Contents8. Seismic Response Aspects for Design and Assessment....................................................2

8.1 Introduction...............................................................................................................2

8.2 Design Methodologies (Jim)......................................................................................2

8.2.1 Free-field ground motion (Boris).........................................................................2

8.2.2 Soil material behavior characterization for design basis earthquake (DBE) and for beyond design basis earthquake (BDBE) (Alain)........................................................3

8.2.3 Foundation modelling (Alain).............................................................................4

8.2.4 Structure modelling (Boris).................................................................................6

8.2.5 SSI models (considerations) (Boris)....................................................................6

8.2.6 Uncertainties in all aspects of SSI need to be taken into account. Methods to do so are probabilistic and deterministic. (Jim)......................................................................7

8.3 Assessment Methodologies (Jim) and same as for design......................................7

8.3.1 Free Field (probabilistic and/or deterministic)..........................................................7

8.3.2 Site response (probabilistic and/or deterministic)................................................7

8.3.3 Foundation modeling – for assessment:...............................................................8

8.3.4 Structure modeling...............................................................................................8

8.3.5 SSI models (considerations).................................................................................8

8.3.6 Uncertainties in all aspects of SSI need to be taken into account. Methods to do so are probabilistic and deterministic.................................................................................8

8.4 Sensitivity studies and benchmarking (Jim).........................................................10

8.5 Deterministic analysis (linear and non-linear) (Alain).........................................14

8.6 Probabilistic analysis (linear and non-linear) (Jim).............................................14

8.7 Structural design quantities (Alain).......................................................................15

8.8 Seismic input to sub-systems (equipment, distribution systems, etc.) (Jim)......15

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8. Seismic Response Aspects for Design and Assessment

8.1 Introduction - Roles and responsibilities - Multi-disciplinary that requires upfront planning and interaction - Link between safety requirements and SSCs performance- Set goals for analysis- Discussion on the target of analysis and what are the expectations

(target values) and how to do probabilistic analysis to determine the targets

- Introducing available regulatory requirements, codes and standards and guidance documents related to SSI (need input and support from expert and IAEA- seeks input through a survey and questionnaire from MSs ASAP)

8.2 Design Methodologies (Jim)

8.2.1 Free-field ground motion (Boris)

1D Case: Wave types as propagated from source to site without distinct site specific features (such as, slanted layers greater than 10o angle, basin effects where basins are very shallow, topographic effects, etc.) influencing the propagation, one can perform 1D site response;- vertical propagating S and P waves in conjunction with soil property variations

accounts for influence/effects of surface waves, and inclined body waves;- ratio between vertical and horizontal motions ;- Effect of high water table on horizontal and vertical ground motion; - Free surface pools, reservoirs and base isolation issues, free motions need special

attention as they are susceptible to effects of low frequency motions. In addition, vertical motions are very important as most base isolation system do not isolate vertical motions, in fact vertical motions can amplify…

2D/3D Case: When special features are present and important (slanted layers, deep basin effects, basin’s edge effects; topographic effects (hills, valleys, sloping ground, location and extend of source, etc.) one has to perform a 2D or 3D site response analysis to assess the effects of these special features. NERA project provided some guidance (reference to be included (Peter Moczo, Pierre-Yves Bard…Recent example of the use of shallow and deep geology for design (For example Cadarache ?)- site with non-horizontal layers perform sensitivity study to assess influence of

non-vertically incident horizontal and vertical waves

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- Free-field ground motion taking into account man-made features at the site, e.g., excavated soil and rock for construction purposes; construction of berms to support structures; etc.

Effect of high water table on horizontal and vertical ground motion; Incoherence of ground motion, especially for high frequencies (remove passing

wave effect to get pure incoherence in 1D?

8.2.2 Soil material behavior characterization for design basis earthquake (DBE) and for beyond design basis earthquake (BDBE) (Alain)

a. Linear, equivalent linear, and nonlinear material behavior; (reference in chapter 4 Alain’s note on use of second invariance of strain to relate from 1D to 3D models!)

b. Decision-basis – excitation level, site properties, categorization of structure (importance based on risk to personnel (on-site/public) and environment, complexity of structure, others;

c. Laboratory tests to define material behavior correlated with material models; d. Correlate field data with laboratory data; e. To evaluate material model sophistication hierarchy for given earthquake levelf. Perform sensitivity studies of material parameters for models chosen above.

2. Soil replacement and backfill – definition and modeling

a. Excavation and soil replacement, backfill, berm build-up – new NPPs require adequate material property specifications and configurations,

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8.2.3 Foundation modelling (Alain)

A. Important considerations (Jim)i. Purpose of the SSI analysis – overall SSI response requires different

foundation modeling than direct calculation of stresses for design; ii. Purpose - DBE design or BDBE assessment in design phase

B. Surface or near surface-founded; (Jim)i. Basemats – rigid/flexible; shear keys formed by sump collection points;

ii. Strip footings, spread footings; iii. Transformers, sliding and rocking (walking, Alains work from some time

ago, non coincidence of center of mass and center of stiffness)

C. Embedded (Boris)i. Basemat – rigid/flexible; shear keys; sliding and uplift of basemats

ii. Side-walls; sliding and gapping (engineered fill, back fill, differences in stiffness between surrounding soil and the backfill (Stiffness)

iii. Partial embedment – less than all four sides; iv. Deeply embedded Small Modular Reactors (SMRs) See material below

D. Pile foundations (defined when ratio length over diameter is larger than 5) (Alain)

i. Single pile within pile group and for testing and developing P-Y and T-Z springs;

ii. Pile groupsiii. Assumptions for design – basemat/base slab maintains contact with or

separates from underlying soil;

E. Caisson foundations (all others (see piles above)) (Boris)Cases of caissons modeled using super/macro elements, need to take into account caisson rotations

SMR: Earthquake Soil Structure Interaction of deeply embedded SMRs requires special considerations. For modeling of SMR, it is important to note extensive contact zone of deeply embedded SMR walls and base slab with surrounding soil. This brings forward a number of modeling and simulation issues for SMRs. listed below, In addition, noted are suggested modeling approaches for each listed issue.

• Seismic Motions: Seismic motions will be quite variable along the depth and in horizontal direction. This variability of motions is a results of mechanics of inclined seismic wave propagation, inherent variability (incoherence) and the interaction of body waves (SH, SV and P) with the surface, where surface waves are developed. Surface waves do extend somewhat into depth (about two wave lengths at most (Aki and Richards, 2002)). This will result in different seismic motion wave lengths (frequencies, depending on soil/rock stiffness), propagating in a different way at the surface and at depth of SMR. As a results, an SMR will experience very different motions at the surface, at the base and in between.

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Due to a number of complex issues related to seismic motions variability, as noted above, it is recommended that a full wave fields be developed and applied to SSI models of SMR.

– In the case of 1D wave propagation modeling, vertically propagating shear waves are to be developed (deconvolution and/or convolution) and applied to SSI models.

– For 3D wave fields, there are two main options:∗ use of incoherence functions to develop 3D seismic wave fields. This option has a limitation as incoherent functions in the vertical direction are not well developed.∗ develop a full 3D seismic wave field from a wave propagation modeling using for example

SW4. This option requires knowledge of local geology and may require modeling on aregional scale, encompassing causative faults, while another option is to perform stress

testing using a series of sources/faults (Abell et al., 2015).• Nonlinear/Inelastic Contact: Large contact zone of SMR concrete walls and

foundation slab, with surrounding soil, with its nonlinear/inelastic behavior will have significant effect on dynamic response of a deeply embedded SMR.

Use of appropriate contact models, that can model frictional contact as well as possible gap opening and closing (most likely in the near surface region) is recommended. In the case of presence of water table above SMR foundation base, effective stresses approach needs to be used, as well as modeling of (possibly dynamically changing) buoyant forces, as described in section 6.4.8 and also below.

• Buoyant Forces: With deep embedment, and (a possible) presence of underground water (water table that is within depth of embedment), water pressure on walls of SMR will create buoyant forces. During earthquake shaking, those forces will change dynamically, with possibility of cyclic mobility and liquefaction, even for dense soil, due to water pumping during shaking (Allmond and Kutter, 2014).

Modeling of buoyant forces can be done using two approaches, namely static and dynamic buoyant force modeling, as described in section 6.4.8.

• Nonlinear/Inelastic Soil Behavior: With deep embedment, dynamic behavior an SMR is significantly influenced by the nonlinear/inelastic behavior of soil adjacent to adjacent SMR walls and foundation slab.

Use of appropriate inelastic (elastic-plastic) 3D soil models is recommended. Of particular importance is proper modeling of soil behavior in 3D as well as proper modeling of volume change due to shearing (dilatancy). One dimensional equivalent elastic models, used for 1D wave propagation are not recommended for use, as they do not model properly 3D effects and lack modeling of volume change.

• Uncertainty in Motions and Material: Due to large contact area and significant embedment, significant uncertainty and variability (incoherence) in seismic motions will be present. Moreover, uncertainties in properties of soil material surrounding SMR will add to uncertainty of the response.

Uncertainty in seismic motions and material behavior can be modeled using twoapproaches, as described in section 6.5. One approach is to rely on varying inputmotions and material parameters using Monte Carlo approach, and its variants. Thisapproach is very computationally demanding and not too accurate. Second approach isto use analytic stochastic solutions for components or the full problem. For example,stochastic finite element method, with extension to stochastic elasto-plasticity withrandom loading. More details are given in section 6.5.

Figure x.xx illustrates modeling issues on a simple, generic SMR finite element model

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(vertical cut through middle of a full model is shown).

Figure x.x: Four main issues for realistic modeling of Earthquake Soil StructureInteraction of SMRs: variable weave field at depth and surface, inelastic behavior ofcontact and adjacent soil, dynamic buoyant forces, and uncertain seismic motions andmaterial.It is important to develop models with enough fidelity to address above issues. It ispossible that some of the issues noted above will not be as important to influenceresults in any significant way, however the only way to determine importance(influence) of above phenomena on seismic response of an SMR is through modeling.

8.2.4 Structure modelling (Boris)

A. Important considerations i. Purpose of the SSI analysis – overall/global SSI response may permit

simpler structure models than detailed stress analysis; Detailed dynamic analysis for NPPs (no stick models preferably…)

ii. Overall kinematic responses sought vs. detailed stresses; iii. Ground motion level;

B. Finite element models; use of sophisticated models instead of stick models

C. Linear or nonlinear material model; in design it is still linear

D. Frequency range of interest – high frequency in particular (50Hz; 100Hz);

8.2.5 SSI models (considerations) (Boris)

8.3 Conservatism (C) vs. realism (R)i. Design (DBE); C;

ii. Beyond design basis earthquake (BDBE) for assessment in design phase; C/R;

8.4 Site rock/soil modeling – irregular profile, geology (shallow and deep) repeat from the free field section above, perhaps make reference and remove from here);

8.5 Soil material behavior – linear; 8.6 Structure-to-structure interaction, light structures next to heavy structures and

take into account changes in motions for light structures; 8.7 Other.

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8.2.6 Uncertainties in all aspects of SSI need to be taken into account. Methods to do so are probabilistic and deterministic. (Jim)

After introducing what uncertainties they are, we need to explain how to deal with

uncertainties described in the previous chapters (3, 5, and 7)

Full probabilistic analysis develops probability density functions (PDFs) for all

variables.

Part of the PDF can be used to assess sensitivity (part near mean/mode) while tails

of the PDF are to be used to develop risk assessment…

Reference to section 6.4 where Jim talked about sources of uncertainty…

Probabilistic modelling in design: Use of broad band (non site specific) ground motions or/and suite of time histories

(multiple number of time histories to be used, for linear analysis min of 3 for

nonlinear min 5 or whatever member state practice/regulation is ) for new NPPs

Incoherence modelling, multiple analysis for different frequencies (usually around 30

analysis)

Simplistic use of bounds for soil properties (stiffness) (3 simulations with material

properties increase and decrease by minimum value of 1.5 of modulus of elasticity)

plus the median/mean.

8.3 Assessment Methodologies (Jim) and same as for design

8.3.1 Free Field (probabilistic and/or deterministic) Free-field ground motion; refer to the design section above for the Free Field

Motions (1D vs 3D) In addition a set of BDBE are to be used, with all the necessary adjustments as seen

in section above All possible earthquake records for the site (free field, rock outcrop?)

8.3.2 Site response (probabilistic and/or deterministic) For material properties we need to get more data, additional site characterization,

obtain as much data as possible in order to develop (good/excellent) models. Photos during construction, additional ground (soil and rock) test data

Observations/records of soil structure system behavior post construction (in order to validate modeling!) mainly to get infor on soil properties, fluctuation of water table,

All possible earthquake records for the site (soil response)

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8.3.3 Foundation modeling – for assessment:

Start with linear and see if nonlinear is warranted/needed Nonlinear modeling to be done if needed (uplift and sliding) (NOTE: not for final

version, in order to reduce demand (which might not actually have, you reduce acc but increase disp. This is just a note, perhaps not use it in final version…

Piles for horizontal and vertical might need to be analyzed using nonlinear methods. Hierarchy of models, a) springs (transpaltions and rotations, impedance functions) b) use P-Y and T-Z springs, c) use full 3D piles model with contacts, etc… impedance tests on piles (as build)

Buoyant forces applied as single vertical force, seasonal variations (water table), for potentially liquefiable soil, use more sophisticated models that take into account pore fluid pressure changes during shaking

Assessment for surface or near surface-founded; (see list in design section above) (surface, embedded, piles, caisons, SMRs (not yet, in future after they are built)

8.3.4 Structure modeling Obtain as built drawings to develop good/excellent momdels, Testing, coring to obtain material properties Use of best estimates for parameters with or without test data Forced and ambient vibration tests on sructureal Review test reports obtained during construction Linear modeling except for special structures or by request Sometimes do a pushover test to assess static nonlinear response Use reduced stiffness (equivalent linear) for structural modeling

8.3.5 SSI models (considerations)

Modeling more sophisticated and less conservative than in design Occasionally done as nonlinear (if possible, for example soil and contacts at least,

possibly structure as well (at least reduced stiffness as noted above).

8.3.6 Uncertainties in all aspects of SSI need to be taken into account. Methods to do so are probabilistic and deterministic.

See notes above for free field, site response, soil parameters uncertaintiesThan more uncertainties:

Variability of structural stiffness, damping, mass (variability more than 20%) Uncertainty ion behavior of equipment and subsystems (behavior in itself,

anchorages…)

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8.4 Sensitivity studies and benchmarking (Jim)

Sensitivity studies in addition to the above (!)1. Sensitivity studies will form an important element in the assessment of issues

identified in A. If possible, the TECDOC should identify sensitivity studies to be performed for assessment of individual issues.

a. Propose acceptance criteria for inclusion or exclusion of particular elements

Example:

There is a general consensus over the last several decades that changes in results that are less than 10% due to modelling changes, or refinements in the models or analyses, are acceptable and introducing such modelling changes or refinements is not required.

Most recently, ASCE 4-16 codifies this principle in Section 3.1.4, which states:

“3.1.4 Alternate MethodsAlternate methods may be used to satisfy the requirements of Chapter 3 provided that it can be demonstrated that the response parameter(s) of interest are not underestimated by more than 10%.”

2. Benchmarking studies whose intent is to investigate under what conditions a SSI feature or modeling consideration is important are extremely effective tools.

a. Examples abound from previous studies. For example, the later listed study by Nakaki et al. documents the comparison of probabilistic response results with deterministically calculated results by an ASCE 4-16 approach that leads to the desired non-exceedance probability. Studies of non-vertically incident waves’ impact on structure response have led to guidance on if and when such phenomena should be included.

b. The TECDOC should identify benchmark studies to be performed for assessment of individual phenomenon.

A. Design challenges and issues

1. Design requires a certain amount of conservatism to be introduced into the process. The amount of conservatism is dependent on a performance goal to be established. ASCE 4-16, ASCE 43-05, and U.S. DOE Standards define specific performance goals of structures, systems, and components (SSCs) in terms of design (DBE) and in combination with beyond design basis earthquakes (BDBEs). Performance goals are established dependent on the critical nature of the SSC and the consequences of “failure” to personnel (on-site and public) and the environment. Performance goals are defined in probability space.

In nuclear power plants (NPPs), meeting the guidelines for annual core damage frequency (CDF) and annual large early release frequency (LERF) are the ultimate

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performance criteria of the NPP. As an intermediate step, performance criteria for SSCs based on their seismic categorization needs to be established.

Example:

Given the seismic design basis earthquake (DBE), the goal of ASCE/SEI 4-16 is to develop seismic responses with 80% probability of non-exceedance. For probabilistic seismic analyses, the response with 80% probability of non-exceedance is selected.

ASCE/SEI 4-16 is intended to be used together, and be consistent with the revision to ASCE/SEI 43-05. The objective of using ASCE 4 together with ASCE/SEI 43 is to achieve specified target performance goal annual frequencies. To achieve these target performance goals, ASCE/SEI 43 specifies that the seismic demand and structural capacity evaluations have sufficient conservatism to achieve both of the following:

i. Less than about a 1% probability of unacceptable performance for the Design Basis Earthquake Ground Motion, and

ii. Less than about a 10% probability of unacceptable performance for a ground motion equal to 150% of the Design Basis Earthquake Ground Motion.

The performance goals will be met if the demand and capacity calculations are carried out to achieve the following:

i. Demand is determined at about the 80% non-exceedance level for the specified input motion.

ii. Design capacity is calculated at about 98% exceedance probability.

a. Establish performance goals, or a procedure to establish performance goals, for seismic design and beyond design basis earthquake assessments for SSCs.

b. Partition achievement of the performance goal into elements, including SSI. c. Develop guidance for SSI modeling and analysis to achieve the performance

goal. d. Solicit the state of practice of Member States for establishing seismic design

criteria and beyond design basis earthquake acceptance criteria.

2. Establish levels of conservatism in current methods of SSI analysis for design purposes

Through the results of existing studies, additional studies, and expert opinion, quantify the conservatism in SSI analysis procedures implemented for design.

Example:

Nakaki, D.K., Hashimoto, P.S., Johnson, J.J., Bayraktarli, Y., Zuchuat, O., “Probabilistic Seismic Soil Structure Interaction Analysis of the Mühleberg Nuclear Power Plant Reactor and SUSAN Buildings,” Paper PVP2010-25343, 2010 ASME Pressure Vessel and Piping Conference, Bellevue, WA, USA, 18-22 July 2010.

Nakaki et al. demonstrated that the ASCE 4-XX SSI analysis criteria when implemented and compared to probabilistic SSI analyses (taking into account

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uncertainty in soil-structure parameters probabilistically) yields the targeted performance goal for the analysis, i.e., about an 80% non-exceedance level for seismic response.

This study addressed some SSI issues, but not the full range of issues raised herein.

a. Partition achievement of the performance goal into elements, including SSI.

B. Assessment challenges and issues

Assessments can be separated into two parts: BDBE evaluations and forensic analysis for recorded earthquakes. This section can be expanded upon following the approach of C above.

1. BDBE evaluations – SSI considerations 2. Forensic analysis for recorded motions

C. Questions and Responses

1. Highest level is establishment of performance goals for DBE and BDBE:

a. Establish performance goals, or a procedure to establish performance goals, for seismic design and beyond design basis earthquake assessments for SSCs.

b. Partition achievement of the performance goal into elements, including SSI. c. Develop guidance for SSI modeling and analysis to achieve the performance

goal.

2. Address each issue with these important considerations

a. Purpose of the SSI analysis – overall SSI response requirements differ from those for detailed stress calculations;

b. Ground motion level; c. Purpose - DBE design or BDBE assessment or forensic engineering for a

recorded earthquake motion (free-field and in-structure response)

3. Sensitivity studies and benchmark analyses – this project (TECDOC) should identify and provide guidance on types of sensitivity studies and benchmark studies are appropriate.

a. Sensitivity studies to be identified either for a generic issue or a site specific issue; Generic issue: wave propagation mechanism for uniform sites; Site specific issues: complex soil profile of slanted layers; locally defined topography; man-made excavation in hilly terrain; sensitivity studies can be reduced scope in terms of model size (two dimensional vs. three dimensional)

b. Benchmark studies to be identified for particular issuesExample is nonlinear vs. equivalent linear soil property representation.

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4. How to choose the methods for design and assessment in order to address these issues? See 1, 2, and 3 just above.

5. What is the use of linear and nonlinear analysis? See 2 and 3 just above.

6. State of practice in the industry to be considered in the appropriate context including nonlinearity, with examples of superseded by current state of practice.

a. Establish and document the state-of-practice of U.S.A, Canada, and France by TECDOC authors based on:

i. Expert knowledge ii. Existing results of sensitivity studies;

iii. New sensitivity studies to be performed; iv. Expert opinion.

b. Solicit the state of practice of other Member States for: i. Establishing seismic design criteria and beyond design basis earthquake

acceptance criteria. ii. SSI modeling approaches, especially treatment of uncertainties;

7. How to build appropriate and reliable models for the design and assessment

a. Guidance exists in various forms in Member States, e.g., U.S. ASCE 4-16 Chapters 2 Free-field ground motion and 5 SSI; U.S. NTTF evaluation guidelines – SPID; other MSs to provide through solicitation;

b. Assessment guidance exists in various documents for Seismic Margin Assessment (SMA) Methodology and Seismic Probabilistic Risk Assessment (SPRA) documents;

8. What is the use of linear and nonlinear analysis?See 2 and 3 just above.

9. How to address the SSI uncertainties in the design and assessment?

a. Through various techniques including probabilistic SSI analysis, deterministic SSI analysis with parameter variations, and sensitivity studies;

b. Another significant question for site soil properties is: How to reduce or minimize soil property uncertainties given lack of available data or ability to generate new data through in-situ testing – e.g., existing sites (ability to bore holes, quality of engineered fill, etc.), new sites more opportunities – perform sensitivity studies and average or envelope results.

c. How to determine if a particular phenomenon is important for a given site – sensitivity studies.

10. Approach and criteria for assessment and design retrofit or modification of existing facilities

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a. Establish and document the state-of-practice of U.S.A, Canada, and France by TECDOC authors;

b. Solicit Member States practice in this regard.

D. Future industry practice in 5-10 years

It is envisioned that High Performance Computing (HPC) will dominate the analysis landscape for a majority of external events for the purposes of defining the design basis and the beyond design basis hazards and, potentially, the seismic demand on the structures, systems, and components (SSCs). HPC has an almost unlimited potential. This is especially true for the probabilistic hazard analysis element of the external event PRA process, and for some of the analyses of the facility’s response to the loads imposed by the external hazard.

This vision emphasizes the ability to perform simulations of external events from source to item of interest in the external event PRA. One visualizes thousands of simulations being performed in rapid calendar time due to the independent nature of each simulation. Distributed memory parallel computers can simultaneously execute large numbers of simulations because each simulation is independent of each other. Ten thousand processors can execute ten thousand simulations simultaneously.

These types of analyses will be routine in this time period.

8.5 Deterministic analysis (linear and non-linear) (Alain)

Purpose of the analysis determines the sophistication of the model

Collecting relevant data in order to develop models,

Model development (for all cases below) develop a hierarchy of models from

linear toward nonlinear (sensitivity studies to determine governing parameters (!)

(Used for all components (free field, site response, structure alone and the SSI

system)

Once model developed, start with static analysis (gravity in all three directions,

modal analysis (fixed base, soil springs, full model if possible)

Time domain (linear and nonlinear) start with linear elastic, and add

nonlinearities slowly, doing sensitivities all along

Frequency domain (Fourier and Laplace transform) (linear only)

8.6 Probabilistic analysis (linear and non-linear) (Jim)

Model selection/development (sophistication, etc.) parameters selection

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All the above modeling suggestions apply here too!

Monte Carlo, LHS, RVT, etc.

Full probabilistic will include sensitivities in results, however sensitivities have to

be extracted (post-processed)

8.7 Structural design quantities (Alain)

Internal forces, moments, displacement, stresses

How you combine results from different responses

8.8 Seismic input to sub-systems (equipment, distribution systems, etc.) (Jim)

In-structure response spectra

Building displacements

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