Earthquakes - Geotechnical Engineering Investigation - page 979

TABLE 11.19
Comparison of Half-Space Analysis with Finite Element Analysis a

Consideration Half-Space Theory or Interaction Springs Finite Element Analysis

Deformability of soil profile Usually assumes that accelerations Can take account of deformations

and variation of are constant with depth of profile and variability of

accelerations with depth accelerations with depths

Characteristics of Usually assumes that the motions Can readily take into account

motion below the below the base of the structure the influence of interaction on the

base of structure (and usually around structure) are the characteristics of the motions

same as those in the free field below the base of the structure

Determination of soil motions Provides no means fordetermining Provides means for determining

adjacent to structure motions adjacent to structure motions adjacent to the structure

Determination of soil Characteristics can only be Characteristics can be determined

deformation characteristics approximated on rational basis

Determination of Damping can only be estimated Damping can be appropriately

damping effects characterized and considered

in analysis.

Effects of adjacent structures Effects cannot be considered Effects can readily be evaluated

Inclusion of high-frequency Effects are appropriately included Effects may well be masked by

effects computational errors, including:

(1) the use of a coarse mesh,
(2) the use of Rayleigh damping,
and (3) the use of too few modes

Lateral extent of model Not a factor in analysis Must be large enough to provide

required boundary conditions

Three-dimensional Can be considered in analysis Must be represented by

configuration two-dimensional model

a

From Seed, H.B. et al., Report No. EERC. 75-25, Earthquake Engineering Center, University of California,
Berkeley, August 1975. With permission.

0.25 g

0.18 g

0.16 g

FIGURE 11.47
Finite-element model of
soil-structure system of Figure
11.46. (From Seed, H.B. et al., Report
No. EERC 75-25, Earthquake
Engineering Center, University of
California, Berkeley, August 1975.
With permission.)

Soils

0.145 g

0.145 g

Rock