A  Study  of  SSI  Effects  Incorpora�ng  Seismic  Wave  Incoherence  within  

the  DOE  Complex    

2014  U.S.  DOE  Natural  Phenomena  Hazards  Mee�ng  

 Carl  J.  Costan�no  and  Associates  

www.cjcassoc.com  

Introduc�on  

  CJC&A  was  commissioned  to  evaluate  the  effects  of  seismic  wave  incoherency  on  the  SSI  response  at  a  DOE  complex  in  the  eastern  US.    The  site  is  characterized  as  a  rela�vely  s�ff  rock  site  with  high  frequency  input  mo�ons.  –  Input  mo�ons  peak  near  30  Hz.  –  Structure  has  a  large  spa�al  area  (160,000  sq.  �.)  –  Site/structure  is  an  excellent  candidate  for  reduc�on  of  high  frequency  mo�ons  due  to  spa�al  varia�on  of  ground  mo�ons.  

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Background    Seismic  wave  incoherence  is  the  spa�al  varia�on  of  the  amplitude  and  phasing  of  seismic  ground  mo�ons  over  an  extended  area.  

  In  terms  of  its  impact  on  SSI,  incoherence  generally  tends  to  reduce  founda�on  mo�ons  rela�ve  to  the  free  field  input  mo�on.  –  Generally,  if  the  founda�on  is  rela�vely  large  and  s�ff,  results  in  reduced  seismic  demands  within  the  structure.    

–  The  amplitude  of  founda�on  mo�ons  generally  decrease  as  the  frequency  of  the  mo�on  increases.  

  Incorpora�on  of  incoherency  in  SSI  has  been  implemented  within  the  nuclear  power  industry  and  is  accepted  by  regulators.  

  Incorpora�on  of  incoherency  in  SSI  within  the  DOE  complex  has  NOT  yet  been  implemented  to  date.  

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Incoherency  Implementa�on    EPRI  has  performed  valida�on  of  the  implementa�on  of  the  U.S.  NRC  approved  methodologies  for  considering  seismic  wave  incoherence  in  CLASSI  and  SASSI.  –  EPRI  report  1015111,  “Program  on  Technology  Innova�on:  Valida�on  of  CLASSI  and  SASSI  Codes  to  Treat  Seismic  Wave  Incoherence  in  Soil-­‐Structure  Interac�on  (SSI)  Analysis  of  Nuclear  Power  Plant  Structures.”  

  This  study  uses  the  SASSI-­‐SRSS  methodology  implemented  in  SASSI2010.  –  Decomposes  the  coherency  matrix  into  spa�al  modes.  –  Transfer  func�ons  are  modified  for  each  spa�al  mode  and  combined  by  the  SRSS  method.  

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Incoherency  Implementa�on  (2)  

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  The  Abrahamson  rock  coherency  func�on  was  used  for  this  study.    This  rock  coherency  model  is  accepted  by  the  NRC  for  both  rock  and  soil  sites.  

Surface  Input  Design  Spectrum  at  the  Site  

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Descrip�on  of  the  SSI  Problem  

  The  SSI  problem  is  characterized  by  the  following:  –  Large  footprint,  squat  shear  wall  structure  –  Soil  profile  is  based  on  excava�on  of  residuum  and  “so�”  weathered  shale  at  the  site,  and  replaced  with  mass  concrete  fill  from  the  top  of  shale  to  the  grade  level.  

– Mass  concrete  is  founded  on  layers  of  weathered  and  unweathered  shale  (3000  fps  <  Vs  <  6000  fps)  

  The  original  SSI  model  incorporates  the  mass  concrete  fill  substructure.  

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Schema�c  of  the  Original  Coherent  SSI  Model  used  for  Design  

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Grade  

Infinite  Halfspace  

Free  Field  Soil  Layers  

Superstructure  

Mass  Fill  Concrete  Wea.  Shale  

Unweathered  Shale  Unweathered  Shale  

Excava�on  Limit  

Substructure  Soil  Model  

Incoherency  Analysis  Models  

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Superstructure  

Mass  Fill  Concrete  Wea.  Shale  

Unweathered  Shale  Unweathered  Shale  

Superstructure  

Free  Field  Soil  Layers  

Superstructure  Founded  on  a  Layered  Halfspace      This  is  the  typical  implementa�on  for  

incorpora�on  of  incoherency  into  SSI.    Interac�on  nodes  are  at  the  surface.  

Interac�on    Nodes  (typ.)  

Superstructure  Founded  on  Embedded  Substructure      This  is  an  embedded  SSI  model.    Interac�on  nodes  are  at  the  sides  and  bo�om  of  

excava�on.  

Incoherency  Analysis  Models  (2)  

  A  simplified  FE  model  of  the  facility  was  generated.    The  FE  model  has  the  following  characteris�cs:  –  Same  dominant  natural  frequencies  of  the  target  facility.  

–  Seismic  mass  and  building  footprint  were  consistent  with  the  target  facility.  

–  Localized  slab/roof  proper�es  around  the  structure  were  varied  to  produce  a  broad  frequency  range  of  response  around  the  structure.  

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Incoherency  Analysis  Parameters    As  previously  shown,  coherent  and  incoherent  SSI  analyses  were  performed  on  the  surface  and  embedded  FE  models.    A  single  determinis�c  SSI  analysis  is  performed  on  each  using  the  BE  soil  profile  for  the  site.    Other  sensi�vity  studies  were  performed  which  are  not  all  covered  here:  – Number  of  spa�al  modes  required.  –  Effect  of  basemat  s�ffness.  –  Rocking  and  torsion  response.  – Other  building  specific  features.  

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Observa�ons  from  the  Surface  Founded  Model  (Superstructure  on  a  Halfspace)  

  The  results  are  consistent  with  industry  experience  and  expecta�ons.    The  large  footprint  and  high  frequency  input  results  in  reduc�ons  in  ISRS  of  up  to  40%  (system  dependent).  –  ZPA  reduces  up  to  30%.  

  No  surprises  or  unusual  behaviors  were  observed.    Addi�onal  rocking  and  torsional  responses  were  observed,  but  were  very  low  magnitude.  –  This  structure  is  highly  symmetric.  

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Coherent

Incoh. 10 Modes

Incoh. 20 Modes

Incoh. 30 Modes

0.5 1.0 5.0 10.0 50.0 100.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Frequency �Hz⇥

SpectralAcc.�g⇥

Incoh. 10 Modes

Incoh. 20 Modes

Incoh. 30 Modes

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

Frequency �Hz⇥

MeanSpectralRatioofIncoh.:Coherent

Coherent

Incoh. 10 Modes

Incoh. 20 Modes

Incoh. 30 Modes

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

Frequency �Hz⇥

SpectralAcc.�g⇥

Incoh. 10 Modes

Incoh. 20 Modes

Incoh. 30 Modes

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

Frequency �Hz⇥

MeanSpectralRatioofIncoh.:Coherent

Typical  Basemat  Level  ISRS  –  Surface  Founded  Structure  

5%  Damped  ISRS   Mean  Spectral  Ra�o  Horizontal  

Ver�cal  

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Coherent

Incoh.

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Frequency HHzL

SpectralAcc.HgL

Incoh.:Coh. Mean

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

Frequency HHzL

MeanSpectralRatioofIncoh.:Coherent

Coherent

Incoh.

0.5 1.0 5.0 10.0 50.0 100.00

1

2

3

4

Frequency HHzL

SpectralAcc.HgL

Incoh.:Coh. Mean

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

Frequency HHzL

MeanSpectralRatioofIncoh.:Coherent

Typical  ISRS  at  Intermediate  Floor  –  Surface  Founded  Structure  

5%  Damped  ISRS   Mean  Spectral  Ra�o  Horizontal  

Ver�cal  

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Example  Results  for  Rocking  and  Torsion  Responses  

Observa�ons  from  the  Embedded  Model  (Superstructure  on  a  Substructure)  

  In  addi�on  to  the  surface  founded  case  (typical  implementa�on),  the  embedded  substructure  model  was  analyzed.  

  The  purpose  was  to  inves�gate  the  impact  of  embedment  effects  on  incoherent  response  rela�ve  to  the  surface  model.  

  In  general,  high  frequency  ISRS  amplitude  reduce  up  to  ~30%.  

  However,  at  several  loca�ons  on  the  basemat  there  are  points  of  increased  ISRS  rela�ve  to  the  coherent  analysis.  

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Superstructure  

Mass  Fill  Concrete  Wea.  Shale  

Unweathered  Shale  Unweathered  Shale  

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Coherent

Incoh.

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Frequency HHzL

SpectralAcc.HgL

Incoh.:Coh. Mean

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

Frequency HHzL

MeanSpectralRatioofIncoh.:Coherent

Coherent

Incoh.

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Frequency HHzL

SpectralAcc.HgL

Incoh.:Coh. Mean

0.5 1.0 5.0 10.0 50.0 100.00.0

0.2

0.4

0.6

0.8

1.0

Frequency HHzL

MeanSpectralRatioofIncoh.:Coherent

Typical  ISRS  at  Intermediate  Floor  –  Surface  Founded  Structure  (X  Direc�on)  

5%  Damped  ISRS   Mean  Spectral  Ra�o  Surface  Structure  

Embedded  Model  

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Basemat  ISRS  –  Basemat  Level  

5%  Damped  ISRS   Mean  Spectral  Ra�o  X  Direc�on  

Y  Direc�on  

Overall  Observa�on  

  This  large  footprint  structure  on  a  s�ff  site  is  an  excellent  candidate  for  the  applica�on  of  seismic  wave  incoherency.  

  High  frequency  ISRS  reduc�ons  are  observed,  consistent  with  industry  experience.  

  The  method  of  modeling  the  SSI  problem  can  impact  the  results:  –  The  standard  surface  implementa�on  of  incoherence  behaves  consistent  with  expecta�ons.  

–  The  applica�on  to  an  embedded  problem  can  introduce  ar�facts  in  the  response  not  observed  in  the  surface  case.  

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