Contents
1 Introduction – (5 pages) (Johnson) 4
1.1 Background 4
1.2 Objectives 4
1.3 Scope of the TECDOC 4
1.4 Document structure 4
2 Elements of SSI analysis (10 Pages) (Johnson) 4
3 Site configuration and soil properties (15 Pages) (Pecker) 4
3.1 Site configuration and characterization 4
3.2 Constitutive models 4
3.3 Physical parameters 4
3.4 Field and laboratory measurements 4
3.5 Calibration and validation 4
3.6 Uncertainties 4
3.7 Spatial variability 4
4 Free field ground motions (seismic wave fields) (30 Pages) (Jeremic) 5
4.1 Purpose of free field response analysis 5
4.2 Seismic wave fields (referenced section 3.1) 5
4.2.1 Recorded data 5
4.2.2 Synthetic (source models) 5
4.2.3 Uncertainties 5
4.3 Analysis models and modelling assumptions 5
4.4 Dynamic soil properties (elastic plastic, equivalent linear elastic; modelling assumptions). Note: will be referenced in Section3.2 5
4.5 Spatial variability of ground motion and seismic wave incoherence 5
4.6 Time domain free field response methods 5
4.7 Frequency domain free field response methods 5
4.8 Limitations 5
5 Seismic Input (15 Pages) (Johnson) 5
5.1 Free-field ground motion 5
5.2 Seismic hazard analysis 5
5.3 Standard and site specific response spectra 5
5.4 Time histories 5
5.5 Uncertainties and confidence level 6
6 Methods and models for SSI analysis (50 Pages) (Pecker & Jeremic as Chapter leads) 6
6.1 Basic steps for SSI analysis 6
6.2 Direct methods (Jeremic) 6
6.2.1 Linear discrete methods (finite elements and finite difference) 6
6.2.2 Non-linear discrete methods (finite elements and finite difference) 6
6.3 Sub-structuring methods (Pecker and Johnson) 6
6.3.1 Continuum methods 6
6.3.2 Discrete methods (finite elements and finite difference) 6
6.3.3 Foundation input motion (Reference to section 4.5 and 5.1) 6
6.4 SSI computational models (Jeremic and Pecker) 6
6.4.1 Soil/Rock linear and nonlinear modelling 6
6.4.2 Structural models, linear and nonlinear: shells, plates, walls, beams, trusses, solids. 7
6.4.3 Contact (soil/rock - foundation) linear and nonlinear modelling, dry and saturated, uplift/gap (geometric nonlinearity), slip (material nonlinearity) 7
6.4.4 Structures with a base isolation system 7
6.4.5 Foundation models 7
6.4.6 Simplified models 7
6.4.7 Seismic loads (input, etc.) 7
6.4.8 Structure-Soil-Structure Interaction (in phase, out phase, distance between structures relative to the surface wave length, etc.) 7
6.4.9 General guidance on soil structure interaction modelling and analysis (Table) 7
6.5 Probabilistic response analysis (random vibration theory, Monte Carlo, Stochastic Finite Element Methods, etc.; modelling assumptions) (Jeremic and Johnson) 7
7 Seismic Response Aspects for Design and Assessment (50 Pages) (Johnson and Pecker) 7
7.1 Introduction (Johnson) 7
7.2 Design and Assessment Methodologies (Need from All Experts input on how to develop this section by April 15, 2016) 7
7.3 Deterministic analysis (linear and nonlinear) (Pecker) 8
7.4 Probabilistic analysis (linear and nonlinear) (Johnson/Jeremic) 8
7.5 Structural design quantities (Pecker) 8
7.6 Seismic input to sub-systems (equipment, components, distribution systems, etc.) (Johnson and Pecker)) 8
8 Available software (10 Pages) (Jeremic) 8
8.1 Examples of commercially available, open source, open use and public domain, etc. 8
8.1.1 Linear 8
8.1.2 Nonlinear 8
8.2 Software support 8
8.3 Education and training 8
8.4 Quality Assurance 8
8.5 Examples of V&V (reference to SASSI V&V Project results, etc.) 9
9 Sustainable capacity building (5 Pages) (Johnson) 9
9.1 Training courses 9
9.2 Refresher (Just-in-time training and computer based) 9
10 References 9
8. Seismic Response Aspects for Design and Assessment (50 Pages) (Johnson and Pecker)
8.1 Introduction (Johnson)
- 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 and Assessment Methodologies (Need from All Experts input on how to develop this section by April 15, 2016)
- Design and assessment challenges and issues
- How to choose the methods for design and assessment in order to address these issues
- What is the use of linear and nonlinear analysis?
- State of practice in the industry to be considered in the appropriate context including nonlinearity, with examples of superseded by current state of practice
- (Appendix using information from MSs and vendors)
- How to build appropriate and reliable models for the design and assessment
- How to address the SSI uncertainties in the design and assessment?
- How Member States deals with uncertainties and the end results?
- Approach and criteria for assessment and design retrofit or modification of existing facilities
- Future industry practice in 5-10 years
8.3 Deterministic analysis (linear and nonlinear) (Pecker)
- Design
- Assessment
- Standards and practice (Appendix using information from MSs and vendors)
8.4 Probabilistic analysis (linear and nonlinear) (Johnson/Jeremic)
- Design
- Assessment
- Standards and practice (Appendix using information from MSs and vendors)
8.5 Structural design quantities (Pecker)
- Seismic member forces
- Seismic stability evaluation
- Seismic soil pressure
8.6 Seismic input to sub-systems (equipment, components, distribution systems, etc.) (Johnson and Pecker))
- In-structure response spectra
- Time histories
- Peak values of acceleration and displacement
8.2 Design and Assessment Methodologies
A. SSI phenomena
1. Free-field ground motion
- Wave types as propagated from source to site without distinct site specific features (such as, slanted layers, topographic effects, etc.) influencing the propagation, i.e., uniform site; for discussion purposes, vertically propagating shear and dilatational waves;
- Free-field ground motion due to natural geological and geotechnical characteristics that differ from 1a – slanted layers of soil and rock; topographic effects (hills, valleys, etc.);
- 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.
- Relationship between vertical and horizontal ground motion;
- Effect of high water table on horizontal and vertical ground motion;
- Incoherence of ground motion, especially for high frequencies;
- Others to be identified.
2. In-situ soil profiles and configurations
- Soil and rock data acquisition through geophysical and geotechnical field investigations – feasible for new sites, limited for existing sites near the NPP power block; generally low strain data;
- Other
3. Soil material behavior characterization due to design basis earthquake (DBE) and beyond design basis earthquake (BDBE)
- Linear, equivalent linear, and nonlinear material behavior;
- Decision-basis – excitation level, site properties, categorization of structure (importance based on risk to personnel (on-site/public) and environment, complexity of structure, others;
- Laboratory tests to define material behavior correlated with material models;
- Correlate field data with laboratory data;
4. Soil replacement and backfill – definition and modeling
- Excavation and soil replacement, backfill, berm build-up – existing plants difficult to obtain adequate construction records to determine material property, new plants – feasible;
5. Foundation modeling –
- Important considerations
- Purpose of the SSI analysis is important – overall SSI response requires different foundation modeling than direct calculation of stresses for design;
- Ground motion level is important;
- Purpose - DBE design or BDBE assessment or forensic engineering for a recorded earthquake motion (free-field and in-structure response)
- Surface or near surface-founded;
- Basemats – rigid/flexible; shear keys formed by sump collection points;
- Strip footings, spread footings;
- Embedded
- Basemat – rigid/flexible; shear keys;
- Side-walls;
- Partial embedment – less than all four sides;
- Pile or caisson foundations
- Pile groups;
- Assumptions for design and assessment – basemat/base slab maintains contact with or separates from underlying soil;
- Separation of basemat and side walls from soil (in-situ, engineered fill, back fill);
- Basemat sliding and/or uplift;
6. Structure modeling
- Important considerations
- Purpose of the SSI analysis is important – overall SSI response may permit simpler structure models than detailed stress analysis;
- Overall kinematic responses sought vs. detailed stresses;
- Ground motion level is important;
- Finite element models;
- Linear or nonlinear material model;
- Frequency range of interest – high frequency in particular (50 Hz; 100 Hz);
7. SSI models (considerations)
- Conservatism (C) vs. realism (R)
- Design (DBE); C;
- Beyond design basis earthquake (BDBE); C/R;
- Forensic analysis – recorded earthquake motion; R;
- Site rock/soil modeling – irregular profile;
- Soil material behavior – linear or nonlinear;
- Structure-to-structure interaction;
- Other.
8. Uncertainties in all aspects of SSI need to be taken into account. Methods to do so are probabilistic and deterministic.
B. Sensitivity studies and benchmarking
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 Methods
Alternate 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.
C. 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 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:
- Less than about a 1% probability of unacceptable performance for the Design Basis Earthquake Ground Motion, and
- 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:
- Demand is determined at about the 80% non-exceedance level for the specified input motion.
- 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 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.
D. 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
E. 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.