceleration at the foundation base level was selected. (The procedure is
described in Schnabel et al., 1971).

Analysis
Representative values for the spring constants were computed and an analysis of struc-
tural response made for damping ratios of 7 and 15%, which led to values of maximum
acceleration at the base of the structure of 0.38 and 0.32 g , respectively.