SEISMIC PERFORMANCE VERIFICATIONGUIDELINES FOR CRITICALUNDERGROUND REINFORCED CONCRETESTRUCTURES IN NUCLEAR PLANTS -2021DRAFT

August 2023

Japan Society of Civil Engineers
Committee of Civil Engineering of Nuclear Power Facilities

本ファイルは、「原子力発電所屋外重要土木構造物の耐震性能照査指針」2021年版（条文とその解説）を英訳したものである。国外での耐震設計・研究、国内での英文作成、国外への情報発信などに広く活用してもらうことを目的として、ドラフトを公開することとした。
正式には、後日、技術付属書を添付した 日英版として刊行する計画としている。

This published draft represents an English translation of provisions and theircomments involved in the 2021 version of the "Seismic Performance
Verification Guidelines for Critical Underground Reinforced Concrete
Structures in Nuclear Power Plants". We have decided to prepare the draftwith the purpose of having it widely used for overseas seismic design andresearch developing related English technical documents, and disseminating
information overseas. We will officially release the Japanese-English version
with a dozen of technical annexes at a later date.

This manuscript has been proofread and partly edited by Uni-Edit (https://uni-edit.net/) with a level of easy for a nativespeaker of English to read in addition to no obvious mistakes.

Seismic Performance Verification Guidelines for Critical Underground Reinforced
Concrete Structures in Nuclear Power Plants -2021CONTENTS
CHAPTER 1 GENERAL································································ 1
1.1 Scope of Application ······························································· 1
1.2 Definitions ······································································ 10

CHAPTER 2 PERFORMANCE SETTINGS FOR CRITICAL UNDERGROUND REINFORCED
CONCRETE STRUCTURES ··············································· 13
2.1 General ········································································ 13
2.2 Performance Requirements for Critical Underground RC Structures·························· 15
2.3 Seismic Performance ······························································ 17
2.4 Steps for Performance Verification ··················································· 21

CHAPTER 3 MATERIALS ···························································· 25
3.1 General ········································································ 25
3.2 Material Properties of Concrete, Reinforcing Bar, and Soil ································· 26

CHAPTER 4 ACTIONS ······························································· 31
4.1 General ········································································ 31
4.2 Combinations of Actions························································· 31

4.3 Seismic Action Considered in Seismic Performance Verification ···························· 35

CHAPTER 5 METHODS OF ANALYSIS ·············································· 38
5.1 General ········································································ 38
5.2 Selection of Analysis ······························································ 38
5.3 Structural Analysis for Calculating Response Values in Seismic Performance Verification················································································· 41

CHAPTER 6 VERIFICATION ······················································· 44
6.1 General ········································································ 44
6.2 Prerequisites of Verification ························································ 44
6.3 Safety Factors ··································································· 46
6.4 Seismic Performance Verification ···················································· 48

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

CHAPTER 1

GENERAL

1.1 Scope of Application(1) The present guidelines shall be used for the performance verification of critical undergroundreinforced concrete (RC) structures at nuclear plants that may be subjected to seismic action
as well as RC structures that require an equivalent level of seismic safety.
(2) The structures covered by the guidelines include:
1) Underground RC structures that require the functioning of supporting of equipment andpiping systems classified as seismic grade “Class S” and2) Underground RC structures that require a seawater delivery function in the event of an
emergency (hereafter referred to as "emergency water delivery function").
[Commentary](1)
The present guidelines provide the principles of seismic performance verification for criticalunderground RC structures at nuclear power plants (hereinafter referred to as "underground RC
structures"). The structural configurations of underground RC structures commonly feature lines
along the longitudinal direction and solid figures. These structural forms have relatively large shear
rigidity in their longitudinal or depth direction, and their seismic behavior is biased towards the weakaxis direction of their structure, which has lower shear rigidity. Therefore, the transverse sections ofthese structures are generally examined for seismic performance verification, and emphasis isaccordingly placed on the application of two-dimensional seismic response analysis throughout theseguidelines. Seismic performance verification of the longitudinal direction of structures should be
performed separately as needed. Furthermore, if seismic performance verification using twodimensional analysis is judged to be unsound from an engineering point of view, the threedimensional seismic response analysis can be employed.
Other RC structures at a nuclear plant (e.g., tsunami protection facilities and reprocessing
facilities) that require equivalent seismic safety levels to underground RC structures are designed inaccordance with the same design practice, i.e., the principles of these guidelines are applied to such
structures. Foundation structures that are handled as mass concrete in design practice (e.g., dieselgeneration fuel storage tank foundations, stack foundations, refueling tank foundations) are beyond
the scope of these guidelines because the seismic stability of such foundations is considered under
the rigid body assumption.
The seismic stability of the supporting ground where the underground RC structures are to beconstructed should be examined as required. An impact of possible ground surface displacement on
an underground RC structure is also beyond the scope of these guidelines. (Technical commentary-1-Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

on this issue can be found in a separate guideline volume.) Although the guidelines should be applied
for performance verification at the design stage of yet to be constructed structures, they can also be
applied to an existing structure under the condition that aging has had a minor impact. Thus, specificconsiderations for existing structures, including the material factor for the concrete used for
performance evaluation and the effects of repair, are provided in the present guidelines whereappropriate. In addition, strengthening effects may be considered in the seismic performanceverification of certain existing structures about seismic upgrading.
In addition to the various methods of performance evaluation outlined in these guidelines, othertools for structural analysis and experimental approaches may also be valid for use in seismic
performance verification. These methods are provided in “The Safety Verification Manual on Seismic
Design of Important Outdoor Civil Structures in Nuclear Power Plants” (September 1992, Nuclear
Civil Engineering Committee of the Japan Society of Civil Engineers (JSCE) (hereinafter referred toas the "the Manuals (1992)". These tools and approaches may be used for the performance evaluationof underground RC structures if they are able to properly assess the real structural performance ofindividual designs.
In Japan, the regulatory system necessary for ensuring nuclear safety has already been established.
The laws and regulations and administrative rules of the regulatory system concerning the seismic
design of underground RC structures are shown in Commentary Figure 1.1-1. Commercial standards(or recommendations/guidelines released by academic societies) can also be found in Fig. 1.1-1. The
laws and regulations and administrative rules specify the rules that must be complied with for nuclear
safety; commercial standards, on the other hand, are used as criteria for evaluating the sufficiency ofthese rules.

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Atomic Energy Basic Act
Law
Laws
and
OrdersAct for the Regulation of
Nuclear Reactors and others

Act for Establishment of the
Nuclear Regulation Authority

Regulations
Regulations on Standards for Location, Structures and Equipment of Commercial Power Reactors and theirAccessories

Administrative
Rules

PublicNotice
Guidelines
NoticeCommercial
Standards

Nuclear Regulatory Commission
Interpretation of Regulations on Standards for Location, Structures and Equipment of Commercial Power
Reactors and their Accessories
Safety Review Guides on Reference Earthquake Motion and Seismic Design Policies and others
Example:Technical Codes for Seismic Design of Nuclear Plants JEAC4601 (JEA)
Standard Specifications for Concrete Structures (JSCE)

Commentary Figure 1.1-1 Regulatory system and commercial standards for underground RC structures

In addition to these guidelines, the seismic performance verification of an underground RC
structure should essentially comply with, refer to, and apply the following regulations andrecommendations:"Regulations on Standards for Location, Structures and Equipment of Commercial Power
Reactors and Their Accessories" (hereinafter referred to as "Regulations on Installation
Permission Standards"); Nuclear Regulation Commission, July 8, 2013 (revised on April 1,
2020) (in Japanese).
"Interpretation of the Regulations on Standards for Location, Structures and Equipment ofCommercial Power Reactors and Their Accessories" (hereinafter referred to as "Interpretation of
the Regulation"); Nuclear Regulatory Commission, June 19, 2013 (revised on April 21, 2021)(in Japanese).
"Regulations on Technical Standards for Commercial Power Reactors and Their Accessories"(hereinafter referred to as "Technical Standards Regulation"); Nuclear Regulation Commission,
July 8, 2013 (revised on April 1, 2020) (in Japanese).
"Interpretation of the Regulation on Technical Standards for Commercial Power Reactors andTheir Accessories"; Nuclear Regulatory Commission, June 19, 2013 (revised on July 21, 2021)(in Japanese).
"Safety Review Guide on Reference Earthquake Motion and Seismic Design Policy" (hereinafterreferred to as "Safety Review Guide for Seismic Design Policy"); Nuclear Regulatory
Commission, June 19, 2013 (revised on April 21, 2021) (in Japanese).
-3-Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-"Construction Authorization Safety Review Guide for Seismic Design" (hereinafter referred to as"Safety Review for Construction Authorization Guide"); Nuclear Regulatory Commission, June
19, 2013 (revised on March 31, 2020) (in Japanese).
"Technical Codes for Seismic Design of Nuclear Plants Japan Electric Association Code(JEAC4601-2015)" (hereinafter referred to as "JEAC4601-2015"); Japan Electric Association(JEA), 2016 (in Japanese).
"Japanese Architectural Standard Specification for Reinforced Concrete Work JASS5N (2013)(Reinforced Concrete Construction at Nuclear Power Plant Facilities)"; Architectural Institute ofJapan (AIJ), 2013 (in Japanese).
"Technical Guidelines for Seismic Design of Nuclear Plants JEAG4601-2015" (hereinafterreferred to as "JEAG4601-2015"); JEA, 2015 (in Japanese).
"Technical Guidelines for Seismic Design of Nuclear Plants -Classification of Importance-,
Allowable Stress- JEAG4601 Supplement 1984" (hereinafter referred to as "JEAG4601-1984
Supplement-1984"); JEA, 1984 (in Japanese).
"Technical Reports on Assessment Methods for Geological and Geotechnical Explorations andTests and Seismic Stability of Ground at a Nuclear Power Station" (hereinafter referred to as the"JSCE Reports on Assessment Method for Seismic Stability of Ground"); Nuclear Power Civil
Engineering Committee, Japan Society for Civil Engineers (JSCE), August, 1985 (in Japanese).
"The Safety Verification Manual on Seismic Design of Important Outdoor Civil Structures in
Nuclear Power Plants" (hereinafter referred to as the "the Manuals (1992)"); Nuclear Power Civil
Engineering Committee, JSCE, September, 1992 (in Japanese).
"Guideline and Recommendation for Seismic Performance Verification of Underground
Reinforced Concrete Structures in Nuclear Power Plants"; Nuclear Power Civil Engineering
Committee, JSCE, June, 2005 (hereinafter referred to as "Guidelines, Manuals, Examples,Technical Commentaries 2005") (in Japanese).
"Standard Specifications for Concrete Structures 2002 [Structural Performance Verification]",JSCE (hereinafter referred to as "Specifications [Structural Performance Verification]").
"Standard Specifications for Concrete Structures 2017 [Principles of Earthquake ResistanceDesign]", JSCE (hereinafter referred to as "Specifications [Principles of Earthquake ResistanceDesign]") (in Japanese).
"Standard Specifications for Concrete Structures 2018 [Maintenance, Construction, Design]",
JSCE (hereinafter referred to as "Specifications [Maintenance, Construction, Design]") (inJapanese).
"Standard Specifications for Concrete Structures 2017 [Construction]", JSCE (hereinafter referred
to as " Specifications [Construction]") (in Japanese).
"Standard Specifications for Concrete Structures 2007 [Design]", JSCE (hereinafter referred to as"Specifications [Design]").
"Standard Specifications for Concrete Structures 2012 and 2017 [Design]", JSCE (hereinafter-4-

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

referred to as " Specifications [Design]") (in Japanese).
"Standard Specifications for Concrete Structures 2017 [Test Methods and Specifications]", JSCE(in Japanese).
"Technical Commentaries for Revision of Standard Specifications for Concrete Structures 2007,2012, and 2017", JSCE (in Japanese).
"Verifications of Seismic Performance of In-ground LNG Structure", Energy Committee,
Concrete Library 98, JSCE, December, 1999.
"Recommendations on Evaluation of Structural Sound Function for Underground Reinforced
Concrete Structures in Nuclear Power plants", Nuclear Power Civil Engineering Committee,
JSCE, October, 2012 (hereinafter referred to as the "Recommendations on Evaluation of
Structural Sound Function") (in Japanese).
"Common Specifications for Infrastructures 2016 [Principals, Performance and Actions,
Structural Planning]", Structural Engineering Committee, JSCE (in Japanese).
"Seismic Performance Verification Guidelines, Manuals and Examples for Underground RC
Structures in Nuclear Power Plants” (revised in October, 2018), Nuclear Power Civil
Engineering Committee, JSCE, June, 2018 (hereinafter referred to as "Guidelines, Manuals,
Examples, Technical Commentaries 2018") (in Japanese).
As these regulations and recommendations are revised, seismic performance verification should
essentially comply with, refer to, and/or apply the content of the most recent versions of these
documents. In addition, these regulations and recommendations must be fully examined andunderstood for valid application.
(2)
To effectively ensure the seismic safety of nuclear power plants, their various facilities areclassified into seismic design grades as explained in Commentary Table 1.1-1 (cited from "Chapter
2 Seismic Grade Classification" (JEAC4601-2015)). It should be noted that underground RC
structures are only indirectly involved with the seismic safety of these facilities (cited from
Commentary Table 1.1-2, "Seismic Grade Classification in Chapter 2" (JEAC4601-2015)) and they
are normally dealt with outside the scope of seismic grade classification. On the other hand, according
to the nuclear equipment category (cited from Tables 1.1-3 and "Chapter 2 Seismic GradeClassification" (JEAC4601-2015)), Underground RC structures constitute a part of the system thatguarantees the seismic safety of a nuclear power plant, and thus an underground RC structureindirectly supports the primary and secondary equipment that directly ensure seismic safety. In otherwords, the structure plays an indirect supporting structural role that supports the primary andsecondary equipment (referred to as “direct support structures”). Accordingly, an underground RC
structure ensures seismic safety equivalent to that of the primary and secondary components in termsof functionality. The primary equipment, secondary equipment, and direct support structures areclassified into Class S, Class B, and Class C seismic grades, and the seismic grades of undergroundRC structures are established in accordance with these facilities. Various equipment and piping-5-Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

systems that have been classified as Class S are commonly installed on the floors or walls of facilities
that have been classified as Class S, B, or C. Of these, the present guidelines will focus on the support
function of facilities classified as Class C. Accordingly, the present guidelines define such a function
simply as “support” without designating it as direct or indirect to avoid confusion.
Considering the above, the following two RC structure types represent the underground RC structures
covered by these guidelines:
1) Underground RC structures that require the functioning of supporting equipment and piping
systems classified as seismic grade “Class S”, and
2) Underground RC structures that require an emergency water delivery function.
The various underground RC structures covered in these guidelines are listed in Commentary Table
1.1-4. For example, the underground RC structures corresponding to a reactor cooling water systemfacility (emergency cooling water intake equipment) include intakes and sea water delivery channels
that require a seawater flow function, piping support ducts that require the support of Class S
equipment and piping systems, and intake pits that require both water delivery and support functions.
A layout and bird's-eye view of a typical underground RC structure are illustrated in Commentary
Figures 1.1-2 and 1.1-3, respectively.
Commentary Table 1.1-1 Definitions for seismic grade classification

Class SClass BClass C

Nuclear structures or components with built-in radioactive material or directly related to a facility
with built-in radioactive material. This includes structures and components that are potentiallyhazardous and could release radioactive material externally due to loss of functionality, thoserequired to prevent such situations from happening, those required to reduce the impact of the releaseof radioactive material externally during the onset of an accident, and those that have a significant
influence on the accidental release of radioactive material.
Any of the above nuclear structures or components that have a minor impact on seismic safety
Nuclear structures or components that require the same degree of seismic safety as general industrial
facilities, excluding Class S and Class B structures.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Commentary Table 1.1-2 Functional classification

Class SClass BClass C(i) Equipment and piping systems that contribute to the "reactor coolant pressure boundary" (asdefined in "Safety Review Recommendations for Safety Designs for Light Water Reactors").
(ii) Equipment for storing spent fuel.
(iii) Facilities for rapidly adding negative reactivity to an emergency shutdown of a nuclear reactor
and facilities for establishing the shutdown of a nuclear reactor.
(iv) Facilities that remove decay heat from the core after reactor shutdown.
(ⅴ) Facilities acting as pressure barriers in the event of damage to the reactor coolant pressure
boundary and directly preventing the release of radioactive material.
(ⅴi) Facilities for controlling the external release in the event of an accident involving the release ofradioactive material other than the facilities covered by (i) to (ⅴ).
(i) Facilities directly connected to the reactor coolant pressure boundary with or capable ofincorporating primary coolant.
(ii) Facilities with built-in radioactive waste except for cases where the built-in amount is small or
the effects of radiation on the public due to damage are sufficiently small compared to the annual
exposure dose outside supervised areas, depending on the storage method.
(iii) Facilities related to radioactive material other than radioactive waste and having the potential tocause excessive radiation exposure to the public and employees when damaged.
(iv) Facilities for cooling spent fuel.
(ⅴ) Facilities for controlling the release of radioactive material to the outside and not belonging toClass S.
Facilities not belonging to Class S or Class B.

Commentary Table 1 1-3 Facility category (cited from Tables 1.1-3 and "Chapter 2 Seismic GradePrimary equipment
Secondary equipment
Direct support structures
Indirect support structures
Facilities deemed to haveconsequential impact

Classification" (JEAC4601-2015)
Equipment directly related to the functions required for the safety assurance of a
nuclear power plant.
Equipment indirectly related to the functions required for the safety assurance of a
nuclear power plant and having an ancillary role to the primary equipment.
Support structures attached directly to the primary or secondary equipment or
directly bearing the load of these facilities.
Support buildings or structures such as reinforced concrete or steel frame structuresthat carry loads transferred from the direct support structures.
Facilities that could consequentially impact the facilities of higher seismic grades
when damaged.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Piping support duct

Intake pit (Screen)
Sea

Diesel-generation fuelstorage tank foundation
Fuel piping duct fordiesel power generation

Inlet
Turbinebuilding

DeliveringchannelWater inlet

Reactorbuilding

Stack

Intake pit (Pump)

OutletWater
outletDrainagechannelDrainage pit

Commentary Figure 1.1-2 Example arrangement of nuclear power plant facilities(Yellow color indicates critical underground RC structures)

Turbine building
Shaft

Piping support duct
Sea water pipe
for auxiliary
cooling

Sea water pipe
for condenser
cooling

Intake pit
Sea water
pump
Deliverychannel

Water
inlet

Inlet
Sea

Commentary Figure 1.1-3 Layout example of a critical underground RC structure-8-

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Commentary Table 1.1-4 Facilities and functions related to typical critical underground RC structures
Equipment Systems and their Functions
Typical Underground RC Structures and their Functions○Reactorcoolingwater
system○Water inlet and water delivery channel
equipment (common equipment for
Ensures the required amount of coolingpressurized-water reactors (PWRs) and Water
water in the event of an emergency.
boiling-water reactors (BWRs)○Intake pit (screen section and others)deliveryRemoves the decay heat (residual heat)
Ensures the required amount of coolingof the reactor and supplies sufficientwater in the event of an emergency.
cooling water required to cool the○Intake pit (pumping room)
emergency equipment and piping systems
Safely supports the equipment (e.g., pumps)
in the event of an emergency.
in the event of an emergency.
○Piping support duct
Safely supports piping in the event of an
emergency.
○Emergency power supply equipment○Diesel-generation
fuelstoragetank(common equipment for PWRs andfoundation
BWRs)
Safely supports the oil tank for diesel power
Supplies the necessary power to safely
generation in the event of an emergency.
shutdown the reactor as well as the power○Fuel piping duct for diesel power generation
to operate the engineered safety
Safely supports the oil piping for diesel
equipment in the event of a loss of thepower generation in the event of an emergency.
Class S
external power supply system.
○Safety injection system and auxiliary○Fuel replacement water storage tankwater supply facilities (PWR facilities)
foundation and condensate tank foundation
Support
Removes decay heat (residual heat) by
Safely supports the respective tank to secure,injecting boric acid water into the reactor
for example, boric acid water to remove decay
core in the event of an emergency; theheat in the event of an emergency.
auxiliary water supply facilities work by○Fuel replacement water piping duct andfeeding water to the secondary system ofcondensate piping duct
the steam onset unit in the auxiliary water
Safely supports the respective piping in the
supply system in the event of anevent of an emergency.
emergency.
○Emergency gas processing facilities○Emergency gas processing-piping duct
(BWR facilities)
Safely supports the piping to prevent air
Safely processes gaseous radiation
leakage from the emergency gas processingmaterial generated in the reactor building
piping in the event of an emergency.
thereby maintaining internal negative○Stack foundationpressure in the event of an emergency.
Safely supports the stack to prevent air
leakage from below the limit altitude in theevent of an emergency.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

1.2 Definitions
The following terms are defined for general use in the present guidelines:
・Critical underground RC structure
Generic term referring to underground RC structures that require the supporting function of
special equipment and piping systems classified as seismic grade Class S and also undergroundRC structures that require an emergency water delivery function.
・Reference earthquake motion SS
An earthquake ground motion defined as appropriate for the assumption that such a motion
would very rarely occur, generating a subsequent severe impact on given nuclear facilities
during their service period with regard also to the geological and geotechnical structures nearby
the nuclear power plant site based on the seismology and seismic activity in the vicinity of theplant site. Reference earthquake motion SS should be defined as an earthquake ground motion
on an outcrop rock surface.
・Outcrop rock surface
A free surface hypothetically assumed to have no subsurface layers or no structures on the
bedrock surface. The bedrock surface is idealized to be almost horizontal and be a half-space
without significant difference of surface elevation. "Bedrock" refers to firm ground with a shear
wave velocity (Vs) of approximately Vs = 0.7 km/s or more and with insignificant weathering.
・Performance requirements
Levels of performance required from facility owners as well as socially accepted standards,
considering the importance and application of a facility, its operating conditions, and thesurrounding environment. These should be intelligible to the public.
・Performance objectives
The engineering interpretation of the performance requirements, expressed as various factorsthat comprehensively satisfy the performance requirements.
・Seismic performance
Performance objectives related to the earthquake resistance capability of a structure during
and after an earthquake.
・Durability performance
Performance objectives related to the durability of a structure at the end of its design life.
・Performance verification
A check that a specified structure satisfies the relevant performance objectives.
・Crisis resistanceIn the case of general infrastructure, the capability of a structure to reduce the probability ofdevastating economic and/or social activity by managing the disruptive failure of the entire
structure, even if an unexpected event occurs and despite the completion of the performance- 10 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

verification stage. In the case of underground RC structures, the capability of a structure to avoid
losing the cooling function of a reactor by preventing the structural damage consecutively
impacting priority nuclear systems, even if the response resultants of the structure exceed theallowable limit state.
・Verification item
Limit state or failure mode of a structure or the members of a structure that need to bechecked. A more descriptive expression of the performance objectives to feasibly check the
performance of a structure. Each verification item has a corresponding response value andlimiting value.
・Response value
A value representing the dynamic responses of a structure (e.g., stress resultant, strain) underseismic action.
・Limiting value
The value to be checked to determine whether a performance objective is satisfied.
・Design life
The period specified in the design for which the structure and/or structural members will
satisfy the specified performance requirements.
・Macro element nonlinear analysis
A method of structural analysis (finite element analysis) in which a structural model istypically divided into an array of beam or shell elements and the stress resultants are calculatedusing nonlinear properties with respect to the member section force defined in the respective
element. In the case of a beam element of reinforced concrete, the relationship between the
flexural moment and the curvature is used to express the nonlinear properties of the beam
elements.
・Material nonlinear analysis
A method of structural analysis (finite element analysis) in which a structural model istypically divided into an array of two- or three-dimensional solid elements and the stressresultants are calculated using nonlinear properties with respect to the stress-strain curves
defined in the respective element. In the case of a structural analysis of a reinforced concrete
structure, various constitutive laws for cracked concrete, reinforcing bars, and the bondcharacteristics between them are applied.
・Pushover analysis
A method of structural analysis in which loads or displacements are loaded incrementally
onto a structural model until the member or structure reaches its ultimate state (load-carrying
capacity). Pushover analysis is typically carried out using only monotonic loading but cyclicloading can be used as needed.
・Relative story deformation angle- 11 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Value obtained by dividing the horizontal relative displacement between the upper and lower
slabs of a structure by the distance between the slabs.
・Repair
Maintenance measures primarily intended to restore or improve the durability of a structure
or to restore the mechanical capability of an earlier age of the structure.
・Strengthening (seismic upgrade)
A maintenance measure to restore or improve the structural performance of a structure, suchas load-carrying capacity and/or stiffness.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

CHAPTER 2 PERFORMANCE SETTINGS FOR CRITICALUNDERGROUND REINFORCED CONCRETE STRUCTURES
2.1 General
Seismic performance shall be appropriately determined in accordance with the performancerequirements for a structure of interest as determined by the performance verification for criticalunderground RC structures during an earthquake.
[Commentary]
In the design stage of civil engineering structures, including critical underground RC structures, it
is a guiding principle that the performance requirements for the safety, serviceability, restorability,and durability of a structure are identified, and verification is carried out in a reasonable manner. Of
these performance requirements, the present guidelines primarily cover performance verification forearthquakes (i.e., seismic performance verification) that would seriously affect the safety
requirements of a nuclear power plant. The guidelines exclude direct examination of the timedependent variation of seismic performance over the design life of a structure; however, theguidelines specify durability-related performance requirements and performance objectives as aprerequisite for ensuring acceptable time dependent variation of seismic performance over the design
life of a structure. The present guidelines address durability performance verification as a prerequisite
for seismic performance verification, in particular providing verification methods in Chapter 6 of
the manuals. (NOTE: The Japanese version of the present guidelines are published as one volume
with their manuals and examples. Hereinafter, “the manuals” refers to the one in the same volume.)
Performance-based design is a design method that allows design engineers to discretionarily selecta suitable method of checking whether the performance requirements have been satisfied byassuming that the selected method meets the requirements in an adequately reliable manner while
also clearly defining the required performance. In other words, performance-based design is intended
to promote the adoption of new materials and technologies.
Generally, the concepts underlying performance-based design can be expressed in a hierarchized
structure, as shown in Commentary Figure 2.1-1, with reference to “Common Specifications for
Infrastructures [Performance and Action]” as an example. The first class designates the "objective"
and the function conforming to the "objective" of the structure. The second class then determines the"performance requirements" that satisfy the function. Based on this, the third class defines theperformance using a technical expression that can be checked using a suitable engineering method
or else it is "performance specified". "Performance specified" may be equivalent to the "performance
objective" defined in the guidelines. Finally, the fourth class verifies fulfillment of the “performancerequirements” (i.e., performance verification). The "performance verification" basically employs the- 13 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

limit state design method. Here, the first, second, and third classes have corresponding regulations
that must be observed. On the other hand, the "performance verification" in the fourth class allows
design engineers to discretionarily select the method of confirming “performance verification” aslong as a verification method with an appropriate scope of application is selected, i.e., one regarded
as “an acceptable method” or “deemed as conforming to specification".
Based on the performance-based design concept, regulatory documents, standards, and otherrelevant factors, seismic performance verification for underground RC structures can be adapted tothe hierarchized structure according to their roles and the scope of application. The chapters of thepresent guidelines are organized in accordance with such a structure. Of these, various analysis andverification methods are provided in Chapters 5 and 6 in the manuals as specific examples ofapplicable methods.

Act for the
Regulation of
Nuclear Rectors
and others
Regulations on
Installation
Permission Standards

Interpretation of theRegulation
Safety Review Guides
JEAC4601
Standard Specifications
for Concrete Structuresinc. the present guidelines
and others

Chapter 1 General
Objectives

Performancerequirements

Performance
criteria

Performanceverificationan acceptable method
as deemed conforming to
specification

Chapter 2 Performance
Settings for Critical
Underground Reinforced
Concrete Structures

Specifiedmatters

Chapter 3 Materials
Chapter 4 Actions

Chapter 5 Methods ofAnalysis
Chapter 6 Verification

Discretionary
matters

Commentary Figure 2.1-1 Hierarchized structure of performance-based design

The "performance requirements" are intended to convey the principle of “performance”,
expressing the content of the performance settings in a manner that is intelligible to the public. The"performance objectives" are an engineering interpretation of the performance requirements,
expressing all the factors that must be considered to satisfy the performance requirements inengineering terminology. The performance objectives are determined by combining the assumed load
action with an acceptable limit state of the structure. Each performance objective is broken down intoa “verification item” (i.e., the “limit state” or “failure mode” of a structure) with regard to practical- 14 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

verifiability, and hence the actual process of checking is performed for a corresponding “verificationitem”. The definitions and examples of these performance expressions are described in Commentary
Table 2.1-1. The performance requirements and performance objectives are described in this chapter,and the verification items are described in detail in Chapter 6 Verification.
Commentary Table 2.1-1 Performance expressions
Performance
Performancerequirements

Performance
objectives

Verification
item

Definition
The performance required for a structure fromboth social and economic perspectives. It is
intended to convey the general principle of therequired “performance”.
The performance levels technically achievable
to meet the performance requirements. It
combines the assumed load action with anacceptable limit state of a structure expressedin engineering terms.
The limit state or failure mode to be examinedfor verification. It is described for eachperformance objectives as specific items (limit
states or failure modes) that can be checked.

ExamplesThe pumping function of an intake pit must
continue to operate. The water delivery
function of the sea water piping for a piping
support duct must continue to operate.
For a reference earthquake motion Ss:
1) A given structure does not collapse.
2) A given structure satisfies the constraints
for the functional capability of its
equipment and piping.
The member of a structure does not reach its
limit state, e.g., flexural failure or shear failure.

2.2 Performance Requirements for Critical Underground RC Structures(1) The performance requirements for underground RC structures shall be determined for either
the supporting function of Class S equipment and piping systems or the emergency water
delivery function, depending on their role, objective, and importance as follows:
1) Underground RC structures that require the functioning of supporting will have thesupporting function for equipment and piping systems.
2) Underground RC structures that require the emergency water delivery function will havethe water delivery function (sea water) established for cooling.
(2) Seismic performance and durability performance shall be determined in the performancerequirements for underground RC structures as objective engineering standards.
[Commentary](1)The “Installation Authorization Standards Regulation” provides that nuclear power reactor
facilities of the highest seismic grade must retain safety functionality against possible seismic actionthat may have a major impact on the nuclear power plant during their service period. Accordingly,
ensuring the required level of performance during an earthquake is to be set as the ultimate
performance requirement, even for the underground RC structures covered by the present guidelines.
To ensure the seismic safety of a nuclear power plant, it is essential to control the release ofradioactive substances into the environment in the event of an emergency. To achieve this, nuclear
power plants must implement 1) a function to “shut down” their reactors, 2) a function to “cool down”- 15 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

their reactors, and 3) a function to “confine” radioactivity, even in the event of an emergency. For
these functions, the reactor cooling water system facilities are responsible for function 2) above, i.e.,
to “cool down” the reactors. Underground RC structures represent civil engineering structures that
require the support of seismically important equipment and piping, for example, reactor cooling water
system equipment or water delivery systems in an emergency. Therefore, the performancerequirements should be specified in relation to the safety function of the supporting facilities or theequipment that could be subjected to consequential impact distinct from the function of the structure
itself.
Underground RC structures may differ in their performance requirements from structures thatbelong to the same system (e.g., emergency cooling water intake equipment), such as intakes andintake channels that require the water delivery function during an emergency or intake pits and piping
support ducts that require that require the support function. Accordingly, the performancerequirements are to be independently determined according to the categories of either the support
function of equipment and piping or the water delivery function.
For structures that require supporting equipment, their performance requirements are to be
determined such that the functioning of the supporting equipment can be ensured. Structures
requiring the water delivery function during an emergency include intakes that are part of theemergency cooling water intake equipment and water delivery channels connecting the coastline tothe pump room. These structures will directly provide seawater for cooling. Given the function of theemergency cooling water intake equipment, the equivalent performance requirements of the relevant
equipment and piping (e.g., seawater pumps and seawater pipes) should be applied to the overall
structure. This determines the performance requirements such that the function of seawater delivery
for cooling can be ensured for structures that require it during an emergency.
The water storage function (with no excessive water leakage) may be required for certain structures.
This requires that underground RC structures such as intake channels and intake pits must store
cooling water when tsunami-induced undertows are created near the shore. When examining such
structures for seismic performance verification, the water storage function must be considered as aperformance requirement.
(2)
The function and performance for the equipment and systems in an underground RC structure mustbe established over the design life of the structure. Here, seismic performance as well as durability
performance should be established for underground RC structures to ensure seismic safety over theirdesign life. For seismic performance, the performance objectives should be specified such that anunderground RC structure can sustain sufficient structural safety against loading actions during anearthquake. For durability performance, the performance objectives should be specified such that therequired seismic performance can be demonstrated over the design life of the structure. Here, theterm "design life" refers to the period over which the performance requirements for a structure must
be met, being determined by a nuclear power operator based on social and economic factors.
- 16 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

The durability performance of an underground RC structure is to be determined such that material
degradation due to environmental impact would have little effect on seismic performance over thedesign life of the structure. The carbonation of concrete or penetration of chloride ions into concrete
may result in material deterioration such as reinforcing bar corrosion, the onset of cracking, andreduction of the reinforcing bar cross-sectional area. In addition, freezing and thawing actions cancause material deterioration, such as the reduction of concrete quality. These may affect seismic
performance depending on the degree of severity. Therefore, the performance objectives of durability
performance are to be specified such that the extent of these material deteriorations would have little
or minor effect on seismic performance over the design life of a structure.
Three types of environmental impact, carbonation, the intrusion of chloride ions, and freezing andthawing action, should be considered for the durability performance of underground RC structures.
The presence of carbon dioxide in the air necessitates examination of the carbonation of concrete forthe underground RC structures commonly exposed to the atmosphere. These structures may be incontact with seawater or soil containing seawater and may also be exposed to airborne salt.
Accordingly, the intrusion of chloride ions should be considered in the durability performanceassessment of such underground RC structures. In addition, freezing and thawing action should beconsidered for underground RC structures in cold regions because of the temperature change.
2.3 Seismic Performance(1) Reference earthquake motion SS shall be employed as the seismic action in the performanceverification of underground RC structures during an earthquake.
(2) The seismic performance of an underground RC structure shall be specified as follows:
1) An underground RC structure that requires the support of equipment and piping systems shallmaintain structural integrity without collapse under reference earthquake motion SS as well
as satisfy the conditions specified for the functional capability of its equipment and piping.
2) An underground RC structure that requires the water delivery function in the event of an
emergency shall maintain structural integrity without collapse under reference earthquake
motion SS.
[Commentary](1)
The technical code JEAC4601-2015 classifies underground RC structures mainly as "indirect
support structures", for which the guidelines drop the term indirectly as discussed in Section 1.1
Scope of Application of Chapter 1. This results in using identical reference earthquake motions forstructures as those used for the earthquake resistant design of "primary equipment", "secondary
equipment", or "direct support structures" that could be significantly affected by damage to anunderground RC structure. The reference earthquake motion is to be defined in accordance with theprovisions of the “Interpretation of the Regulation”.
- 17 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-(2)
Underground RC structures are classified as one of the following four types of system: 1) reactorcooling-water system equipment, 2) emergency-power-supply equipment, 3) safety-injection-system
and auxiliary-water-supply equipment, and 4) emergency gas-processing equipment (seeCommentary Table 1.1-4). According to the technical code "Chapter 2-Seismic Grade Classificationin JEAC4601-2015", the systems labeled 1) to 4) above are classified as seismic grade Class S. In
addition, the performance requirements for underground RC structures are specified distinctly as
either support function or emergency water delivery function. These discussions permit theperformance objectives for underground RC structures to be specified for their respective required
functions. The performance requirements for various structures and the performance objectives being
developed are described in Commentary Table 2.4-1.
The functional capability for equipment and piping should be confirmed under the reference
earthquake motion SS for the underground RC structures requiring the support function of Class S
equipment and piping systems. For this purpose, an underground RC structure needs to secure
installation space, and the structure should provide the required load-carrying capability. Usually, theload-carrying capability can be ensured by demonstrating that the structure maintains its structural
integrity without collapse. Accordingly, the performance objectives for the underground RC
structures requiring the support function of Class S equipment and piping systems should be specified
such that the structure can maintain its structural integrity without collapse under reference
earthquake motion SS. In addition, restrictions on the functional capability of equipment and piping
presented by mechanical or electrical engineers are to be examined during verification.
The systems of underground RC structures requiring the water delivery function in the event of an
emergency are classified as Class S; therefore, the water delivery function should be secured to ensure
proper functioning of the system. This can be achieved with reference to the load-carrying capabilityof the structure. Thus, the performance objectives should also be specified such that the structure canmaintain its structural integrity without collapse under reference earthquake motion SS for
underground RC structures requiring the water delivery function
After the Great East Japan Earthquake, civil engineering communities have recognized theimportance of ensuring crisis resistance such that a structure will never experience catastrophic
damage even if the intensity of a seismic event exceeds the design earthquake ground motion. In thecase of underground RC structures considered as a part of the seismic risk management at a nuclear
power plant, it is desirable to avoid structural collapse due to the exceedance of the specified limit
state defined in the verification process that may affect the upper system and result in the loss of thecooling function of a reactor. Currently, no concepts have been yet established for designing andbuilding power plant facilities (facilities and their environment) to achieve crisis resistance; however,crisis resistance may be approached via seismic design planning. It is also possible to examine crisis
resistance using certain measures, e.g., preventing the brittle failure of a structure by identifying its
failure limits using a pushover analysis or controlling the structural damage of an underground RC- 18 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

structure from the interference of possible recovery activities during accident management. In any
case, the fracture behavior of individual facilities and structures as well as their consequences need
to be understood to comprehensively enhance crisis resistance.

- 19 -

Capabilities of a nuclear power plant during an earthquake

Capabilities of equipment and piping systems

Class S equipment andpiping highly integrated
into an underground RC
structure↓
To “shut down”
Equipment, buildings, andother objects that execute a"shut down", "cool down", or"confine" function normallyoperate without any effectfrom seismic action.
↓
Seismic grade classifications:- 20 -

Securing safety for anuclear power plant during
an earthquake includes:
To “shut down”, to “cooldown” a reactor, and to“confine” radioactivity.

Class S・Facilities with the functionality
required to shut down a reactor andcool down a reactor core in
response to a possible event due toseismic action.
・Facilities that are directly related
to equipment that could diffuse
radioactive substances to theexterior due to the loss offunctionality.
・Facilities necessary to mitigate
the impact of accidents resulting
from the loss of function andreduce the impact of radiation on
the environment.

(1) Emergency-powersupply facilities
The facilities that supply
power are required to safely
shut down a reactor in the eventof a loss of the external power
supply system and to operate
the engineered safety facilities.
(Common for PWR and BWR)

To “cool down”(2) Reactor-cooling-watersystem facilities
In the event of an emergency,
the facilities remove the reactor
decay heat (residual heat) andsupply sufficient cooling water
to cool the emergency nuclear
equipment.
(Common for PWR and BWR)

to “confine”:(3) Emergency gasprocessing facilities
The facilities safely process
the radioactive waste gas
generated inside the reactor
building in an emergency, while
maintaining negative pressure.
(Peculiar for BWR)

Performance requirements forequipment and piping during
an earthquake:To ensure the pumping
capability of pumps.
To ensure the water delivery
capability of pipes.
To ensure the liquid storage
function of tanks.
To ensure the ventilation
capability of pipes.
To ensure the functioning ofstacks.
Performance objectives forequipment and piping:
For reference earthquake
motion Ss:
Piping maintains required
functionality under the loadconditions considering loads undernormal operation and loads arisingfrom transient abnormalities andaccidents during operation andseismic action under the reference
earthquake motion.
・Even when plastic strain
develops in the facilities under aloading as described above, theamount of strain remains at a verysmall level. Thus, the facilities
should have sufficient safetymargin in their load carrying
capacity to remain unaffected intheir functionality as required forthe facilities.
・Equipment establishes therequired functions or dynamic
function against dynamic
responses induced by reference
earthquake motion Ss.
Specifically, the allowable limits
shall be established as functional
capability acceleration as
determined by verification tests.

Capabilities of Underground RC structures
Performance requirements
Performance objectives
For reference earthquake
To safely support equipment
motion Ss:
and piping
Underground RC(1) Structures maintain(Assurance of support
structures that require thestructural integrity without
function)
supporting function ofcollapse.
Class S equipment and(2) Structures satisfy the
The equipment and piping
piping.
requirements for the
maintain operation without
functional capability ofloss of functionality･Intake pits
equipment and piping as(Common for PWR and BWR)
(Notes)an underground RC
Diesel-oil-pipe duct
Underground RC structures are
structure.
(Common for PWR and BWR) civil engineering structures that
(In coordination with･Piping-support ducts
require the supporting function of
mechanical and/or(Common for PWR and BWR) seismically important equipment
electrical engineeringand piping or the water delivery･Emergency gasinput)function of seawater in case of anprocessing-pipe duct
emergency. Their seismic design↓(Peculiar for BWR)
requirements are to be established(1) Verification using
in view of the safety function of therelative story deformation※Tanks and stack foundations
support structures and facilities(massive concrete) areangle, shear capacity, andbeing possibly affected bybeyond the scope of theseother factors.
significant damage distinct fromguidelines.
(2) Checking equipment
the overall structural functions.
(“JEAC4601-2015”)
and piping conditions.
Structures

Underground RC
structures that require theemergency water delivery
function･Water delivery channel，
intake pits

(Common for PWR and BWR)

Delivering the required
amount of sea water(Securing the form of crosssection needed to deliver sea
water)For reference earthquake
motion Ss:(1) Structures maintainstructural integrity without
collapse.
↓(2) Verification using
relative story deformation
angle, shear capacity, andother factors.

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Commentary Table 2.4-1. Required performance for critical underground RC structures developed
from the performance requirements for a nuclear power plant during an earthquake

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

2.4 Steps for Performance Verification
For performance verification during an earthquake, the performance settings are first discussed
considering the performance requirements for a considered structure and the design conditions.
Second, methods of analysis and verification are selected based on the material properties,
seismic action, and environmental impact. Finally, seismic performance verification is carriedout in a distinct manner depending on whether the structure requires the supporting function ofcertain equipment and piping systems or whether the structure requires the emergency water
delivery function.
[Commentary]
Typical flowcharts of performance verification for both underground RC structures requiring the
support function of equipment and piping system as well as the emergency water delivering function
are shown in Commentary Figures 2.6-1 and 2.6-2, respectively. As clearly indicated, the present
guidelines cover the steps in the box with the bold-dashed-line based on the assumption that a survey,
preliminary assessment, and ground stability assessment have already been implemented at a site.
The main objective of performance verification for the underground RC structures requiring the
support function of Class C equipment and piping system is to establish the functional capability of
its equipment and piping under the assumed load conditions as shown in Commentary Figure 2.6-1.
This specifies the constraints required for an underground RC structure as the “conditions given” toensure functional capability of their equipment and piping. In addition, matters incapable of being
directly checked are to be examined as part of an integrity evaluation of the equipment and piping
system outside the flow of the underground RC structure verification process. Specifically, in theunderground RC structures requiring the support function, the seismic safety of equipment and piping
is to be evaluated based on the floor response acceleration. The performance settings for anunderground RC structure may be reset depending on such floor response evaluations. These steps
complete the verification including verification for the seismic safety of the equipment and piping.
From the viewpoint of streamlining the structural design process, it is desirable to check the seismic
safety of equipment and piping using a coupled-structural-system analysis that is incorporated into
the process of the overall structure, but such numerical computation is not possible at present. Thus,
the guidelines identify the boundaries between the equipment and piping and an underground RC
structure, providing a framework for the checking process. For example, if equipment is anchored toa certain part of a structural member that was damaged during an earthquake, the impact of thedamage on the anchor or the equipment and piping should be examined using an appropriate approach.
On the other hand, performance verification for the underground RC structures requiring the
emergency water delivering function can be carried out in accordance with the use and purpose of
the structure, and the verification is considered complete without assessment of the seismic safety ofthe equipment and piping, as indicated in Commentary Figure 2.6-2.
Aging related to the carbonation of concrete, intrusion of chloride ions, and freezing and thawing
action may degrade the seismic performance of underground RC structures. Ideally, the seismic- 21 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

response analysis conducted during seismic performance verification should consider the state ofaging about the design life of a structure; however, the current methods of structural analysis areunable to model this. Accordingly, the present guidelines ensure seismic performance through theindependent verification of durability performance and seismic performance. Hence, limit states,
such as “aging condition having little or minor effects on the seismic performance of a structure", aredetermined during the durability performance verification. This verification confirms the assumption
made during the seismic performance verification that the conditions of a structure (e.g., material
properties, bond characteristics between reinforcing bar and concrete, and stiffness) are the same as
immediately after completion of the structure.
Although the present guidelines only cover performance verification in the event of an earthquake,
underground RC structures are essentially required to ensure water delivery functionality for coolingseawater even under normal conditions (i.e., under stationary loading). In other words, it needs to beensured that the quality of materials assumed during the seismic performance verification of anunderground RC structure is unaffected by the continuous action that would have occurred undernormal conditions. In addition, the structural design of underground RC structures for normal
conditions needs to be streamlined. Therefore, the contents of structural performance verification
under normal conditions are also provided in Chapter 6 of the manuals.

- 22 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Start

Survey and preliminary assessment (including environmental impact)
Ground stability assessment (sliding, bearing capacity, liquefaction potential
evaluation, etc.)

Given conditions: Classification of structures, loads (reference earthquake
motion Ss), geotechnical characterization, environment, etc.)
Setting of performance objectives: Conditions for functional capability of anunderground RC structure, given the design conditions of equipment andpipingCondition settings: Material properties, cross-sectional parameters, structural
shapes and dimensions, etc.

Checking
Seismic performance verification
Setting of limit states(States of local damage)Setting of limiting
valuesCalculation oflimiting values
Limiting relative story
deformation angle.
Limiting strain
Shear capacity.

Settings of Analysis
Conditions
Material model
Structural model

Nonlinear dynamic
response analysis

Calculation of
response values
Relative story
deformation angle.
StrainStress resultant.

Response values≦limiting values

Yes
Checking of equipment and piping

Assessment of the effect ofdeterioration on structural
performance

Structural
integrity
assessment
under stationaryloads

No (Reset)

No (Reset)
Scope of application of
the guidelines

Yes
End

Durability performanceverification

Structural
performanceverification
under normal
conditions(seethe manuals)

Note: The limiting and response values presented here indicate the limiting values for verification
and response values for verification as discussed in “Chapter 6. Verification”, and structural factor
only briefly presented here should also be considered in the process.

Commentary Figure 2.6-1 Flow chart of performance verification (support function)- 23 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Start

Survey and preliminary assessment conditions (including environmental
impact)Ground stability assessment (sliding, bearing capacity, liquefaction potential
evaluation, etc.)

Given conditions: Classification of structures, loads (reference earthquake
motion Ss), geotechnical characterization, environment, etc.)
Setting of performance objectives: Conditions for functional capability ofunderground RC structures, given the design conditions of equipment andpipingCondition settings: Material properties, cross-sectional parameters, structural
shapes and dimensions, etc.

Checking
Seismic performance verification
Setting of limit states(States of local damage)Settings of analysis
conditions
Material model
Structural model

Setting of limiting
valuesNonlinear dynamic
response analysis

Calculation oflimiting values
Limiting relative story
deformation angle.
Limiting strain
Shear bearing capacity.

Calculation of
response values
Relative story
deformation angle.
StrainStress resultant.

Response values≦limiting values

Durability performanceverification
Assessment of the effect ofdeterioration on structural
performance

Structural
performanceverification
under normal
conditions(seethe manuals)
Structural
integrity
assessment
under stationaryloads

No (Reset)

Yes
Scope of the applicationof the guidelines

End

Note: The limiting and response values presented here indicate the limiting values for verification
and response values for verification as discussed in “Chapter 6. Verification”, and structural factor
only briefly presented here should also be considered in the process.

Commentary Figure 2.6-2. Flow chart of performance verification (water delivery function)- 24 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

CHAPTER 3

MATERIALS

3.1 General
Material properties shall be determined using characteristic values of the materials andmechanical properties, such as the stress-strain relationship, according to the application todetermine the structural response values and estimate the limiting values.
[Commentary]
When structural analysis is carried out during seismic performance verification for a criticalunderground RC structure, the material properties of the soil, concrete, and reinforcing bar are
required to assess the loads (e.g., earth pressure and self-weight), structural responses, and limiting
values of the structure. “Material properties” refers to the characteristic values of a material (such asconcrete strength) and a mechanical model of a material (such as the constitutive law of concrete).
In the present guidelines, the mechanical properties of materials mainly cover the flexural momentcurvature relationship of the cross section of a reinforced concrete member used in macro element
nonlinear analysis and the stress-strain relationship (in a compressional, tensile, or shearing field)
used in material nonlinear analysis. Most mechanical models of a material have one characteristic
value as the parameter of the model. Providing the characteristic values of a material often enables
the evaluation of its limiting values using an appropriate formula, whereas structural response
analysis requires a mechanical model of the material. These mechanical properties should beestablished considering the accuracy and limitations of the formulas used to calculate the limiting
values and the tools for structural analysis used in the verification process. Since the characteristic
values of a material are usually scattered to some extent, they should be specified under the mostunfavorable conditions of the structure. The mechanical property models are described in Chapter
5. Methods of Analysis.
Safety verification for existing structures offers narrower uncertainty factors for the construction
work than for the design stage. The measured values of material properties also usually havevariations depending on the data acquisition method. Accordingly, properly managed construction
and data acquisition methods allow the characteristic values of materials to be successfully employed
as measured values.

- 25 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

3.2 Material Properties of Concrete, Reinforcing Bar, and Soil
(1) Characteristic values for concrete materials shall be determined based on tested values
obtained using actual concrete materials and mix proportions.
(2) Characteristic values for reinforcing bars shall be determined based on tested values obtainedusing actual reinforcing bars.
(3) Characteristic values for soil materials shall be determined based on tested values obtained
from in situ tests at the site and laboratory tests of samples taken from the site.
[Commentary](1)Concrete with appropriate mix proportions (such as strength, slump, etc.) has relatively little
variation in its inherent material properties; however, its actual material properties under practicalmix proportions can vary depending on the conditions of the aggregate and mixer kneading.
Accordingly, the characteristic values for concrete materials are to be determined based on tested
values obtained using actual materials and mix proportions.
The material properties of concrete are characterized by compressive strength, tensile strength,bond strength, Young's modulus, Poisson's ratio, stress-strain curve, etc. They can also becharacterized by thermal properties, drying shrinkage, and creep, depending on the environmental
conditions around the structure of interest. Concrete material properties that are not time dependent
are to be generally determined based on strengths obtained from 28-days tests; however, they may
be determined based on tests using other appropriate ages depending upon factors such as the durationof the principal loading, the construction schedule, materials, mix proportion, and construction
Characteristic values for concrete strength may generally
be obtained using Eq. (3.2-1), which has been established byprobability to have tested values less than the characteristic
value and distribution shape of tested values in considerationof the variation arising from the manufacturing process ofconcrete.

Frequency
頻度

technique.

kσ
fk

fm

f

試験値
TestedValue(3.2-1)

𝑓𝑓𝑘𝑘 = 𝑓𝑓𝑚𝑚 − 𝑘𝑘𝑘𝑘 = 𝑓𝑓𝑚𝑚 (1 − 𝑘𝑘𝑘𝑘)

Commentary Figure 3.2-1
Concept of Characteristic Values

where fk is the characteristic value, fm is the mean of tested
values, σ is the standard deviation, δ is coefficient of variation of tested values, and k is the factor.
The other characteristic values for concrete material are to be essentially in accordance with the“Specification [Design Edition]”. The following is an outline of the respective characteristic values
provided in the “Specification [Design Edition]”.
a. Compressive strength
The characteristic values for the compressive strength of concrete are to be specified such that the- 26 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

probability of a tested value being smaller than a characteristic value be less than or equal to 5%.
When this is the case and a normal distribution is assumed, then the coefficient 𝑘𝑘 becomes 1.645(see Commentary Figure C3.2-1)

Characteristic values for the compressive strength of concrete are, in general, to be based on 28-

days tests in accordance with JIS A 1108 "Method of Test for Compressive Strength of Concrete".
Specimens should be prepared in accordance with JIS A 1132 "Methods of Making and Curing
Concrete Specimens". When JIS A 5308 "Ready-Mixed Concrete" is used, however, the nominal
strength specified by the purchaser may be used as the characteristic compressive strength of theconcrete.
When testing compression strength using drilled concrete cores to identify the characteristic valuesof the strength of existing structures, JIS A 1107 "Method of Sampling and Testing for Compressive
Strength of Drilled Cores of Concrete" should be followed. The use of measured results in theverification of existing structures is provided for in the "Recommendations on Evaluation of
Structural Sound Function" and "JEAC 4601-2015". The conditions and methods for incorporatingthem into the safety factors are described in Chapters 2 and 3 of the manuals.
b. Tensile strength
The characteristic values for the tensile strength of concrete are to be determined using appropriate
tests or Eq. (3.2-2). Tests for the tensile strength of concrete are to be carried out in accordance with
JIS A 1113 "Method of Test for Splitting Tensile Strength of Concrete."2⁄3(3.2-2)′
𝑓𝑓𝑡𝑡𝑡𝑡 = 0.23𝑓𝑓𝑐𝑐𝑐𝑐

where ftk is the tensile strength of concrete and fʹck is the basic design strength.
c. Bond strength

The characteristic values for the bond strength of concrete are to be determined using appropriate
tests. Alternatively, deformed bars that meet the provisions of JIS G 3112 "Steel Bars for Concrete
Reinforcement" may be obtained using:2⁄3(3.2-3)′𝑓𝑓𝑏𝑏𝑏𝑏𝑏𝑏 = 0.28𝑓𝑓𝑐𝑐𝑐𝑐

2

where fbok is the bond strength of concrete and is less than or equal to 4.2 N/mm .
d. Flexural cracking strength
The flexural cracking strength of concrete may be obtained using:where(3.2-4)𝑓𝑓𝑏𝑏𝑏𝑏𝑏𝑏 = 𝑘𝑘0𝑏𝑏 𝑘𝑘1𝑏𝑏 𝑓𝑓𝑡𝑡𝑡𝑡
𝑘𝑘0𝑏𝑏 = 1 +0.55

𝑘𝑘1𝑏𝑏 = 4√ℎ1

(3.2-5),0.85+4.5(ℎ⁄𝑙𝑙𝑐𝑐ℎ )(3.2-6)(≥ 0.4),and k0b is the coefficient representing the relation between tensile strength and flexural cracking- 27 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

strength on account of the tension softening characteristics of concrete, k1b is the coefficient
representing the reduction in crack strength caused by drying, heat of hydration, etc., h is height ofthe member (m) and lch is the characteristic length (m) (lch = GFEc/f2tk), Ec is Young’s modulus, GF
is fracture energy, and ftk is tensile strength. Fracture energy and Young's modulus are to beobtained in accordance with "f" and "g" below.
e. Stress-strain curve
Several curves have been developed for various stress-strain behaviors. It is desirable to select anappropriate stress-strain curve after reviewing their features and limitations. When examining
ultimate limit states under the large deformation of a structure, it is advisable to use a stress-strain
curve that can be expected to provide an elaborate solution in the softening region. In general,
concrete surrounded by hoops or shear reinforcement is known to have greater compressive strengthand ultimate strain on account of the confining effect of those reinforcements. If such a confining
effect of reinforcement is identified via experiment, the experimental results may be used as thestress-strain curve. Appropriate hysteresis curves consisting of unloading and reloading curves
should be selected based on previous studies or experiments.
Since the stress-strain curves under biaxial or triaxial stress differ from those under uniaxial stress,
the effect of such stress is to be taken into consideration for examination of the ultimate limit states.
In addition, a concrete constitutive model, which will be provided later in the present guidelines, isrequired to handle stress-strain curves under biaxial or triaxial stress.
The stress-strain relationship of concrete used for earthquake response analysis is described in
Chapter 3, "3.2 Calculation of response values using material nonlinear analysis" of the manuals.
f. Tension softening properties
Fracture energy, 𝐺𝐺𝐹𝐹 , of normal concrete may be obtained using:
11�
3(3.2-7)
where 𝑑𝑑𝑚𝑚𝑚𝑚𝑚𝑚 is the maximum size of coarse aggregate (mm), and fʹck is the characteristic value of′
𝐺𝐺𝐹𝐹 = 10(𝑑𝑑max ) �3 ∙ 𝑓𝑓𝑐𝑐𝑐𝑐

compressive strength (basic design strength).
g. Young's modulus

The Young's modulus of concrete is to be obtained from appropriate tests. The test is to be
conducted in accordance with JIS A 1149 “Method of Test for Static Modulus of Elasticity ofConcrete”. In general, the values of the Young's modulus for concrete may be taken to be equal tothat shown in Commentary Table 3.2-1 provided in the “Specification [Design Edition]”.
Commentary Table 3.2-1. Young's modulus of normal concrete′
𝑓𝑓𝑐𝑐𝑐𝑐

𝐸𝐸𝑐𝑐(N/mm2）

18

24

30

40

50

60

70

80

(kN/mm2）

22

25

28

31

33

35

37

38

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

h. Poisson`s ratio
The Poisson`s ratio of concrete may, in general, be set as 0.2 within the elastic range.
(2)
In general, the strength of reinforcing bars varies in quality even when the manufacturing process
is properly controlled. Thus, the material properties of reinforcing bars are to be determined basedon the strengths obtained by tensile tests, including reinforcing bars specified by JIS (the JapaneseIndustrial Standards).
The material properties for reinforcing bars are determined by yield strength, tensile strength, andthe stress-strain relationship under tension, compression, and shear. In addition, the thermal
properties are to be considered as material properties in accordance with the environmental
conditions of the structure of interest. The material properties for reinforcing bars are to be specified
in accordance with the “Specification [Design Edition]”. These specifications are outlined below:
a. Tensile yield strength and tensile strength
The characteristic values of tensile yield strength and the tensile strength of reinforcing bars are to
be determined based on the strengths obtained from tensile tests. The tensile tests of reinforcing bars
are to be carried out in accordance with JIS Z 2241 “Metallic Materials-Tensile Testing-Method of
Test at Room Temperature”.
In general, reinforcing bars that meet JIS provisions have lower limit values specified as theircharacteristic values. The nominal cross-sectional area of a reinforcing bar specified in JIS may be
used for the purposes of design.
b. Compressive yield strength
The characteristic values of the yield strength of reinforcing bars in compression may be taken to
be the same as that in tension.
c. Stress-strain curve
The stress-strain curve of reinforcing bars is to be assumed to have a suitable form for the purposeof structural analysis. Several stress-strain curves have been developed, including a fully elastoplastic
model with tensile yield points and a bilinear model with secondary stiffness connecting tensile yieldand tensile strength points. The heterogeneously developed stress and strain of reinforcing bars inconcrete increase at the crack position and decrease elsewhere. Thus, recent studies have formulated
the stress-strain relationship of reinforcing bars as an average stress-strain curve. When analyzing
concrete areas in which cracks are dispersed, the use of such a model improves the accuracy of theanalytical results. Preferably, a hysteresis curve consisting of unloading and reloading curves should
be selected from an appropriate model based on previous studies or experiments.
d. Young's modulus and Poisson's ratio
It is a principle that the Young's modulus of reinforcing bars is to be determined from the resultsof the stress-strain curve obtained by the tests for tensile yield strength. Reinforcing bars that meet
JIS provisions may generally be set as 200 kN/mm2. Similarly, the Poisson`s ratio for reinforcing
bars, in general, may be set as 0.3.
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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-(3)
In general, geotechnical profiles vary from site to site with variation in soil properties as well.
Therefore, the characteristic values for soil materials are to be determined based on strengths andmoduli obtained from in situ tests as well as laboratory tests of undisturbed samples taken from thesite. The characteristic values of the soil parameters may also be taken from proven data specified in
geotechnical recommendations or standards or previous test data available elsewhere in certain cases
in which the applicability of the tools for structural analysis or the estimation accuracy of seismic
actions or limiting values clearly conform to the safety requirements of the verification process.
In situ tests and laboratory tests to determine the material properties of soil parameters are to be
conducted in accordance with the "Report on Soil Stability Evaluation Method of JSCE" (Part 3,
Ground Survey and Testing Method) or "JEAG 4601-2015". Testing practices are to be carried out inaccordance with widely recognized standards such as JGS (Societies of Geological Engineering
Standards) or JIS.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021

CHAPTER 4

ACTIONS

4.1 General
For performance verification under seismic action, the actions likely to occur during the design
life of a structure shall be determined according to the performance requirements and concerning
the level of intensity experienced by the structure. The seismic action shall be specified using
earthquake motions established based on the seismicity and ground characteristics around the site.
The performance requirements for a structure shall be identified according to the actions
expected to occur during the design life of the structure.
[Commentary]
In the present guidelines, “action” refers to any event that could cause stress and/or deformationof a structure or members of a structure, as well as any change in material properties over time due
to deterioration. Typical combinations of actions and actions required for structural performanceverification under normal conditions and durability performance verification are described in Section
4.2 Combinations of Actions. Actions to be considered in seismic performance verification and thereference earthquake motion for seismic action are described in Section 4.3 Seismic Action in
Seismic Performance Verification.
Seismic action refers to the effects of a reference earthquake motion on any dynamic behavior ofa critical underground RC structure. The reference earthquake motion is to be established inaccordance with “The Interpretation of the Regulation” and defined under the conditions given forseismic performance verification, being deliberately determined in accordance with appropriate
provisions other than the present guidelines. The reference earthquake motion adopted as the seismic
action for an instance of seismic performance verification is to be determined based on theperformance requirements for the underground RC structure of interest as discussed in Section 2.3
Seismic Performance.

4.2 Combinations of Actions(1) Combinations of actions shall be determined considering the characteristics of individual
actions, the probability of their simultaneous occurrence, and the degree of their impact on
the limit state.
(2) Environmental actions that induce material deterioration such as carbonation, chloride ion
intrusion, and freezing and thawing action shall be determined, in principle, based on fieldobservation data at the site.
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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021(3) Permanent and variable actions shall be accounted for in durability performance verificationand structural performance verification under normal conditions.

[Commentary](1)
In the present guidelines, the combinations of actions tabulated in Commentary Table 4.1-1 shouldbe generally specified according to the performance verification being conducted.
Commentary Table 4.1-1 Combinations of design actions
Seismic performanceverification
Durability performanceverification
Structural performanceverification for normal
conditions

Permanent action + seismic action (variable action is excluded)
Permanent action＋variable action
Permanent action＋variable action

In the present guidelines, load actions are generally divided into permanent actions, variableactions, and seismic actions. Permanent action refers to a load that acts continuously on a structureand has variability of negligible magnitude compared with the average magnitude. Permanent action
includes self-weight (dead load), installation load, overburden load, permanent imposed load, earthpressure at rest, exterior hydrostatic pressure, and interior hydrostatic pressure. underground RC
structures are constructed in coastal areas, where the groundwater level depends on the sea level.
Hydrostatic pressure acts on these structures below the groundwater level, thereby varying with thesea level of the tide; however, the sea water level can be seen as stationary over the long term. This
results in hydrostatic pressure being considered a permanent action here. Variable action refers to aload that varies frequently or continuously and the variations are of non-negligible magnitude
compared with the average load. Variable actions include variable loading due to the effect oftemperature, snow loading, and vehicle traffic; however, when snow fall is frequently observed in acold region, the snow load should be considered a permanent action.
For typical loading actions other than seismic action, their characteristic values are to be
determined so as to provide the most unfavorable conditions for the limit states of a structure
considering the expected variation of the individual loads during the design life, the combinations ofactions, and the limit state being examined.
The action considered during verification is firstly selected from one of seismic action, structural
performance verification under normal conditions (permanent action + variable action), or
environmental action. Next, the combinations of actions are specified according to the category ofverification. Finally, the characteristic values for the respective actions are evaluated and thenmultiplied by action factors to define the design action. Commentary Table 4.1-2 outlines the type of- 32 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021

and calculation methods for the load actions under consideration. Specific characteristic values may
be obtained based on design drawings or the “Specifications [Design Edition]”, or from elsewhere;
however, specific values are to be used if they have been obtained from testing or observed data.
Commentary Table 4.1-2. Outline of load (action) types and calculation methods
Type

Load
Self-weight
Stationary
load

Permanent
actionEquipment andpiping load
Overburden
loadPermanent
imposed load

Earth pressure at rest
Exterior hydrostatic pressure
Interior hydrostatic pressure

Variable action

Variable
imposed load
Snow load
Effect oftemperature
Horizontal
earthquake
ground motion

Seismic action*1

Verticalearthquake
ground motion

Method of Calculation
The volume of the structure multiplied by the unit weight of thematerial based on the design drawings.
The loads specified by the mechanical designs etc., calculated
from the weight of equipment and piping.
The weight of the ground above the crown of the structure.
The weight of objects permanently placed above the groundsurface.
The direction (positive or negative) in which the earth pressure atrest contributes to the stress resultant and accounting for the extent
of variation. Specifically, the coefficient of earth pressure at rest
uses upper and lower limits to determine appropriate action
factors.
Hydrostatic pressure based on the groundwater level.
Hydrostatic pressure acting on the inner wall of a structure, suchas that generated by a water delivery channel or an intake pit.
The live load of traffic transferred to the structure.
Calculation based on the design depth of snow above the ground,unit design weight of snow, and gradient of the structure’s surface.
If the effect of temperature is a subsidiary load, the rigidity of thestructure may be reduced by 50% of the total cross-sectional
rigidity in calculating thermal stresses.
The time history accelerations are specified in accordance with
provisions other than the present guidelines. They are also used asgiven conditions in the present guidelines.
Time history accelerations are specified as given conditions in thesame manner as the horizontal earthquake ground motions. The
seismic coefficient of vertical ground motion is generally set to be
50% of the horizontal maximum acceleration amplitude of thereference earthquake motion defined at the site rock outcrop
surface.
Appropriately modeled by dynamic response analysis.

Dynamic waterpressure* 1) In principle, structural analysis is carried out via time-history earthquake response analysis in the present guidelines.
When other methods are used for structural analysis, the seismic action should be treated as follows:
Dynamic earthpressure
Inertial force
Dynamic waterpressure during
an earthquake

Dynamic earth pressure is, in principle, estimated using dynamic response analysis. Other methods
may be used if proven tools are available.
The effect of a horizontal motion is calculated by multiplying the horizontal acceleration response ofa structure obtained from dynamic analysis by its mass. The effect of a vertical motion is determinedusing a seismic coefficient equal to 50% of the horizontal maximum acceleration amplitude for thereference earthquake motion at the site.
Dynamic water pressure during an earthquake can be estimated using Westergaard’s formula and otherproven methods.

(2)
Environmental actions examined during performance verification are to be identified about theenvironmental conditions in which the structure of interest is constructed. The constructed- 33 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021

environments of the underground RC structures in Japan have carbonation, intrusion of chloride ions,and freezing and thawing processes as their associated environmental actions; however, the actual
environmental conditions may differ.
For the carbonation of concrete, the required conditions (e.g., the wetting conditions) are first
determined to estimate depth of carbonation. Carbonation-induced deterioration, which frequently
occurs on concrete structures almost everywhere in Japan, should be considered as an environmental
action.
For the intrusion of chloride ions, parameters such as the outdoor temperature, humidity, dry-wet
cycle conditions, and the distance from a shoreline are determined based on field observation data toassess the diffusion of salinity from the concrete surface to the interior. Japanese nuclear power plants
constructed on the coast need to consider the intrusion of chloride ions for their associatedenvironmental actions.
For freezing and thawing action, freezing days and freezing and thawing action frequency are
determined based on a survey of local weather conditions (e.g., outdoor temperature); however,freezing and thawing action need not be considered in environments where no freezing actions areobserved.
Concerning durability performance verification, an estimation of the cracking width of a developed
crack in a concrete structure is required as a prerequisite condition for the consideration of the
intrusion of chloride ions. This requires a load action to calculate the cracking width.
(3)
In addition to individual variable actions, the simultaneous action of the effect of temperature andvariable imposed loading can be considered. All other combinations, e.g., the effect of temperatureand snow load, snow load and variable load, etc., are not considered because the probability of theirsimultaneous occurrence is judged to be negligible.
In durability performance verification, the checking of concrete cracks is to be the first item. The
reason for this is that this information is required to set the diffusion coefficient for chloride ions insubsequent verification of chloride-induced deterioration. In addition, if the cracking width is below
or equal to the allowable value, the crack can be ignored for the verification of carbonation.
Structural performance verification under normal conditions can be carried out via the stresscentered checking of strengths of reinforcing bar and concrete. This verification method assumes thatthe rigidity of the structural members is higher than that in seismic performance verification, which
considers the large deformation of structural members. Therefore, the effect of temperature shouldbe accounted for in verification under normal conditions.
Commentary Table 4.3-1 shows the combinations of actions (loads) considered in durability
performance verification and structural performance verification under normal conditions. The datain the table are supplemented with the following outlines:
a. Individual variable action
Characteristic values for a subsidiary variable load, snow load, and the effect of temperature may- 34 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021

be determined to be one-half of the load estimated using commonly accepted methods.
b. Combination of the effect of temperature and a variable loadSignificantly small probabilities of the simultaneous occurrence of these loads can further reducethe characteristic values already modified according to “a” above. This reduction factor can begenerally assumed to be 0.7. In this case, the method for calculating the characteristic values for eachload are:
1) Variable imposed load: 9.8 kN⁄m3 × 0.5 × 0.7 = 3.43 kN⁄m22) Effect of temperature: (estimated effect of temperature) × 0.5 × 0.7

Commentary Table 4.3-1 Combinations of load actions for durability performance verification and structural
performance verification under normal conditions
Type of combinations

a. Individual variable actions
b. Combination of the effect oftemperature and a variable load

Variable imposedload
Applicable
N/A
N/A
Applicable

Variable action
N/A
Applicable
N/A

Effect oftemperature
N/A
N/A
Applicable

N/A

Applicable

Snow load

4.3 Seismic Action Considered in Seismic Performance Verification(1) Seismic action shall be evaluated by time-history seismic response analysis with soilstructural interaction using a reference earthquake motion.
(2) The combination of permanent action and seismic action shall be considered in the seismic
response analysis.
[Commentary](1)
The seismic action considered in seismic performance verification is evaluated based on a timehistory seismic response analysis with soil-structure interaction using a reference earthquake motion
defined at the site rock outcrop surface. As shown in Chapter 5. Methods of Analysis of the present
guidelines, the dynamic behavior of an underground RC structure during an earthquake should be, in
principle, evaluated using nonlinear time-history seismic response analysis that can handle soil andstructure interaction.
For reference earthquake motions, the effects of the combined response values of horizontal andvertical seismic actions are to be appropriately considered in accordance with the provisions of “TheInterpretation of the Regulation”.
The effects of seismic action on an underground RC structure are generally dominated by thehorizontal earthquake ground motion. The relatively large stiffness and load carrying capacity along
the longitudinal direction means that the transverse-section perpendicular to the water flow governs- 35 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021

the seismic responses of intake pits and piping support ducts under horizontal earthquake motions
from all directions. This seismic response characteristic usually leads to idealization of the structurein two- dimensions, and then a time-history earthquake response analysis is performed for in-plane
excitation. On the other hand, when the seismic response of the transverse-section of a structure isinsignificant or when the response behavior of a structure needs to be rigorously evaluated, anappropriate method of performance checking about the seismic action needs to be conducted basedon a longitudinal-section model or a three-dimensional model as needed. In seismic response analysis
using a three-dimensional model, orthogonal horizontal input motions are usually required; however,if the predominant direction of the earthquake ground motions can be distinguished from the lessdominant directions, a single input motion may be applied. It should be noted that even if theearthquake ground motion is applied in a single direction, multiple-axis responses may develop in
certain structural configurations.
If the horizontal and vertical components of the earthquake ground motions used for verification
are both given by time-history accelerations, a seismic response analysis should be performed undersimultaneous input motions. The effects of vertical earthquake ground motions (upward anddownward directions) should be considered in accordance with “The Interpretation of theRegulation”. When examining the effects of vertical earthquake ground motions as vertical seismic
coefficients, two types of seismic coefficient, those in the upward and downward directions, should
be adopted.
(2)
In seismic performance verification, the permanent action is set as the initial load on the structure
being examined and the seismic action is then examined under that condition. In the event of a seismic
action, the response of the structure may enter a nonlinear region, and thus the stresses and strainscaused by each of the permanent action and the seismic action cannot be superimposed. Therefore,
the initial state of the stress and strain in the structure developed from the permanent action is to be
first calculated, and then the time-history earthquake response analysis is to be carried out using areference earthquake motion. These steps enable the superimposition of the permanent action andseismic action. Furthermore, since the probability of the simultaneous occurrence of the seismic
action and variable action is judged to be considerably small, any variable action can be ignored in
the seismic performance verification.
Seismic action also includes dynamic earth pressure and ground deformation acting on a structure.
These should be estimated using the soil properties or appropriate soil parameters prepared forseismic response analysis.
In general, soil properties significantly vary with the type of ground, the inhomogeneous nature ofground, and the extent of soil compaction for filled ground. “Soil properties” here refers to several
generic terms such as the coefficient of earth pressure at rest, shear wave velocity, stress-strain
relationships, and hysteresis loop parameters. Soil properties are also scattered depending on themethod used in the in situ or laboratory tests. Therefore, it is essential to determine the characteristic- 36 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021

values for soil properties in such a way that the effects of the characteristic values on a structure
would be the most unfavorable, including the shear deformation and stress resultant (shear force,
flexural moment, and axial force). Generally, the mean value of the tested data for soil properties(equivalent to the statistical median) does not match their characteristic values. When soil properties
have been determined to provide a safer estimation on the failure mode of a structure (bending or
shear), a single set of soil properties may be used. When the specified soil properties are deemed to
be dangerous for the structural responses yielded at the characteristic value setting stage, it isadvisable to establish multiple sets of soil properties based on the mean values of available tested
data or proven data obtained from previous research.
In addition, dynamic water pressure is considered as a seismic action for water storage structures.
In the seismic response analysis for these structures, dynamic water pressure is to be estimated either
by modeling storage water as an array of added masses or by discretizing the storage water into an
arrangement of fluid elements. Detailed methods are provided in Section 2.2 "Calculation of
Response Values Using Macro Element Nonlinear Analysis" of the manuals.

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Seismic performance verification guidelines for critical

CHAPTER 5

underground reinforced concrete structures in nuclear power plants -2021-

METHODS OF ANALYSIS

5.1 General
Appropriate tools for structural analysis using reliable and accurate models shall be selected to
calculate response values in seismic performance verification for critical underground reinforced
concrete (RC) structures.
[Commentary]
The structural analysis methods to be used in seismic performance verification can be generally
divided into linear and nonlinear analyses depending on the nonlinear stress-strain behavior of thesoils and structures of interest, including damping. Much experience in linear analysis has enabled
the calculation of structural responses with a reliable solution close to the linear stress-strain behaviorof structural members. Nonlinear analysis, on the other hand, has a wider scope of application thanthat of linear analysis but the analytical results are frequently scattered to some extent. Therefore, itis essential to confirm the applicability of nonlinear analysis before using it. Commonly usedstructural analyses for the seismic performance verification of underground RC structures andmethods of deterioration prediction for durability performance verification are provided in Section
5.2 Selection of Structural Analysis. In addition, preferable structural analyses for seismic
performance verification are discussed in Section 5.3 Structural Analysis for Calculating
Response Values in Seismic Performance Verification. Methods of prediction to estimate thedeterioration of concrete for structural performance verification under normal conditions anddurability performance verification are described in Chapters 6 and 7 of the manuals, respectively.
5.2 Selection of Analysis(1) Appropriate tools for structural analysis shall be selected to calculate response values forseismic performance verification in accordance with the verification conditions associatedwith the geotechnical characterization and the structure, the nonlinear behavior of the soilstructure interaction, and the relevant limit states.
(2) Appropriate methods for the prediction of the deterioration mechanism of concrete induced
by environmental action shall be employed to determine the design values.
[Commentary](1)
The structural analysis tools to be used in the seismic verification should specify reasonable- 38 -

Seismic performance verification guidelines for criticalunderground reinforced concrete structures in nuclear power plants -2021-

dimensions from an engineering perspective according to the verification conditions associated withthe geotechnical characterization and the structure. Two-dimensional analyses are generally used inseismic performance verification for underground RC structures because the transverse-sections ofthese structures have relatively low shear rigidity under horizontal seismic forces and areconservatively considered in the verification process. On the other hand, the selection of threedimensional structural analysis is advantageous for evaluations of seismic responses in thelongitudinal direction and structures that exhibit complicated dynamic behavior. The dimensions forstructural analysis can be selected by considering the earthquake motions to be applied and the
profiles of soils, rocks, and/or structures (refer to Chapter 1 of the manuals).
Accurate assessment of the dynamic behavior of a structure (deformation and/or stress resultants)
during and after an earthquake is required to efficiently check the seismic performance of anunderground RC structure. For this purpose, it is essential to use tools for structural analysis that arecapable of calculating the actual dynamic behavior of a structure as accurately as possible. Thus,
the applicability and accuracy of the selected structural analysis method is to be confirmed throughhighly reliable experimental verification in advance (see Section 3.3.2 of the manuals). This step,
however, may be skipped if the method has been verified elsewhere.
The types of structural analysis used in seismic response analysis can be generally divided intolinear, equivalent linear, and nonlinear analyses depending on how they handle the dynamic nonlinear
behavior of soils and structures, including damping. Appropriate tools for structural analysis should
be selected depending on the limit state being examined. Generally, the use of a sophisticated analysis
method provides more accurate verification than the use of a simplified analysis method. This leads
to the recommendation of nonlinear time-history seismic response analysis as an appropriate tool for
structural analysis; however, simplified analyses have sufficient technical documentation and many
practical demonstrations, thereby having proved their validity in practice. Therefore, the present
guidelines permit the use of simplified analysis for verification. Since items such as the calculated
outputs, accuracy, and the scope of application differ between structural analysis tools, i.e., between
linear analysis, equivalent linear analysis, and nonlinear analysis, the most preferable tool should beselected according to the limit state to be examined. Commentary Table 5.1-1 shows the descriptive
correlations between seismic performance categories with their limit states and favorable tools forstructural analysis as an example. Generally, sophisticated analyses cover the application scope of
simplified analyses.
In calculating response values, it is important to select tools for structural analysis whose reliability
and accuracy has been verified and to realistically idealize the examined structure as well as its
surrounding soil and/or rock layers. Generally, such idealization allows more essential response
behavior to be assessed on account of the soil-structure interaction, leading to reliable verification.
As a result, the structural design is further streamlined. Accordingly, it is emphasized that the soil
and/or rock layers, structures, and their interfaces should be discretized depending on the site
conditions and structural configurations in an appropriate manner. In addition, special care must be- 39 -

Seismic performance verification guidelines for criticalunderground reinforced concrete structures in nuclear power plants -2021-

paid to matching the performance objectives specified according to Chapter 2 Performance
Settings for Critical Reinforced Concrete Structures and the feature and application scope of themechanical properties used in the selected tool for structural analysis.
Commentary Table 5.1-1 Correlation between seismic performance and analysis tools
Performance objectives
Category
Limit state

1

Members of astructure will notreach yield strength.

2

A structure will notreach maximum
bearing capacity.

3

A structure will
maintain structural
integrity without
collapsing.

Tools for structural analysis to be selectedLinearanalysis

Simplified approach.
Computational load: light
Equivalentlinear analysis
Macro element
nonlinear analysis
Material
nonlinear analysis

Sophisticated approach.
Computational load:(2)
For durable performance verification, a method of prediction capable of representing the concrete
deterioration due to environmental action such as carbonation, the intrusion of chloride ions, or
freezing and thawing action is to be used, and the respective design values are to be determined.
In an examination of carbonation, a prediction method of carbonation depth should be used thatconsiders the quality and environmental conditions of the concrete. Generally, the √𝑡𝑡 law, or squareroot relationship between time and carbonation depth is to be used. When factoring in a reduction in
the possible effect of carbonation due to rainfall, the method of infiltration of water may be used.

Concerning the intrusion of chloride ions, this can be physically treated as a diffusion phenomenon
allowing the chloride ion content to be predicted. The prediction method derived from a solution of
the one-dimensional diffusion equation should be used. When setting the limit state described as "No
cracks due to reinforcement corrosion will occur", the degree of corrosion of steel over time should
be predicted considering variations of the chloride ion content around the position of reinforcement,
and then the presence or absence of cracks due to corrosion should be checked. Currently, no methods
are available to accurately assess the degree of corrosion of steel and the subsequent onset of cracks.
Therefore, these predictions are to rely on empirical methods based on recorded data, provided in
Chapter 5 of the manuals.
When frost damage sustained by concrete is concerned, no appropriate prediction methods have
- 40 -

Seismic performance verification guidelines for criticalunderground reinforced concrete structures in nuclear power plants -2021-

yet been established to estimate deterioration induced by freezing and thawing action over time basedon actual environmental conditions and/or mix proportion conditions. Accordingly, the deteriorationof concrete quality due to freezing and thawing action should be assessed using appropriate indicessuch as the relative dynamic elastic modulus obtained from a laboratory material test using concrete
test specimens of the same mix proportion (JIS A 1148 (Method A) "Concrete Freezing and Thawing
Action Test Method (Underwater Freezing and Thawing Action Test Method)").
The cracking width of the concrete will limit the scope of application of these prediction methods.
Therefore, estimating the cracking width triggered by permanent and variable loads should be carriedout prior to the assessment of individual environmental actions, and thus the scope of application of
the above methods of deterioration prediction should be confirmed.
It has been recommended that "In maintaining the underground RC structures, appropriate remedial
measures shall be taken to satisfy the performance requirements throughout the design life bycarrying out inspection and predicting the degradation of structural performance considering the
progress of deterioration even before the defects on concrete surface becomes apparent” (see the"Recommendations on Evaluation of Structural Sound Function"). This requires using an appropriate
prediction method to meet such requirements. In addition, the above prediction methods may be
applied to members or parts of existing structures where repair or serviceability restoration are
assumed. The effects of repair materials and cross-sectional repairing materials may vary dependingon the quality of the concrete to which these methods are applied and the conditions during
construction. Therefore, the effects on the deterioration prediction methods should be appropriately
considered by identifying the actual concrete quality and construction conditions of the structures ofinterest.

5.3 Structural Analysis for Calculating Response Values in Seismic Performance Verification(1) Time-history seismic response analysis with dynamic soil-structure interaction shall be used
to calculate the response values for seismic performance verification.
(2) Constitutive models of materials used for the time-history seismic response analysis shalldemonstrate the nonlinear stress-strain behavior of soils and structures, including damping.
[Commentary](1)
The performance of an underground RC structure depends heavily on the site response during anearthquake. Accordingly, seismic response analysis with soil-structure interaction should be used to
calculate the response values of an examined structure. The analysis is expected to represent the
dynamic stress-strain behavior of the soils and structures and their interaction behaviors. In addition,
ensuring seismic safety with structural ductility constitutes a basic issue in the verification processprovided in the present guidelines because response values may exceed the elastic limit in a stress- 41 -

Seismic performance verification guidelines for criticalunderground reinforced concrete structures in nuclear power plants -2021-

strain curve. This enables the application of time-history seismic response analysis using step-bystep time integration to appropriately evaluate the absorption of input energy through the hysteresis
curve due to structural deformation and the deformation characteristics of the nonlinear behavior ofstructural members.
Static analysis may be used to calculate response values if the seismic coefficient approach has
been validated. This is discussed in Chapter 4 "Seismic Performance Verification Using Pushover
Analysis" of the manuals.
(2)
Commentary Table 5.2-2 shows the classification of the tools for structural analysis with respect
to the constitutive models of the materials. Although the application of nonlinear structural analysis
is treated in the present guidelines, linear structural analysis is presented for the purposes ofcomparison.
Commentary Table 5.2-1 Classification of seismic response analyses with soil-structure interaction
Item

Classification of
tools

Soil
Material
Model
Structure

Response values to be
calculated

Seismic response analysis
Linear analysis

Nonlinear analysis

Equivalent linear analysis
using strain dependent
rigidity and damping

Nonlinear models based on total stress or effective stressInitialrigidityStressEquivalentrigidity(degraded
stiffness)

Macro element nonlinear modelFlexuralmoment/axial force

Flexural moment，axial force
curvature，relative story
displacement

Shear force

Shear force

Material nonlinear modelFlexural moment，axial force,
concrete strain on thecompression edge，curvature，
relative story displacement
Shear force
shear strain

Suggested toll for structural analysis

In discretizing the geotechnical and structural profiles into finite element meshes, the following
points should be considered when setting the constitutive models of the materials for the soil andstructure as well as the mechanical model of the interface between the two:(i) Material models of soil elements
Soil elements should retain appropriate constitutive models of materials capable of properly
assessing dynamic soil properties depending on their strain levels assumed from the intensity of thereference earthquake motions used for verification. The effect of groundwater on the dynamic soil
properties is to be considered in case of a high groundwater table in the subsurface layers. In
particular, the decrease of soil stiffness due to the generation of excess pore water pressure or soilliquefaction should also be appropriately considered in the saturated sand layers (details of which
are provided in Section 1.3 of the manuals).
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Seismic performance verification guidelines for criticalunderground reinforced concrete structures in nuclear power plants -2021-(ii) Material models of structural elements
The basic intention is to ensure the seismic safety of structures during verification using energy
absorption from input ground motion through the hysteresis curve due to structural ductility.
Consequently, the structural elements require appropriate constitutive models of materials capableof properly assessing concrete cracks and reinforcement yield under the combination of permanentloads and earthquake ground motions.
(iii) Material models of the interface elements between soil and structure
Complicated behavior such as sliding and separation may occur at the interface between the soil
and structure depending on the strength of the reference earthquake motion used for verificationand the magnitude of the relative shear rigidity between the subsurface layers and the cross-sectionof the structure. These will affect the global dynamic response of the structure. Therefore, theinterface elements between soil and structure require appropriate models capable of representing
this behavior.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

CHAPTER 6

VERIFICATION

6.1 General
Verification shall be carried out by setting appropriate verification items, using appropriate
safety factors, and confirming that the response values for verification do not exceed the limiting
values. A set of verification items and their indices (limiting values) shall be specified according
to the tools selected for structural analysis and used to calculate the response values andperformance objectives.
[Commentary]
The principle of seismic performance verification is to conform with the "Specification [Design
Edition]." Chapter 6 contains Section 6.2 Prerequisites of Verification followed by a detailed
description of seismic performance verification in Section 6.3 Safety Factors and Section 6.4
Seismic Performance Verification". The tools for structural analysis used to calculate the response
values are described in Chapter 5 Methods of Analysis and the manuals. For descriptions of theperformance objectives and limit states, see Chapter 2. Performance Settings for Critical
Reinforced Concrete Underground Structures.
6.2 Prerequisites of Verification(1) Verification of an underground RC structure under seismic action shall be carried out on theassumption that durability and structural performance under normal conditions have beenverified in advance.
(2) For durability performance verification of an underground RC structure, appropriate
verification items and their indices (limiting values) shall be specified to demonstrate thatmaterial degradation due to environmental action would have an insignificant effect on the
structure’s seismic performance over its design life for three types of environmental action:
carbonation, the intrusion of chloride ions, and freezing and thawing action.
[Commentary](1)
The performance required of a structure must be maintained over its design life, and hence thepresent guidelines assume that both durability and structural performance under normal conditions
have already been verified. Durability performance verification and structural performanceverification under normal conditions are described in Chapters 6 and 7, respectively, of the manuals.
(2)- 44 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Irreversible deterioration of a concrete structure generally shows most significantly at the end of
the design life of the structure. The seismic performance of the structure should be checked, taking
into consideration the state of deterioration at the time of checking; however, it is difficult to performseismic response analysis even under the assumption of aging of the structure. Therefore,
demonstration of the insignificant impact of deterioration on seismic performance at the end stage ofa structure’s design life allows for the exclusion of the temporal change associated with the seismic
performance.
The deterioration condition with insignificant effects on the seismic performance is defined as thecondition that does not have cracks from reinforcing bar corrosion and in which the concrete hasretained its required quality. Under this condition, a set of verification items and indices (limiting
values) can be determined. The present guidelines specify that the following three deterioration
mechanisms are to be checked to verify the durability performance of a structure: carbonation,
chloride-induced deterioration, and freezing damage.
The progress of reinforcing bar corrosion due to concrete carbonation or the intrusion of chloride
ions generates cracks in the concrete and further deterioration results in peeling; however, thecondition just before the onset of cracking is widely accepted to experience little reinforcement
corrosion as well as exhibiting the unchanged dynamic behavior of the original reinforced concrete
structure.
In coastal environments where underground RC structures are in service, the speed of intrusion ofchloride ions is generally much faster than the rate of concrete carbonation. Thus, in many cases theformer is the dominant deterioration mechanism, which can be accordingly regarded as the primary
deterioration mechanism. Thus, the limiting values for verification are to be determined such that no
cracks be generated by reinforcing bar corrosion due to the intrusion of chloride ions. In addition, thedepth of carbonation should be checked in such a way that the depth of carbonation stays within theconcrete superficial layer, thus eliminating its effect on the progress of reinforcing bar corrosion due
to the intrusion of chloride ions. When the progress of carbonation seems to have been slowed byrainfall or similar weather conditions, the verification process is to be carried out by focusing on
reinforcing bar corrosion.
For freezing and thawing action, the quality of concrete is generally examined using the relative
dynamic elastic modulus.
The methods of calculation for the above-mentioned limiting values are presented in “Chapter 5
Durability Performance Verification” of the manuals.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

6.3 Safety Factors(1) Seismic performance verification shall employ five safety factors: the structural factor,
material factor, action factor, structural analysis factor, and member factor.
(2) Appropriate values of the safety factors shall be determined based on the methods used to
calculate the response and limiting values as well as the verification items corresponding to
the performance requirements.
[Commentary](1)
The basic concept of seismic performance verification can be expressed as:𝑆𝑆(6.2-1)𝛾𝛾𝑖𝑖 ⋅ 𝑅𝑅𝑑𝑑 ≤ 1.0
𝑑𝑑

where 𝑆𝑆𝑑𝑑 is the response value for verification (𝑆𝑆𝑑𝑑 = 𝑆𝑆�𝛾𝛾𝑓𝑓 , 𝛾𝛾𝑚𝑚 � ⋅ 𝛾𝛾𝑎𝑎 ), 𝑅𝑅𝑑𝑑 is the limiting value forverification (𝑅𝑅𝑑𝑑 =𝑅𝑅(𝛾𝛾𝑚𝑚 )
), 𝑆𝑆
𝛾𝛾𝑏𝑏

is the characteristic value of the response value, 𝑅𝑅 is the characteristic

value of the limiting value, 𝛾𝛾𝑖𝑖 is the structural factor, 𝛾𝛾𝑚𝑚 is the material factor, 𝛾𝛾𝑓𝑓 is the action
factor, 𝛾𝛾𝑎𝑎 is the structural analysis factor, and 𝛾𝛾𝑏𝑏 is the member factor

When setting the safety factors, partial safety factors should be considered related to both the

response values and limiting values depending on relevant uncertainties such as the material
properties and/or actions. The basic concepts guiding safety factor settings are as follows:
The structural factor is to be determined by considering the importance of a structure, as determinedby the social impact of the structure reaching the limit state. Advisably, underground RC structuresof relatively high importance should allow the structural factor to be generally taken as greater thanor equal to 1.0.
The material factor is to be determined by considering the unfavorable deviations of material
strength from the characteristic values, the difference between test specimens and actual structures,the effects of material properties on the specific limit state, and time dependent variation of materials.
The action factor is to be determined by considering the unfavorable deviation of actions (loads)
from the characteristic values, uncertainty in the evaluation of actions, time dependent variation ofactions over the design life of a structure, the effect of actions on the limit state, and variation ofenvironmental actions.
The structural analysis factor is to be determined by considering the uncertainty of computational
accuracy of calculating response values through structural analysis, the difference between realized
models and actual structures, and other uncertainties involved in the computation of response values.
The member factor is to be determined by considering the uncertainties in the computation of the- 46 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

capacities of members, the severity of the dimensional error of members, the importance of members,
and the effects on the entire structure of a member reaching certain limit states.
Multiplication and division of the safety factors are used to guard against unfavorable deviation,and this arithmetic operation should be appropriately applied according to the relevant verification
indices, such as the load carrying capacity, stress, deformation, and strain.
(2)
The safety factor is to be defined in each seismic performance verification, durability performanceverification, and structural performance verification under normal conditions. Although a set ofsafety factors and the terms used to refer to the response values may differ between performanceverification categories (for example, “response value” is usually designated by “design value” when
it does not apply to seismic performance verification), the principle guiding the safety factor settings
should conform to that of seismic performance verification. Standard values for the safety factors areprovided in the manuals.
In examining the seismic performance of existing structures, the characteristic values for materials
may be judged as the design basis strength, and the material factor may be deduced from the values
determined at the design stage if the construction process of the structure has been confirmed to havebeen properly managed.
The safety factors to be considered for durability performance verification are to be distinctly
determined from those for seismic performance verification because they involve reference tospecified limit states associated with the generation of reinforcement corrosion due to carbonation or
the intrusion of chloride ions, crack development due to reinforcement corrosion, and quality
deterioration due to freezing and thawing action. These safety factors include the material factor, thesafety factor considering the computational accuracy of the design equation concerned withdurability (equivalent to the assessment of response values), and the structural factor. They also
include the safety factor for the prediction accuracy of the carbonation velocity factor and the design
chloride ion diffusion coefficient. Determining these safety factors for durability performanceverification requires consideration of the constructability of concrete, the accuracy of the respective
deterioration prediction methods, and the importance of the structure.
When verification is carried out using strains calculated from material nonlinear analysis, theassessment of the response values is affected by uncertainty, unfavorable deviation, and error
accounted for by the material factor and the member factor because of the considerable influence of
nonlinear material properties on calculating the response values. On the other hand, the assessmentof limiting values is also affected by the uncertainty of the computational accuracy of structural
analysis, accounted for by the structural analysis factor. This means that the safety factors examinedin seismic performance verification using material nonlinear analysis need to be appropriately
determined depending on the associated set of verification items and indices (limiting values), taking
the principle of the safety factors discussed above into consideration.

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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

6.4 Seismic Performance Verification(1) For the reference earthquake motion Ss, a set of verification items and indices (limiting values)
to demonstrate that an underground RC structure "maintains its structural integrity withoutcomplete collapse of structure" shall be appropriately determined according to the tools forstructural analysis used for the calculation of the response values.
(2) In the seismic performance verification of the transverse-section of a structure, a set ofverification items and indices (limiting values) shall be generally used as follows:・Flexural failure: concrete strain on the compression edge at the time of concrete peeling or
the corresponding amount of deformation (limit curvature/limit relative story deformation
angle);
・Shear failure: shear capacity of a structural member.
(3) The use of material nonlinear analysis may permit use of verification indices (limiting values),
such as strains, to represent the local damage of materials or detailed deformation behavior.
(4) Structures requiring the support of equipment and piping systems shall be checked using a setof verification items and indices (limiting values) as well as other given function-related
indices (limiting values) to ensure the functional capability of such installations.
[Commentary](1)
Commentary Figure 6.3-1 shows a schematic load-deformation relationship for a simple rigid
frame box structure with flexural yielding failure preceding shear failure of its members. The area ofthe cover concrete spalling in Commentary Figure 6.3-1 is defined as the limit state for flexural
failure considering the applicability of macro element nonlinear analysis and the ease of illustrative
understanding. Previous experimental studies and examples of seismic damage have shown thatreinforced concrete structures retain their ductile nature without rapid softening and compressive
buckling of reinforcing bars unless cover concrete spalling occurs. Thus, the limit state defined insuch a way is expected to avoid the occurrence of fatal damage, such as the complete collapse of thestructure.

- 48 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

Load

Maximum load carrying capacity

Softening
Member yield

Cover concrete spalling

Reinforcing bar buckling
Toughness limits

Crack onset

Collapse
Displacement or deformation

Commentary Figure 6.3-1 Illustration of the process of flexural failure of a reinforced concrete structure

The “Specification [Design Edition]” defines the following three seismic performance standards
that should be met by a structure:
Seismic Performance Standard 1: full functional capacity during an earthquake and operational
without having undergone repair after the earthquake.
Seismic Performance Standard 2: full functional capacity can be restored in a short to moderate
time and structure remains operational without strengthening.
Seismic Performance Standard 3: the entire system of a structure does not collapse due to theseismic force.
In general, the state of Seismic Performance Standard 1 is equivalent to structural behavior within
the elastic range or before reinforcing bar yield along its load and displacement curve. This state is
judged to have little effect on the earthquake resistance capability of a structure unless the state enters
Seismic Performance Standard 2. This enables the safe use of Seismic Performance Standard 1 fortrend monitoring and repair in a normal maintenance routine. In addition, the seismic performancethat "structures maintain structural integrity without complete collapse" (which is described in
Chapter 2 Performance Settings for Critical Reinforced Concrete Underground Structures ofthe present guidelines) is generally equivalent to Seismic Performance Standard 3.
Most verification practices are examined against the failure of the cross-section of structuralmembers by setting a set of verification items for flexural failure and shear failure for individual
sections/parts or the overall structural members. In addition, checking for local material damage also
needs to be considered based on the stress-strain outputs from finite element analysis. On the other
hand, the use of a pushover analysis allows checking of the overall behavior of a structure bycalculating the limiting values representing its horizontal loads or displacements. This broadly
evaluates the overall behavior of the structure and makes no distinction between the failure modes
such as flexural failure or shear failure associated with the cross-section of a member.
(2)
For underground RC structures, the performance objectives are specified such that the floor slabs
will not collapse, and the sidewalls and partition walls will not lose capacity in accordance with theseismic performance where "the structure maintains structural integrity without complete collapse ".
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Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

This can be satisfied by preventing the transformation of the state of plastic deformation into the stateof near collapse (hereinafter referred to as "transformation of the structure to a near collapse state")and shear failure of the cross-section of the members. In checking the seismic performance in thetransverse-section direction of an RC underground structure, respective verification indices (limiting
values) may be specified for the amount of deformation corresponding to the transformation of thestructure to a near collapse state from flexural failure and the shear capacity of a member.
Verification indices for flexural failure may include the relative story deformation angle andcurvature in addition to concrete strain on the compression edge. The deformation angle is obtainedby dividing the relative displacement between the upper and lower horizontal members constituting
the story by the distance between the same members. The seismic performance of a simple rigid
frame box structure having shear deformation modes can be easily checked based on the deformation
angle. On the other hand, the curvature or the concrete strain on the compression edge should be
carefully selected when an irregular deformation is surmised; the possible deformation stems fromthe complex shape of the structure or the localized deflection around the center of the member length,
possibly due to earth pressure. Even when such local deformation is specified as a verification index(limiting value), however, it is still desirable to assess the degree of deformation of the overall
structure using a macro deformation index such as the relative story deformation angle.
On the other hand, shear failure is characterized by an abrupt reduction in the load carrying
capacity of the members after the onset of failure. In particular, the out-of-plane shear failure of
sidewalls and partition walls is significantly brittle compared with ductility-related failure such as
flexural failure. Accordingly, shear failure must be avoided in statically determined structures. Shear
failure of sidewalls and partition walls generally leads to loss of the inner space of the structure, butmacro element nonlinear models are incapable of evaluating the state beyond shear failure. Therefore,
the limit state for shear failure is to be specified such that no shear failure occurs in any member ofthe structure.
(3)
Material nonlinear models can simulate the damage process in more detail compared with macro
element nonlinear models and have the advantage of extended capability to handle the complex shapeof the members or structure. Three-dimensional configurations can also be used. The present
guidelines offer a choice in the specification of verification indices based on the use of material
nonlinear analysis because there is accumulated experience using this type of analysis in bothresearch and practice, improving its reliability and capability. Material nonlinear models use a wide
variety of constitutive laws to model elastic-plastic mechanical properties of concrete, reinforcing
bars, and their boundaries. Therefore, verification indices should be selected and corresponding
limiting values should be determined in consideration of the scope of application of the material
nonlinear models.
When a verification item depends on a limit state discussed in the durability performanceverification, it is necessary to consider the dependency of the durability when determining the- 50 -

Seismic performance verification guidelines for critical underground reinforced concrete structures in nuclear power plants -2021-

limiting values.
If studies by multiple researchers have proposed rational and reliable evaluation methods forcalculating limiting values through experimental validation, they may be applied for verification.
(4)
Concerning the seismic performance of structures that require functioning of support for nuclear
components and piping networks, it is required that they "satisfy the constraints against structuraldimensions to establish functional capabilities of equipment and piping system (e.g. establishing theoperation of equipment and the elastic deformation of piping) provided through technicalcoordination with mechanical engineers."
Alternative limit states may be assumed to ensure the functional capability of equipment andpiping systems in certain underground RC structures that require to support installations ranked asseismic grade Class S. The limit state may be specified as bearing more moderate damage than thatof the limit state associated with the seismic performance that a structure "maintains its structural
integrity without complete collapse ". Additional verification indices may also be needed to ensure
the functional capability of the equipment and piping. For instance, the following verification indicesmay be selected: floor response accelerations to ensure the functional capability of intake pumps, therelative story displacement between floors through which intake pumps are installed, and the relative
displacement between piping supports. In addition, the effects of the local damage sustained by astructure on the anchorage capability of equipment and piping should be examined as needed.

- 51 -

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