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SC SOLUTIONS Copyright 2010 SC Solutions, Inc., All Rights Reserved IAEA/NEA

Soil Structure Interaction (SSI) Knowledge and Effect on theSoil Structure Interaction (SSI) Knowledge and Effect on the

Seismic Assessment of NPPs Structures and ComponentsSeismic Assessment of NPPs Structures and Components

Recent Advances in Seismic Soil-Structure

Interaction Analysis of NPPs

Mansour Tabatabaie, Ph.D., P.E.

SC Solutions

NEA/CSNI-IAGE / IAEA ISSC

SSI Workshop

Ottawa, Canada

October 6-8, 2010




SSI ANALYSIS PROCEDURE

Seismic soil-structure interaction (SSI) analysis of nuclear power plants

(NPPs) in the United States is often performed in the frequency domain

using SASSI. This enables the analyst to properly:

a) Address the effects of wave propagation in an unbounded soil

media

b) Incorporate strain-compatible soil shear moduli and damping

properties

c) Specify input motion in the free field using the deconvolution

method and/or spatially variable incoherent ground motions




SSI ANALYSIS PROCEDURE

The original code was published in 1981 and consisted of 6 program

modules. Over the past 30 years, the program SASSI has evolved. A

current version of the code incorporates many enhancements that have

culminated in 28 program modules that perform a variety of tasks.




LAYOUT OF ORIGINAL SASSI CODE




TAPE

2 SITE

CNTRL

POINT

MOTOR

HOUSE

TAPE

3

TAPE

1

TAPE

4

TAPE

9

TAPE

8

TAPE

8

TAPE

8

COMBN8 TAPE

8

TAPE

4

TAPE

4

MOTION STRESS

RANDOM

TAPE

12

TAPE

14,15

TAPE

12

CONVER

TAPE

8n

FBASE

TAPE

1

TAPE

5,10 AXSYM

TAPE

8

ANALYS

MATRIX

LOADS

TAPE

5 SOLVE

TAPE

6

TAPE

10

TAPE

7

TAPE

5 RIMP

PARTIAL LAYOUT OF MTR/SASSI PROGRAM




LARGE-SCALE SSI MODEL

Seismic response of NPPs in the United States is often required for

frequencies up to 33 Hz. In addition, NPPs founded on hard rock in the

Eastern United States are now required to be analyzed to frequencies up

to 50 Hz.

For a typical nuclear island about 100m x 100m in plan dimensions,

embedded 10m in engineered backfill (Vs = 250 m/s) and founded on

hard rock, 3-D detailed FE SSI analysis using SASSI requires:

> Max soil element size ~ 250 / 5 / 50 = 1 m

> No. of interface soil nodes ~ 100 x 100 + 4 x 100 x 10 = 14,000

> No. of virtual soil nodes ~ 100 x 100 x 10 14,000 = 98,600

There could another 60,000 nodes in the structure model to provide

adequate mesh refinement to extract response spectra.




CONVENTIONAL SASSI MODELING &

ANALYSIS CONSTRAINTS

Seismic SSI analysis of NPPs using SASSI has had two main constraints

in the past:

a) Because of large storage and processing requirements, SASSI has

been constrained to coarse soil and structure FE models, which has

made it difficult to address the high frequency passing requirements

for SSI analysis.

b) Because SASSI uses frequency domain procedure, it is limited to

equivalent linear systems, which makes it difficult to address local

nonlinear effects in the structure such as base sliding and/or uplift,

component isolation, etc.




HOW TO ADDRESS PROBLEM SIZE ?




LARGE CORE SOLUTION (LCS) MODEL

The LCS model takes advantage of modern computer technologies,

which now makes it possible to efficiently analyze large-scale, deeply-

embedded nuclear island models in SASSI.

Advantages of LCS:

a) Total SSI system can be analyzed in one step.

b) Provides highly efficient and accurate solution.

c) Eliminates large file storage requirements.

d) Structure model can be a duplicate copy of corresponding FE

model used for design without need for renumbering the nodes

and/or elements. This facilitates model development, translation,

calibration and maintenance for SSI analysis.




Plan View of EPRTM Nuclear island

Nuclear Island

APPLICATION OF LCS MODEL TO LARGE-SCALE

NUCLEAR ISLAND MODEL




GENERIC SOIL PROFILES & PROPERTIES

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