Environmental Engineering Reference
In-Depth Information
Nuclear power plant
Free-field motion
Free-field motion
Control motion
Should be
alike
g
g
g
g
Period
Period
Period
Period
Fuel
bldg.
Reactor
bldg.
C
adjustad
adjustad
t
1
t
2
t
2
t
2
Base motion
Base motion
Base motion
g
g
g
Same
Same
Period
Period
Period
(c)
(b)
(a)
FIGURE 11.49
Representation of general analysis procedure used in soil-structure interaction analysis: (a) soil deposit model
used in deconvolution analysis to obtain compatible motion of base rock; (b) soil deposit representing free
field with modified soil properties (resulting from turbine building); (c) finite-element model of soil-structure
system. (After Idriss, I.M. and Sadigh, K.,
Proc. ASCE, J. Geotech. Eng. Div.,
102, 663-682, 1976.)
The stress-strain trajectories are cyclic.
●
There are no residual displacements.
●
There is no soil liquefaction.
●
Step 2
: Base-rock motion from Step 1 is then used for analysis of the soil-structure sys-
tem, leading to an evaluation of the motions at any selected points such as the base of the
structure, operating floor, etc.
A number of other computer programs are available that allow for nonlinear dynamic
analysis, as well as consideration of excess pore pressures and the presence of structural
elements. PLAXIS and SASSI 2000 are based on the finite-element method (FEM).
Conclusions
Comparison with Half-Space Analysis
In the FEM model illustrated in
Figure 11.47
of the problem illustrated in
Figure 11.46,
Seed et al. (1975) found that the maximum acceleration at the base of the structure would
be only 0.16
g
, or roughly half that found by the half-space analysis.
Damping was appropriately characterized by soil modulus values compatible with the
strains that developed in the different elements representing the soil deposit. A compari-
son between the FEM and the half-space approach for buried structures is summarized in
Table 11.19.
From the study comparing half-space analysis with the FEM, Seed et al. (1975) con-
cluded that there is still much to be learned about SSI that even sophisticated analyses do
not have the capability to incorporate all aspects of reality, and that considerable judgment
is required in evaluating the results obtained in any analysis.
