Foundation Input Motion

DOE-STD-1020-2002

foundations consisting of individual column or wall footings, consideration of relative

displacement between footings may also be required.

Foundation Input Motion. Developing foundation input motion including the

variation of ground motion over the width of the foundation necessitates consideration of

nonvertically propagating earthquake wave motion. Vertically propagating shear and

compressional waves may be assumed for an SSI analysis provided that torsional effects due to

nonvertically propagating waves are considered. An accidental eccentricity of 5 percent can be

implemented to incorporate possible effects of non-vertically incident waves (Ref. C-37).

Variation of amplitude and frequency content with depth may be considered for embedded

structures. Input motion to the boundaries of soil models shall be compatible with the design

earthquake specified at the finished grade in the free-field. The motions shall be established as a

function of the soil properties, the type of waves propagating during the earthquake, and the type

of boundary assumed. The analyses to establish boundary motions shall be performed using

mathematical models and procedures compatible with those used in the SSI analysis. The design

earthquake control motion defined at the free-field surface may be input to the massless rigid

foundation in an impedance method SSI analysis. When the control motion is used as the input,

rotational input due to embedment or wave passage effects need not be considered.

Alternatively, the input motion to the massless rigid foundation may be modified from the

control motion at the free-field surface to incorporate embedment or wave passage effects,

provided the corresponding computed rotational inputs are also used in the analysis.

Soil Properties and Modeling. Subsurface material properties shall be determined by

field and laboratory testing, supplemented as appropriate by experience, empirical relationships,

and published data for similar materials (refer to DOE-STD-1022 Reference C-38). Soil

properties needed for conducting equivalent-linear analyses include: shear modulus, damping

ratio, Poisson's ratio, and total unit weight. The shear modulus and material damping ratio used

to evaluate foundation impedance shall be values compatible with the shear strain level induced

in the foundation medium during earthquake excitation. The shear modulus decrease and

damping increase with increasing shear strain in soils shall be accounted for when performing an

SSI analysis. The behavior of soil, though recognized to be nonlinear with varying soil shear

strain, can often be approximated by linear techniques. Nonlinear soil behavior may be

accounted for by: (1) using equivalent linear soil material properties typically determined from

an iterative linear analysis of the free-field soil deposit; or (2) performing an iterative linear

analysis of the coupled soil-structure system. The variation in shear modulus and damping as a

function of shear strain for sands, gravelly soils, and saturated clays can be found in References

C-42, C-43, and C-44. At very small strains (percent), the material (hysteretic) damping ratio

shall not exceed 2% of critical. In no case should the material damping ratio exceed 15% of

critical (Ref. C-30). Poisson's ratio, in combination with shear modulus defines the Young's

modulus of the material in accordance with the theory of elasticity. For saturated soils, the

behavior of the water phase shall be considered in evaluating Young's modulus and in selecting

values of Poisson's ratio.

Determination of Foundation Impedances. Foundation impedances may be evaluated

by mathematical models or by published formulas giving soil spring and dashpot coefficients.

C-26