Soil-Structure Interaction

DOE-STD-1020-2002

(vi)

Analysis of the coupled soil-structure system by solving the appropriate equations of

motion.

Acceptable methods for considering SSI include multi-step impedance function

approaches and single step direct methods as described in Sections 3.3.3 and 3.3.4 of ASCE 4

(Ref. C-16). SSI is further addressed in Wolf, 1985, 1988 (Refs. C-31 and C-32). SSI analysis

methods and computer programs commonly used include:

(i)

Soil spring or lumped parameter methods representing foundation impedances by soil

springs and dashpots (see Ref. C-16) and using a two step solution procedure consisting

of impedance analysis and SSI response analysis;

(ii)

CLASSI computer program (Ref. C-33) employing 3-D continuum-halfspace model and

multiple step analysis technique consisting of fixed base structure modal extraction

analysis, foundation impedance and scattering analysis, and SSI response analysis;

(iii)

SASSI computer program (Ref. C-34) employing a 3-D finite element foundation model

and multiple step analysis technique consisting foundation impedance analysis and

combined scattering and SSI response analysis; and

(iv)

FLUSH (2-D) and ALUSH (axisymmetric) computer programs (Refs. C-35 and C-36)

using a discretized finite element halfspace foundation model and solving for SSI

response in a single step.

Horizontal spatial variations in ground motion result from nonvertically propagating

shear waves and from incoherence of the input motion (i.e., refractions and reflections as

earthquake waves pass through the underlying heterogeneous geologic media). In lieu of a more

sophisticated SSI evaluation, the following reduction factors may be conservatively applied to

the input ground response spectra to account for the statistical incoherence of the input wave for

a 150-foot plan dimension of the structure foundation (Ref. C-37):

Fundamental Frequency of the Soil-Structure System (Hz)

Reduction Factor

1.0

10.00

0.90

25.00

0.80

For structures with different plan dimensions, a linear reduction proportional to the plan

dimension should be used: for example, 0.95 at 10 Hz for a 75-foot dimension and 0.8 at 10 Hz

for a 300-foot dimension (based on 1.0 reduction factor at 0-foot plan dimension). These

reductions are acceptable for rock sites as well as soil sites. The above reduction factors assume

a rigid base slab. Unless a severely atypical condition is identified, a rigid base slab condition

may be assumed to exist for all structures for purposes of computing this reduction. For

C-25