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Seismic Soil-Structure Interaction Analysis: A Walk Through Time – Past,
Present, and Future
OECD/NEA IAGE / IAEA ISSC Workshop on
Soil Structure Interaction (SSI) Knowledge and Effect on the Seismic Assessment of NPPs Structures and Components
Ottawa, Canada, 6-8 October 2010
Sponsored by:
OECD Nuclear Energy Agency
International Atomic Energy Agency/
International Seismic Safety Centre (ISSC)
Presentation by:
Dr. James J. Johnson
James J. Johnson and Associates
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The SSI Problem
•Given the free-field motion at the site, determine the dynamic response of soil, structures, and components
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Topics of Presentation
•Historical Perspective
• Elements of SSI
• Present State of Practice
•Anticipated Future Developments
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SSI Analysis Methodologies: Historical Perspective
•Vintage 1960s to 1970s – Rigid disk founded on the surface of a uniform half-space – machine
vibration methods applied to the earthquake problem – inertial interaction (impedances)
– Simple lumped mass, spring, dashpot representations of the behavior of the foundation/soil (soil spring method)
– Treatment of composite damping of soil/structure
– Time domain solutions using standard analysis tools
• Linear and localized nonlinear analyses (uplift, )
– No spatial variation of free-field ground motion assumed
– Active research on all fronts (numerical methods and finite element methods)
• US regulatory requirements – limitation on composite damping values (20%)
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SSI Analysis Methodologies: Historical Perspective
• Vintage 1970s/early 1980s – Research
• UC Berkeley (Seed/Lysmer Group)
• UCSD/USC (Luco/Wong)
• MIT (Roesset/Kausel/Christian)
• NRC – SSMRP (LLNL) – NUREG/CR-1780 “Soil Structure Interaction: The Status of Current Analysis Methods and Research” (1980)
• Nonlinear soil material models (cap model, multi-surface plasticity models, )
– Simplified soil spring methods
– Direct finite element methods (frequency domain) • LUSH, ALUSH – 2D and axisymmetric representations
• PLAXLY
• FLUSH – pseudo-3D representations
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SSI Analysis Methodologies: Historical Perspective
• Vintage 1970s/early 1980s – Substructure methods
• CLASSI (1980)
• SASSI (1981)
– Controversial
– US regulatory requirements – perform SSI analyses by the soil spring approach and the finite element method and envelope the results
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SSI Analysis Methodologies: Historical Perspective
• Vintage 1980s to 1990s
– Soil-structure interaction tests (Lotung and Hualien, Taiwan)
– Established the validity of different methods when applied to the same model
• Frequency domain solutions
• Surface-founded
• Embedded foundations - additional data on the spatial variation of motion – depth in the soil
• Responses compared within engineering accuracy
– US regulatory requirements - relaxed the requirement to perform SSI analyses with multiple approaches and envelope the results
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SSI Analysis Methodologies (Modeling and
Parameters): Historical Perspective
•Vintage 1990s to Present – Substructure Approaches
– Relies on superposition (linear assumption)
– SASSI, CLASSI, SUPELM, others
– Three dimensional
– Earthquake acceleration time histories define control motion (3 components)
– Arbitrary wave fields
– Linear or equivalent linear material behavior
– Frequency domain solutions
– Simpler methods for standard designs
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SSI Analysis Methodologies: Present Perspective
• Design of New Nuclear Power Plants
• Evaluation for Beyond Design Basis Earthquake Motion
– Seismic PRA (PSA) required for New Plants in US - Probabilistic response analyses defining seismic demand (Nakaki et al.)
• Evaluation of Nuclear Power Plants Experiencing Significant Earthquake Ground Motion at the Site (Forensic engineering)
• Japan NPPs
– Well instrumented in free-field and in-structure
– Experience significant earthquake ground motion
• Other countries
• Design vs. Analysis of a Facility experiencing earthquake ground motion
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SSI Analysis Methodologies: Present Perspective
• Standard Designs - World-Wide Vendors and Sites – broad-banded design basis ground motion to envelope high percentage of NPP sites
– Certified Standard Designs (US) – Certified Seismic Design Response Spectra (CSDRS)
– Standard Designs (EPR- Europe) – Site Dependent Response Spectra (EURH, EURM,EURS)
– ACR Standard Designs (Canada)
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Standard Design: Broad-Banded Design Basis Ground Response Spectra
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.1 1 10 100
Sp
ectr
al A
ccele
rati
on
(g
)
Frequency (Hz)
Horizontal Spectra 5% Damping
RG 1.60
US EPR, EUR Hard
US EPR, EUR Medium
US EPR, EUR Soft
AP1000 RG 1.60
ACR, CSA Soil
ACR, CSA Rock
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SSI Analysis Methodologies: Present Perspective Site Specific Seismic Hazard
•PSHA – Typical Procedure
– Generate hard rock ground motion (US Vs 9,200 fps rock)
– Perform probabilistic site response analyses (Simulations time/frequency domain, RVT)
– Ground motion on soil surface or at foundation depth
– Issue – Relationship between large family of site profiles probabilistically determined (60 or more) and limited number of profiles to be used in SSI analyses
(3 or more) – FIRS
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SSI Analysis Methodologies: Present Perspective Site Specific Seismic Hazard
•Empirical based – Input to PSHA, DSHA
• Fault Modeling
– Numerical simulation of fault mechanism and transmission of waves from source to site
• Japan – required approach
• US – significant effort over last decade or more (3 or more) –
FIRS
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EPR EURH, EURM, EURS, UHS CEUS Rock Sites
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.1 1 10 100
Spe
ctra
l Acc
ele
rati
on
(g)
Frequency (Hz)
Horizontal Spectra 5% Damping
UHS-Rock-1
UHS-Rock-2
UHS-Rock-3
UHS-Rock-4
UHS-Rock-5
UHS-Rock-6
UHS-Rock-7
UHS-Rock-8
UHS-Rock-9
UHS-Rock-10
UHS-Rock-11
UHS-Rock-12
5% Damp EUR Hard 0.25g
5% Damp EUR Medium 0.25g
5% Damp EUR Soft 0.25g
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Elements of the SSI Analysis Chain
•Free-Field Ground Motion
• Defining the Soil Profile
– Low Strain
– Earthquake Strain Compatible Properties
• Soil-Structure Interaction Modeling and Parameters
• Structure Model
•SSI Analysis
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Free-Field Ground Motion or Seismic Input: General Requirements
• Control Motion (Amplitude and Frequency Characteristics)
– Response Spectra (site independent, site dependent)
– Site Specific Response Spectra (PSHA – GMRS, performance-based DRS)
– Time Histories (recorded motion, simulations, deaggregated scenario earthquakes)
• Control Point
• Spatial Variation of Motion
– Over the depth and width of the foundation and the embedded portion of the structure
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Design Basis Earthquake: Historical PerspectiveVintage 1960s to 1970s
• Control Motion
– Housner Average Response Spectra
– Recorded Acceleration Time Histories (Golden Gate, El Centro,)
– Standard Response Spectra
• NUREG/CR-0098 – Newmark-Hall (median, 84%NEP) (rock, soil)
• US NRC Regulatory Guide 1.60 (1973)
• Control Point
– At foundation
• Spatial Variation of Motion
– No consideration
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Design Basis Earthquake: Historical Perspective Vintage 1970s to early 1980s
• Control Motion
– Standard Response Spectra
• NUREG/CR-0098 – Newmark-Hall (median, 84%NEP) (rock, soil)
• US NRC Regulatory Guide 1.60 (1973)
• Japan (Ohsaki)
– Probabilistic Seismic Hazard Analysis
• Initiated by US NRC (LLNL) and EPRI
• Control Point
– Foundation level in free-field
• Spatial Variation of Motion
– Wave propagation from foundation level to surface
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Standard Design: Broad-Banded Design Basis Ground Response Spectra
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Design Basis Earthquake: Historical Perspective Vintage 1980s to 1990s
• Control Motion
– Standard Response Spectra
• US NRC Regulatory Guide 1.60
– Probabilistic Seismic Hazard Analysis (PSHA)
• EPRI and US NRC (LLNL)
• US NRC Regulatory Guide 1.165
• Control Point
– On a Free Surface of Soil or Rock – actual or hypothetical outcrop on the upper most in-situ competent material
• Spatial Variation of Motion
– Wave propagation mechanisms from control point to other points in the free field
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Issues
• PSHA – Typical Procedure
– Relationship between large family of site profiles probabilistically determined (60 or more) and limited number of profiles to be used in SSI analyses (3 or more) – FIRS
• High frequency ground motion for rock sites
– Filter during hazard study (e.g., CAV)
– Account for incoherence of ground motion in SSI analyses
• Vertical ground motion corresponding to horizontal PSHA and DSHA
– V/H ratios
– Fault modeling - numerical simulations of source and source to site transmission
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Modeling the Soil Profile
• Soil Configuration
– Layering and stratigraphy
• Soil Material Behavior
– Equivalent linear viscoelastic material (earthquake level dependent)
– Nonlinear material models
• Field Exploration
– Borings
– In-situ tests
• Laboratory Tests
• Correlation of Field and Laboratory Data
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Issues
•Defining soil profile heterogeneity (number and location of bore holes)
• Material models other than visco-elastic equivalent linear, e.g., nonlinear
– Functional form
– Parameters of model
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SSI Modeling and Parameters: State of Practice
•Methodologies
– Substructure approach (programs, characteristics)
– Other
•Foundation Models
• Structure Models
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SSI Modeling and Parameters: Methodologies
•Methodologies – Substructure approaches
– Relies on superposition (linear assumption)
– SASSI, CLASSI, SUPELM, others
– Three dimensional
– Earthquake acceleration time histories define control motion (3 components)
– Arbitrary wave fields
– Linear or equivalent linear material behavior
– Frequency domain solutions
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SASSI SSI Calculational Steps: Schematically
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Elements of the Substructure SSI Analysis as Implemented in CLASSI Programs
Free-Field MotionFoundation Input Motion
Kinematic Interaction
M
F
Soil Profile
Site Response AnalysisImpedances SSI
Structural Model
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Modeling the Foundation
•Embedment
• Stiffness
• Geometry
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Structure Models: General
•Detail and sophistication of the structure model is determined by it’s purpose
– Overall dynamic response characteristics
• First step in a multi-step process
• More detailed dynamic and/or static models used to calculate responses for design and qualification (force and moment quantities, ISRS, ) – input are responses from SSI model
– Detailed in-structure responses for design and qualification of structures, systems, and components
– Combination
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Structure Models: BWR Reactor Building and Internals
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Structure Models: Detailed EPR NI Model
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Issues
• Effect of Nonlinear Behavior on Soil/Structure Response
– Design levels, beyond design levels
– Forensic engineering (recorded earthquake ground motion and structure response)
• Validation
– Complex SSI Models (approaches include validation of individual elements or analysis, sensitivity studies encompassing fixed-base to SSI, soil property variations, Peer Review, others)
– Complex Structure Models
• Foundation/soil interface nonlinear effects (separation, sliding, uplift, )
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Future Developments
•Further Define and Validate Performance-Based Design Criteria
• Forensic Evaluations of NPP Site and Structure Response Subjected to Actual Earthquake Motions (Continued)
• Integrated Models and Analyses
– Fully probabilistic from source to structure response
– Validate design-based approaches (simpler user friendly analyses)
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Future Developments
• Integrated Models and Analyses
– Source mechanism simulations (Japan, US, )
– Source to site transmission of motion
• In the large (wave propagation mechanisms, )
• In the small (site response analyses – including nonlinear soil behavior, scattering, )
– Nonlinear behavior in the neighborhood
• Nonlinear soil material behavior
• Nonlinear geometric effects (sliding, separation, )
– Structure response for structure design and capacity determination
– Structure response for input to systems, equipment, components
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Site Response and SSI as a Learning Process
• Kashiwazaki-Kariwa Nuclear Power Plant Response to the NCOE and aftershocks
– Site response – not as simple to model as one might surmise even with 5 downhole recordings of aftershocks
– Significant influence of embedment – all reactor buildings deeply embedded
– Seismic margin in demand is significant
– IAEA/ISSC KARISMA benchmark on-going investigation
• Incoherency of ground motion, i.e., high freaquency ground motion effects on structure response
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Site Response and SSI as a Learning Process
• Incoherency of ground motion, i.e., high frequency ground motion effects on structure response
– Revised thinking on definition of rock for SSI purposes
• Vs = 6,000 fps vs. 3,500 fps
– Effect of accounting for SSI effects for coherent ground motion is significant – incoherency effects are in addition
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Envelope ISRS: HR (+19.5m), Horizontal
Envelope Spectra, CLASSI Fixed Base and SSI with Coherent & Incoherent Scattering vs.
AREVA Soil Springs, 4% damping, Reactor Building HR, Level +19.50m, Horizontal
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10.0
20.0
30.0
40.0
50.0
60.0
1 10 100
Frequency (Hz)
Accele
rati
on
(m
/s2)
Fixed Base, HR +1950 Horizontal Envelope
Coherent SSI, HR +1950 Horizontal Envelope
Incoherent SSI, HR +1950 Horizontal Envelope
AREVA Soil Spring, HR +1950 Horizontal Envelope
AREVA Fixed Base, HR +1950 Horizontal Envelope
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Envelope ISRS: HR (+19.5m), Vertical
Envelope Spectra, CLASSI Fixed Base and SSI with Coherent & Incoherent Scattering vs.
AREVA Soil Springs, 4% damping, Reactor Building HR, Level +19.50m, Vertical
0.0
5.0
10.0
15.0
20.0
25.0
30.0
1 10 100
Frequency (Hz)
Accele
rati
on
(m
/s2)
Fixed Base, HR +1950 Vertical Envelope
Coherent SSI, HR +1950 Vertical Envelope
Incoherent SSI, HR +1950 Vertical Envelope
AREVA Soil Spring, HR +1950 Vertical Envelope
AREVA Fixed Base, HR +1950 Vertical Envelope
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Thank you
• IAEA/ISSC (Ovidiu Coman et al.)
• OECD/NEA IAGE, IAEA/ISSC, and CNSC for organizing and sponsoring the Workshop