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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

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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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30.0

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60.0

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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

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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