Soil-Structure Interaction

DOE-STD-1020-2002

slab interface corresponding to a shear wave velocity, vs, of 3500 fps or lower. For very stiff

structures (i.e., fixed base fundamental frequency of about 12 hz), the effects of SSI may be

significant at shear wave velocities in excess of 3500 fps. In such a case, a fixed base support

would not be appropriate.

Various aspects of soil-structure interaction (SSI) result in reduced motion of the

foundation basemat of a structure from that recorded by an instrument on a small pad. Such

reductions are conclusively shown in Reference C-37 and the references cited therein. These

reductions are due to vertical spatial variation of the ground motion, horizontal spatial variation

of the ground motion (basemat averaging effects), wave scattering effects, and radiation of

energy back into the ground from the structure (radiation damping). These effects always result

in a reduction of the foundation motion. This reduction tends to increase with increasing mass,

increasing stiffness, increasing foundation plan dimensions, and increasing embedment depth.

Soil-structure interaction also results in a frequency shift, primarily of the fundamental

frequency of the structure. Such a frequency shift can either reduce or increase the response of

the structure foundation. It is always permissible to do the necessary soil-structure interaction

studies in order to estimate more realistic and nearly always lesser foundation motions. It is also

permissible, but discouraged, to ignore these beneficial SSI effects and assume that the DBE

ground motion applies at the foundation level of the structure. However, any frequency shifting

due to SSI, when significant, must always be considered. If SSI effects are considered, the

seismic analysis should be peer reviewed.

For structures subjected to earthquake excitation, the solution of the dynamic response

of the coupled soil-structure system involves the following basic elements:

(i)

Characterization of the site including evaluation of local soil/rock stratigraphy,

low-strain soil and rock dynamic properties and soil nonlinearities at

earthquake-induced strain levels, ground water location, and backfill configuration and

dynamic properties.

(ii)

Evaluation of free-field input excitation including the effects of local soil conditions.

DBE ground motion including the effects of local soil conditions were discussed in

Section C.3.

(iii)

Development of a model adequately representing the mass, stiffness and damping of

the structure.

(iv)

Evaluation of foundation input excitation including scattering (modification) of the

free-field motions due to the presence of the foundation soil excavation and behavior at

the structure-foundation interface. This step is sometimes called the kinematic

interaction problem.

(v)

Evaluation of foundation stiffness or impedance functions defining the dynamic

force-displacement characteristics of the soil.

C-24