NUREG/CR-6919

 BNL-NUREG-77174-2006

Recommendations for

Revision of Seismic

Damping Values in

Regulatory Guide 1.61

Brookhaven National Laboratory

U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Washington, DC 20555-0001
 

 

 

 

 NUREG/CR-6919 

 BNL-NUREG-77174-2006

Recommendations for

Revision of Seismic

Damping Values in

Regulatory Guide 1.61

Manuscript Completed: September 2006

Date Published: November 2006

Prepared by

R.J. Morante

Brookhaven National Laboratory

Upton, NY 11973-5000

H.L. Graves, NRC Program Manager

Prepared for

Division of Fuel, Engineering and Radiological Research 

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC Job Code N6185
iii

ABSTRACT

This report provides recommendations developed by Brookhaven National Laboratory (BNL) for

revision of the damping values in U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide

1.61, "Damping Values for the Seismic Design of Nuclear Power Plants,” Rev. 0, issued October

1973. The recommendations included herein are based on the results of various damping

studies, as well as NRC licensing actions related to the resolution of damping issues.

Recommendations of the NRC Damping Task Force, and additional work performed by BNL to

address staff questions have been incorporated into the final recommendations.

The recommendations in this document considered recently published guidance on damping for

seismic analysis included in ASCE Standard 43-05, "Seismic Design Criteria For Structures,

Systems, and Components in Nuclear Facilities" and ASME Boiler and Pressure Vessel Code,

Section III, Division 1, Non-Mandatory Appendix N, "Dynamic Analysis Methods", 2004 Edition.

vii

TABLE OF CONTENTS

Page No.

ABSTRACT.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

FOREWORD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

TABLE OF CONTENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

LIST OF FIGURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

LIST OF TABLES..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

EXECUTIVE SUMMARY.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

ACKNOWLEDGMENTS..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

ABBREVIATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

1. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Proposed General Qualifications for Use of Regulatory Guide 1.61 Revision. . . 1

2. PROPOSED INTRODUCTION FOR REVISED REGULATORY GUIDE 1.61. . . . . . . . . . . . 3

3. STRUCTURAL DAMPING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1 Proposed Structural Damping Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.1 Safe Shutdown Earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.2 Operating Basis Earthquake.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.3 Correlation of Structural Response Level with SSE Damping.. . . . . . . . . 6

3.2 Justification for Proposed Structural Damping. . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.1 Safe Shutdown Earthquake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.2 Operating Basis Earthquake.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.3 Correlation of Structural Response Level with SSE Damping.. . . . . . . . . 8

4. PIPING DAMPING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.1 Proposed Piping Damping Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2 Justification for Proposed Piping Damping.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. ELECTRICAL DISTRIBUTION SYSTEMS DAMPING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.1 Proposed Cable Tray/Conduit Damping Values.. . . . . . . . . . . . . . . . . . . . . . . . 15

5.2 Justification for Proposed Cable Tray/Conduit Damping. . . . . . . . . . . . . . . . . . 16

5.2.1 Cable Trays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.2.2 Conduits.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6. HEATING VENTILATION AND AIR CONDITIONING (HVAC) DUCT DAMPING.. . . . . . . . 21

6.1 Proposed HVAC Duct Damping Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
viii

6.2 Justification for Proposed HVAC Duct Damping. . . . . . . . . . . . . . . . . . . . . . . . 21

7. MECHANICAL AND ELECTRICAL COMPONENT DAMPING.. . . . . . . . . . . . . . . . . . . . . . 23

7.1 Proposed Component Damping Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

7.2 Justification for Proposed Component Damping. . . . . . . . . . . . . . . . . . . . . . . . 24

7.2.1 Motor, Fan, and Compressor Housings. . . . . . . . . . . . . . . . . . . . . . . . . 24

7.2.2 Pressure Vessels, Heat Exchangers, Pump and Valve Bodies.. . . . . . . 24

7.2.3 Welded Instrument Racks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.2.4 Electrical Cabinets, Panels, and Motor Control Centers (MCCs). . . . . . 24

7.2.5 Metal Atmospheric Storage Tanks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

8. CONCLUSIONS AND RECOMMENDATIONS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

9. REFERENCES.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
ix

LIST OF FIGURES

Page No.

Figure 1 Frequency-dependent damping. ... ... ... ... ... ... ... ... ... ... ... . 12
x

LIST OF TABLES

Page No.

Table 1 Proposed structural damping values for a safe shutdown earthquake. . . . . . . 5

Table 2 Proposed structural damping values for an operating basis earthquake.. . . . . 6

Table 3 Damping values for piping systems.... ... ... ... ... ... ... ... ... ... . . 11

Table 4 Damping values for electrical cable tray and conduit systems. . . . . . . . . . . . 15

Table 5 Damping values for HVAC duct systems.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table 6 Damping values for mechanical and electrical components. . . . . . . . . . . . . . 23
xi

EXECUTIVE SUMMARY

The U.S. Nuclear Regulatory Commission (NRC) issues Regulatory Guides to provide guidance

for licensees on how to meet various regulations for the design, construction, licensing, and

operation of nuclear power plants. These Regulatory Guides are developed using the best

technical information available at the time. However, as experience is gained in the nuclear

industry, and progress is made in the development of new technologies, Regulatory Guides

must be updated to incorporate this new information.

Regulatory Guide 1.61, "Damping Values for the Seismic Design of Nuclear Power Plants,”

provides information on the proper selection of damping values for use in the seismic analysis of

nuclear power plants. This Regulatory Guide was first issued in October 1973. Since that time,

many advances have been made in the understanding of seismic analysis and its application to

nuclear power plants. Various studies have been performed since the original issue of

Regulatory Guide 1.61, that provide new insights into the appropriate selection of damping

values used in seismic analyses.

In November 1995, Brookhaven National Laboratory (BNL) submitted recommendations to the

NRC Office of Nuclear Regulatory Research (NRC/RES) for revision of the damping values in

Regulatory Guide 1.61. These recommendations were developed under a NRC/RES program

entitled, "Application of Damping Research." The technical inputs for this effort were the results

of damping studies and NRC licensing actions related to resolution of damping issues,

conducted since the release of Regulatory Guide 1.61 in 1973. The scope did not include any

new testing or detailed evaluation of raw test data.

The NRC Damping Task Force, which is comprised of NRC staff members from licensing and

research, reviewed the submitted recommendations. BNL presented the recommendations to

the Task Force at a working session held on November 20, 1995. A meeting report,

summarizing the results of the working session, was prepared by BNL and submitted to the

NRC Program Manager on January 3, 1996. The meeting report was the final deliverable for the

project.

In June 2005, NRC/RES requested that BNL provide technical assistance for a revision to

Regulatory Guide 1.61. The staff indicated that the starting point for the revision would be BNL's

recommendations submitted in November 1995. NRC/RES Contract Job Control Number (JCN)

N6185 was initiated in February 2006, to obtain BNL's technical assistance. 

This report is an update to the November 1995 technical report. It incorporates staff

recommendations from two (2) recent working sessions of the NRC Damping Task Force, held

on December 15, 2005 and March 6, 2006, and additional work performed by BNL to address

staff questions. The NRC Damping Task Force has reviewed and concurs with the final

recommendations.

The recommendations in this report considered recently published guidance on damping for

seismic analysis included in ASCE Standard 43-05, "Seismic Design Criteria For Structures,

Systems, and Components in Nuclear Facilities" and ASME Boiler and Pressure Vessel Code,

Section III, Division 1, Non-Mandatory Appendix N, "Dynamic Analysis Methods", 2004 Edition.
xii

The recommendations proposed herein do not include any radical changes from the damping

values specified in Regulatory Guide 1.61, Rev. 0. The primary improvements are (1) explicit

guidance for damping of mechanical and electrical components and for damping of non-piping

distribution systems; (2) explicit guidance for structural damping at low response levels; and (3)

consideration of experimental data, significant staff licensing actions related to damping, and

revisions to codes and standards related to damping, developed since Revision 0 was issued in

1973. 

Issuance of Revision 1 to Regulatory Guide 1.61, incorporating the recommendations contained

herein, will help to streamline the application and staff review process by minimizing the number

of case-by-case assessments required. 
xiii

ACKNOWLEDGMENTS

The author wishes to acknowledge the support of Herman L. Graves, NRC Program Manager,

in facilitating the development of the final recommendations, as BNL’s interface with the NRC

Damping Task Force. 

The author also wishes to acknowledge the critical review and recommendations provided by

the NRC Damping Task Force.

Finally, the author wishes to acknowledge the support of Robert J. Lofaro, Brookhaven National

Laboratory, for his editorial comments and assistance in preparing this report in NUREG/CR

format.
xiv

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xv

ABBREVIATIONS

ASCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . American Society of Civil Engineers

ASME. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . American Society of Mechanical Engineers

BNL.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brookhaven National Laboratory

DCM..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Damping Correction Multiplier

FSAR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Final Safety Analysis Report

HVAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heating Ventilation and Air Conditioning

JCN.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Job Control Number

MCC..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Motor Control Center

NRC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nuclear Regulatory Commission

OBE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operating Basis Earthquake

PGA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peak Ground Acceleration

RES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . U.S. NRC, Office of Nuclear Regulatory Research

SER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safety Evaluation Report

SRP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standard Review Plan (NUREG-0800)

SSE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safe Shutdown Earthquake

TVA.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tennessee Valley Authority

ZPA.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zero-Period Spectral Acceleration
xvi

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1

1. INTRODUCTION

1.1 Background

In November 1995, Brookhaven National Laboratory (BNL) submitted recommendations (Ref. 1)

to the Nuclear Regulatory Commission, Office of Nuclear Regulatory Research (NRC/RES) for

revision of the damping values in Regulatory Guide 1.61, "Damping Values for the Seismic

Design of Nuclear Power Plants" (Ref. 2). These recommendations were developed under a

NRC/RES program entitled, "Application of Damping Research." The technical inputs for this

effort were the results of damping studies and NRC licensing actions related to resolution of

damping issues, conducted since the release of Regulatory Guide 1.61 in 1973. The scope did

not include any new testing or detailed evaluation of raw test data.

The NRC Damping Task Force, comprised of NRC staff members from licensing and research,

reviewed the submitted recommendations. BNL presented the recommendations to the Task

Force at a working session held on November 20, 1995. A meeting report, summarizing the

results of the working session, was prepared by BNL and submitted to the NRC Program

Manager on January 3, 1996. The meeting report was the final deliverable for the project.

In June 2005, NRC/RES requested that BNL provide technical assistance for a revision to

Regulatory Guide 1.61. The staff indicated that the starting point for the revision would be BNL's

recommendations submitted in November 1995. NRC/RES Contract Job Control Number (JCN)

N6185 was initiated in February 2006, to obtain BNL's technical assistance. 

This report is an update to the November 1995 technical report. It incorporates staff

recommendations from two (2) recent working sessions of the NRC Damping Task Force, held

on December 15, 2005 and March 6, 2006, and additional work performed by BNL to address

staff questions. The NRC Damping Task Force has reviewed and concurs with the final

recommendations.

This update considered recently published guidance on damping for seismic analysis included in

American Society of Civil Engineers (ASCE) Standard 43-05, "Seismic Design Criteria For

Structures, Systems, and Components in Nuclear Facilities" (Ref. 3), and American Society of

Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section III, Division 1,

Non-Mandatory Appendix N, "Dynamic Analysis Methods", 2004 Edition (Ref. 4).

1.2 Proposed General Qualifications for Use of Regulatory Guide 1.61 Revision

This report includes proposed damping values for inclusion in Regulatory Guide 1.61 that are

intended to be used in the seismic analysis of various nuclear power plant systems and

components. The following qualifications apply to the damping values provided herein:

• If there is any deviation from current NRC criteria and guidance for seismic

analysis, then the damping values used in the analysis will require specific

detailed justification; review and evaluation will be on a case-by-case basis.
2

• Damping values higher than those specified herein may be used, if supported by

specific, applicable test data. This will be subject to review and evaluation on a

case-by-case basis.

• Except as noted under specific damping categories, the tabulated damping

values are applicable to time-history, response spectrum, and equivalent static

seismic analyses.

• When inelastic analysis methods are used, the selected viscous damping values

should be limited to the damping values specified herein for the Operating Basis

Earthquake (OBE). The use of different damping values will be subject to review

and evaluation on a case-by-case basis.

• Special limits on damping values apply to time-history analyses of building

structures used for structural design and for generation of in-structure response

spectra. These are noted where applicable.

Section 2 of this report presents a proposed revision to the introduction for Regulatory Guide

1.61. Sections 3 through 7 of this report present recommendations for structural damping;

piping damping; damping for electrical distribution systems; damping for heating, ventilation,

and air conditioning ducting; and damping for mechanical and electrical components,

respectively.
3

2. PROPOSED INTRODUCTION FOR REVISED REGULATORY GUIDE 1.61

To more clearly state the objectives and technical basis for Regulatory Guide 1.61, and how it

relates to the NRC’s Standard Review Plan (SRP), NUREG-0800, (Ref. 5), it is proposed that

the introduction be revised to read as follows:

DAMPING COEFFICIENTS FOR SEISMIC ANALYSIS OF

NUCLEAR POWER PLANT STRUCTURES, SYSTEMS, AND COMPONENTS

Introduction

The specified damping coefficient values are applicable to design calculations

performed in accordance with current NRC criteria and guidance for seismic

analysis (i.e., NUREG-0800, latest revision). They are specifically intended for

elastic seismic analysis where energy dissipation is approximated by viscous

damping (i.e., proportional to velocity). This is the most common analysis

methodology applied to design.

For design load combinations which include the Safe Shutdown Earthquake

(SSE), structural response which somewhat exceeds the elastic response limit is

typically permitted. The specified SSE damping coefficient values, for use with

elastic analysis, take into consideration the fact that some energy dissipation

occurs due to inelastic structural response. Consequently, the specified SSE

damping values are not applicable to analyses that explicitly include inelastic

structural behavior.
4

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5

3. STRUCTURAL DAMPING

This section discusses structural damping values for use in the analysis of containment

structures, containment internal structures, and other seismic category I structures.

3.1 Proposed Structural Damping Values

Proposed structural damping values are discussed in the following subsections for the safe

shutdown earthquake (SSE) and the operating basis earthquake (OBE).

3.1.1 Safe Shutdown Earthquake

Table 1 presents the structural damping values proposed for containment structures,

containment internal structures, and other seismic category I structures for a SSE:

Table 1 Proposed structural damping values for a safe shutdown earthquake

Structure Proposed Damping Value

Reinforced Concrete 7%

Reinforced Masonry 7%

Prestressed Concrete 5%

Welded Steel or Bolted Steel with Friction

Connections

4%

Bolted Steel with Bearing Connections 7%

Note: For steel structures with a combination of different connection types, use the lowest

specified damping value, or as an alternative, use a "weighted average" damping value

based on the number of each type present in the structure.

3.1.2 Operating Basis Earthquake

If the design-basis OBE ground acceleration is greater than one-third of the design-basis SSE

ground acceleration, then a separate OBE analysis is required. For the OBE analysis, the

damping values presented in Table 2 are applicable.
6

Table 2 Proposed structural damping values for an operating basis earthquake

Structure Proposed Damping Value

Reinforced Concrete 4%

Reinforced Masonry 4%

Prestressed Concrete 3%

Welded Steel or Bolted Steel with Friction

Connections

3%

Bolted Steel with Bearing Connections 5%

3.1.3 Correlation of Structural Response Level with SSE Damping

The SSE equivalent viscous damping ratios specified in Table 1, for linear dynamic analysis of

structures, were selected based on the expectation that the structural response due to load

combinations that include SSE will be close to applicable code stress limits, as defined in SRP

Section 3.8. 

However, there may be cases where the predicted structural response to load combinations that

include SSE is significantly below the applicable code stress limits. Because equivalent viscous

damping ratios have been shown to be dependent on the structural response level, it is

necessary to consider the possibility that the SSE damping values specified in Table 1 may be

inconsistent with the predicted structural response, and that lower damping values may be more

appropriate.

Consequently, the following guidelines are provided to assess the appropriateness of using the

SSE damping values specified in Table 1 and, if necessary, to perform a re-analysis using

reduced damping values:

(1) If the significant stresses due to load combinations that include SSE are at least

80% of the applicable code stress limits, then use of SSE damping levels for the

seismic analysis is appropriate and acceptable.

(2) If the significant stresses due to load combinations that include SSE are less than

80% of the applicable code stress limits, then using SSE damping levels may

under-predict the structure’s response to seismic loads. In this case, structural

evaluation and development of in-structure response spectra should be based on

a seismic analysis utilizing the OBE damping values specified in Table 2.

(3) As an alternative to (2), the applicant/licensee may submit a plant-specific

justification for using damping values higher than the OBE damping values

specified in Table 2, but less than the SSE damping values used initially.
7

3.2 Justification for Proposed Structural Damping

The main reference for the proposed damping values is the structural damping study performed

by EQE for NRC/RES (Ref. 6). This proposal represents Brookhaven National Laboratory’s

(BNL's) interpretation of the EQE study results. Damping values are specified in "whole

percents", consistent with the current Regulatory Guide 1.61. The following subsections provide

the basis for the proposal.

3.2.1 Safe Shutdown Earthquake

Based on EQE’s extensive study of structural damping, the current SSE damping values

provided in Regulatory Guide 1.61 should remain essentially unchanged with one (1) major

exception. A distinction is made between "friction-bolted" and "bearing-bolted" connections for

steel structures. EQE’s recommendation, to treat friction-bolted connections as welded

connections appears to be well founded, based on the design intent of friction-bolted

connections. Use of pre-loaded high-strength bolts, with reduced allowable shear stresses, is

intended to preclude slip of the joint. From the perspective of energy dissipation, this type of

connection would appear to perform more like a welded connection than a bearing connection.

Consequently, EQE’s recommendation is incorporated in the proposed SSE damping values.

EQE has concluded that reinforced masonry behaves similarly to reinforced concrete. EQE's

recommendation has been incorporated in the proposed SSE damping values.

EQE recommended that SSE damping for "bearing-bolted" connections in steel structures be

specified at 6%. This represents a 1% decrease in damping from the current Regulatory Guide

1.61 value for "Bolted Steel Structures". Discussion on this point with EQE at an NRC/RES

presentation on September 11, 1992 indicated that this recommendation is based on very

limited data. Considering the very modest impact of this change on SSE structural qualification

and the absence of conclusive data to support the reduction, the SSE damping for "Bolted Steel

with Bearing Connections" is retained at 7%.

3.2.2 Operating Basis Earthquake

The proposed OBE damping values are consistent with EQE’s study results for stress levels in

the range of one-half yield stress. They reflect a 1% increase over the current Regulatory Guide

1.61 values for prestressed concrete and steel. Reinforced concrete and reinforced masonry are

specified at the current Regulatory Guide 1.61 value for reinforced concrete.

For welded and friction-bolted steel structures, EQE concluded that damping is relatively

insensitive to stress level, exhibiting 3% damping at low seismic excitation, and up to 4%

damping at the SSE level of seismic excitation. In accordance with the decision to specify only

"whole percents", 3% was selected.

For bearing-bolted steel structures, EQE concluded that the range of damping values is from

4% at low seismic excitation to 6% at the SSE level of seismic excitation. Based on this, 5% was

selected for the proposed OBE damping value.
8

For prestressed concrete structures, EQE concluded that the range of damping values is from

2% at low seismic excitation to 5% at the SSE level of excitation. Based on this, 3% was

selected for the proposed OBE damping value.

3.2.3 Correlation of Structural Response Level with SSE Damping

Damping has been observed to be directly dependent on the response level. The SSE seismic

analysis of building structures is typically performed using damping values that are consistent

with response levels at or close to the maximum allowable response level for SSE load

combinations. In some cases, however, the predicted response may be significantly lower than

the maximum allowable response. Consequently, the damping value utilized in the SSE time

history analysis may be inconsistent with the predicted response level due to SSE load

combinations.

This potential inconsistency is highlighted in Regulatory Position C.3 of the current Regulatory

Guide 1.61. The correlation between damping and the response level has been recognized for

many years, and is illustrated by the different damping values specified for OBE and SSE in

Regulatory Guide 1.61. This dependence is particularly important when generating in-structure

response spectra for subsequent response spectrum analysis of systems and components. Use

of SSE damping values is not appropriate if the structure response is significantly lower than the

maximum allowable response for SSE load combinations. The objective is to achieve a

damping-compatible structural response This will yield more realistic estimates of internal

forces, moments, displacements, rotations, accelerations, and velocities in the structure.

BNL developed a procedure to provide guidance on this subject. The proposed procedure

involves determining the need to make a damping adjustment and, if necessary, either scaling

the initial results obtained using SSE damping, or performing additional time-history analysis for

SSE, with reduced damping values.

BNL's proposed scaling method is based on standard scaling methods, such as presented in

ASCE 4-98 (Ref. 7), for interpolation of spectral acceleration values for intermediate damping

values. Based on a numerical study, a damping correction multiplier (DCM), defined as

DCM = [0.8 x maximum PGA(ZPA) / design PGA(ZPA)] ; DCM $ 1.0 a

was determined to provide a convenient method for direct scaling of results obtained using SSE

damping.

The procedural steps are:

Step 1: Analyze the structure for the combined effect of the design-basis SSE input in 3

directions, using the SSE damping value specified in Table 1.

Step 2: By linearly scaling the initial SSE analysis results obtained in Step 1, determine

the maximum peak ground acceleration (PGA) or zero-period spectral

acceleration (ZPA) that satisfies the SRP Section 3.8 load combination

acceptance criteria, for the type of structure being analyzed.
9

Step 3: If the design PGA (ZPA) is greater than or equal to 0.8 times the maximum PGA

(ZPA), the SSE damping value specified in Table 1 is considered appropriate.

Use the initial analysis results obtained in Step 1 for both structural evaluation

and generation of in-structure response spectra.

Step 4: If the ratio of the design PGA (ZPA) to the maximum PGA (ZPA) is less than 0.8,

the SSE damping value specified in Table 1 may not be appropriate. Implement

Step 5.

Step 5: Where site conditions dictate the need for a soil-structure interaction analysis, the

soil stiffness typically controls the fundamental horizontal response ("rigid body"

rocking/horizontal translation) and the fundamental vertical response ("rigid body"

vertical translation) of the structure. While these response modes impart

significant acceleration to the structure, the contribution to the elastic response of

the structure is independent of the equivalent viscous damping value that is

assigned to the structure, because the structure responds essentially as a rigid

body in these modes.

The equivalent viscous damping value that is assigned to the structure primarily

affects the amplitude of the fundamental structural modal responses in the two

(2) horizontal directions. Their frequencies typically fall within the amplified region

of the design-basis SSE response spectrum. For higher frequency horizontal

modes of the structure, the importance of the assigned damping value

diminishes.

The frequency of the fundamental structural modal response in the vertical

direction is typically above the frequency region of significant amplification in the

design-basis SSE response spectrum. The minor variations in spectral

acceleration as a function of damping value have a relatively minor effect on the

overall response of the structure to 3-directions of seismic loading.

(A) For fixed-base analyses, excitation of the fundamental horizontal modes of the

structure are the major contributors to the total response of the structure. Using

the appropriate level of structural damping is important, because it has a

significant effect on the structure's response due to excitation of these

fundamental horizontal modes. In lieu of performing re-analysis using a reduced

damping value, an acceptable approach is to apply a damping correction

multiplier (DCM) to the dynamic analysis results obtained in Step 1. DCM is

defined as follows:

DCM = [0.8 x maximum PGA(ZPA) / design PGA(ZPA)]a

As a lower bound, DCM = 1.0 when the design PGA(ZPA) $ 0.8 x maximum

PGA(ZPA).

(i) As an example, if the design PGA(ZPA) = 0.4 x maximum PGA(ZPA),

then DCM = (0.8/0.4) = 1.26. Use of the DCM produces results a

consistent with re-analysis using the OBE damping value.
10

(ii) As a second example, if the design PGA(ZPA) = 0.6 x maximum

PGA(ZPA), then DCM = (0.8/0.6) = 1.10. Use of the DCM produces a

results consistent with re-analysis using a damping value midway

between the SSE and OBE damping values.

(iii) As a third example, if the design PGA(ZPA) = 0.2 x maximum PGA(ZPA),

then DCM = (0.8/0.2) = 1.59. Use of the DCM produces results a

consistent with re-analysis using a damping value equal to 1⁄2 x OBE

damping value.

The scaled dynamic results are then used for both the structural evaluation and

the generation of in-structure response spectra.

(B) When soil-structure interaction is included in the analysis of structural response,

it is conservative but acceptable to scale the dynamic analysis results obtained in

Step 1 in accordance with (A) above.

(C) An acceptable approach for both fixed-base analyses and soil-structure

interaction analyses is to perform one or a number of re-analyses using reduced

structural damping values, and provide a technical justification for the structural

damping value selected for the design-basis analysis used for both the structural

evaluation and the generation of in-structure response spectra.

After review of BNL’s proposal, the NRC Damping Task Force concluded that a simpler

approach would be more appropriate for inclusion in the Regulatory Guide 1.61 revision. This

simpler approach is delineated in 3.1.3 above. It is consistent with the approach recommended

in ASCE Standard 43-05 (Ref. 3), when use of SSE damping values cannot be justified.

However, the damping adjustment is applied for both structural evaluation and generation of instructure response spectra.
11

4. PIPING DAMPING

This section discusses damping values for use in the analysis of piping systems.

4.1 Proposed Piping Damping Values

Table 3 presents the constant damping values specified for SSE and OBE (where required)

analyses of piping systems. They are applicable to time-history, response spectrum, and

equivalent static analysis procedures for piping qualification.

Table 3 Damping values for piping systems

Category

Damping Value

SSE OBE > SSE/3

Piping Systems 4% 3%

As an alternative, for response spectrum analyses using an envelope of the SSE response

spectra at all support points (uniform support motion), frequency-dependent damping, as shown

in Figure 1, may be used, subject to the following restrictions:

• Frequency-dependent damping should be used completely and consistently, if

used at all. (For equipment other than piping, the damping values specified in

Regulatory Guide 1.61, “Damping Values for Seismic Design of Nuclear Power

Plants,” are to be used).

• The damping values specified may be used only in those analyses in which

current seismic spectra and procedures have been employed. Such use is to be

limited only to response spectral analyses. The acceptance of the use with other

types of dynamic analyses (e.g., time-history analyses or independent support

motion method) is pending further justification.

• When used for reconciliation work or for support optimization of existing designs,

the effects of increased motion on existing clearances and on-line mounted

equipment should be checked.

• Frequency-dependent damping is not appropriate for analyzing the dynamic

response of piping systems using supports designed to dissipate energy by

yielding.

• Frequency-dependent damping is not applicable to piping in which stress

corrosion cracking has occurred unless a case-specific evaluation is made and is

reviewed by the NRC staff.
12

4.2 Justification for Proposed Piping Damping

The subject of piping damping for seismic analysis of nuclear power plant systems received

considerable attention over the twenty years following the original issuance of Regulatory Guide

1.61 in 1973. Reference 8 provides a detailed summary through 1990. The basic issues can be

summarized as follows:

• Regulatory Guide 1.61 piping damping is conservative.

• Utilization of these conservative values has not improved overall plant safety; to

the contrary, pipe stresses due to repeated operational loads are generally

increased by conservative seismic support requirements.

• Specification of equivalent viscous damping values for piping system analysis is

based as much on philosophy and "gut feel" as it is on hard data.

• Industry proposals, including PVRC Damping, have their detractors on both the

conservative side and liberal side.

Figure 1 Frequency-dependent damping
13

• While equivalent viscous damping (function of velocity) is mathematically

convenient for analysis, it is not representative of the actual energy dissipation

mechanisms occurring during seismic excitation of a piping system; this is

especially true at high excitation levels, where the primary energy dissipater is

inelastic material behavior.

• There is very significant data scatter when piping test results are used to backcalculate the equivalent viscous damping; however, this is not surprising,

considering that no comprehensive test program has ever been conducted for the

sole purpose of defining damping levels and the primary drivers.

Based on discussion with several NRC personnel and an NRC consultant involved in damping

studies, there would appear to be significant support on the regulatory side for constant 4% SSE

damping, with no restrictions on its application. Two previous studies (Refs. 11 and 12) would

support that this is appropriate if a single value of damping is to be specified.

The role of the OBE in system design is specifically being reduced for new reactors; i.e., there

are no specific OBE design requirements if OBE # SSE/3. However, since the plant designer

has the option to exceed the above criterion, guidance for an OBE analysis must still be

provided in the Regulatory Guide revision. If analysis of loading combinations that include OBE

are required, the allowable stresses are typically limited to the yield strength of the pipe material.

Consequently, the equivalent viscous damping for use in conjunction with elastic analysis must

be specified at a lower value. A constant damping value of 3% is recommended for load

combinations that include OBE, with no restrictions on its application.

Based on assessment of several evaluations performed on the existing pipe testing data base

(Refs. 8, 13, 14,and 15), there does not appear to be a compelling technical justification to vary

piping damping as a function of pipe diameter. There appears to be a relatively weak correlation

between damping and pipe diameter. A stronger case can be made for "fundamental" mode vs.

higher modes. However, potential analytical complications arising from identification and

interpretation of the "fundamental" mode render this a relatively poor parameter for varying the

damping.

For simplicity, constant damping values of 3% OBE and 4% SSE are recommended for seismic

analysis of piping systems. These would be applicable to time-history, response spectrum, and

equivalent static analyses, and would be free of other restrictions.

The NRC previously accepted ASME Code Case N411-1 damping (Ref. 9), with qualifications in

accordance with Regulatory Guide 1.84 (Ref. 10). At the time the qualifications were initially

specified, the NRC had intended to conduct studies aimed at evaluating the validity of these

qualifications, and as appropriate, remove some of the restrictions on N411 damping. However,

the required studies were not conducted.

ASME has annulled Code Case N411-1, because Non-Mandatory Appendix N to Section III

currently recommends 5% damping at all frequencies, for both OBE and SSE (Ref. 4). The staff

had previously accepted 5% SSE damping for AP1000, for uniform support motion, response
14

spectrum analysis of piping systems (Ref. 16). The staff invoked restrictions on its use,

consistent with the qualifications formerly in Regulatory Guide 1.84 for Code Case N411-1.

The staff continues to accept former Code Case N411-1 damping subject to the restrictions

identified in Regulatory Guide 1.84. The staff considers acceptance of 5% damping for AP1000

to be a case-specific determination.
15

5. ELECTRICAL DISTRIBUTION SYSTEMS DAMPING

This section discusses damping values for use in the analysis of electrical distribution systems.

5.1 Proposed Cable Tray/Conduit Damping Values

The constant damping values presented in Table 4 are specified for SSE and OBE (when

required) analyses of cable tray and conduit systems. They are applicable to response spectrum

and equivalent static analysis procedures for structural qualification.

The analysis methodology must consider the flexibility of supports in the determination of

system response to seismic excitation.

Table 4 Damping values for electrical cable tray and conduit systems

Category Configuration

Damping Value

SSE OBE > SSE/3

Cable Tray

Systems 4

Maximum cable loading 1 10% 7%

Empty 2 7% 5%

Sprayed-on fire retardant or other

cable restraining mechanism 3 7% 5%

Conduit Systems 4 1 Maximum Fill 7% 5%

Empty 2 5% 3%

Notes:

1. Maximum cable loadings, in accordance with the plant design specification, must

be utilized in conjunction with these damping values.

2. Spare cable tray and conduit, initially empty, may be analyzed with zero cable

load and these damping values. (Note: Re-analysis is required when put into

service.)

3. Restraint of the free relative movement of the cables inside a tray reduces the

system damping.

4. When cable loadings less than maximums are specified for design calculations,

the selected damping values must be justified and will be reviewed for

acceptance on a case-by-case basis.

The damping values specified above are applicable to all types of supports, including welded

supports. Use of higher damping values for cable trays with flexible support systems (e.g.,

rod-hung trapeze systems, strut-hung trapeze systems, and strut-type cantilever and braced
16

cantilever support systems) is permissible, but will be subject to staff review on a case-by-case

basis.

5.2 Justification for Proposed Cable Tray/Conduit Damping

The justifications for the proposed damping values for cable trays and conduits are discussed in

the following subsections.

5.2.1 Cable Trays

Regulatory Guide 1.61, released in 1973, does not provide damping values for cable tray

systems. Historically, the nuclear power industry has used the values for bolted steel structures

(4% OBE, 7% SSE) for seismic design of cable tray systems.

In early nuclear plant designs, seismic loads were not considered for cable tray systems.

Traditional design methods from fossil power plants were carried over to nuclear power plants.

In the late 1970's and early 1980's, the need to evaluate these early installations for seismic

adequacy led to two major cable tray test programs. The first by BECHTEL/ANCO and the

second by URS/BLUME for the Systematic Evaluation Program Owners Group. As part of these

programs, the damping characteristics of typical cable tray systems were evaluated. These

results are summarized in Reference 17.

In the late 1980's the NRC questioned the seismic adequacy of the cable tray systems at

Comanche Peak. This led to another cable tray test program, conducted by ANCO for TU

Electric. Key objectives of this program were to justify the use of bolted steel structures damping

for cable tray systems with welded steel supports, and also to verify that response spectrum

analysis using these damping values produced conservative predictions of seismic response

(Refs. 18 and 19).

The results of these three test programs have been reviewed as a basis for the current

proposal. In addition, the recommendation for seismic margins studies (Ref. 20) has been

reviewed.

The current proposal attempts to address the following considerations:

• Applicable to response spectrum and equivalent static analysis procedures

• Simple to apply in seismic design calculations

• Sufficient conservatism to account for the scatter in experimentally derived

damping values

• Straightforward, to facilitate NRC safety evaluations

Based on the test results cited above, a number of parameters have been identified which affect

cable tray system damping. The most significant ones are:

• Excitation level
17

• Quantity of cables in the tray

• Presence of sprayed-on fire retardant, which "bundles" the cables

• Support stiffness

The first three parameters are specifically addressed in the current proposal. The fourth

parameter, support stiffness, is not considered in the specification of damping values. However,

the proposal requires that the cable tray analysis methodology include consideration of support

flexibility effects in the prediction of dynamic response.

The existing data on damping versus support configuration is not sufficiently comprehensive nor

consistent to apply in a straightforward manner. Therefore, the proposed damping values were

selected to cover all types of cable tray supports. This approach also eliminates potential

confusion and inconsistencies in the analysis of mixed support systems.

The cable tray tests summarized in Reference 17 included rod-hung trapeze systems,

strut-hung trapeze systems, and strut-type cantilever and braced cantilever support systems.

Welded supports were not covered. Based on the test results, cable tray damping values as

high as 20-25% were recommended for heavily loaded cable trays at high excitation levels.

Reduced damping values were recommended for unloaded or lightly loaded trays (5-7%) and

for trays with sprayed-on fire retardant (5-10%).

The tests conducted for Comanche Peak on cable tray systems with welded supports provided

valuable input for this proposal (Refs. 18 and 19). Damping values were derived from the

dynamic response of six (6) test configurations; most were tested for a range of cable fill level

and two (2) excitation levels. There is considerable scatter in the data. However, the data are

sufficient to support the proposed damping values for both fully loaded and empty cable trays, at

the OBE and SSE excitation levels, for welded support systems.

The proposed damping values for fully-loaded trays (7% OBE, 10% SSE) are conservative for

bolted supports, when compared to the results reported in Reference 17. However, the

difference in floor spectral accelerations between 10% damping and 20% damping is typically

small. Specifying a single set of damping values for all types of supports is judged to be a

higher priority. In addition, the option to specify and justify higher damping values is always

available to the licensee.

Reference 20 also includes guidance to industry on the subject of cable tray damping. For use

in seismic margin evaluations, 15% damping is recommended. Taking into consideration that

the system response level in a seismic margin assessment can significantly exceed the

response level permitted by design criteria, there is reasonable consistency between this

proposal and the recommendation of Reference 20.

The proposed damping values for empty cable trays provide a lower bound to the available test

data, based on References 18 and 19. The intent is to provide some regulatory guidance for

cases where design calculations assume less than the maximum design cable fill. Note 4 to

Table 4 would allow case-by-case justification of damping for intermediate cable fill levels.
18

It is desirable from a regulatory perspective that the maximum design cable fill be used in all

design calculations, regardless of the actual cable fill level. This proposal is intended to promote

this course of action by future licensees.

5.2.2 Conduits

Similar to the situation discussed above for cable trays, Regulatory Guide 1.61 does not

currently provide damping values specifically for conduit systems. Values specified in Final

Safety Analysis Reports (FSARs) and accepted in NRC Safety Evaluation Reports (SERs) have

typically been taken from Regulatory Guide 1.61 values for bolted steel structures, welded steel

structures, or piping. The values initially specified were perceived to be representative of

expected conduit system damping. Because of the advantages of higher damping values for

design calculations, the licensees usually tried to justify the use of bolted steel structure

damping (4% OBE, 7% SSE). This, in turn, led to regulatory concerns when the conduit system

support details did not clearly fit the description of bolted steel structures. A lengthy process of

justification and review usually ensued, oftentimes involving the generation and/or interpretation

of test data.

At this point in time, there is sufficient test data and regulatory precedence to explicitly define

conduit system damping in a revision to Regulatory Guide 1.61. This proposal is based on

review of available test data (Refs. 17, 21, 22, and 23) and the history of two safety evaluations

(Refs. 24 and 25). The proposed damping values have been selected to satisfy the same four

(4) considerations discussed previously for cable trays.

The parameters which are generally accepted as influencing the damping of a conduit system

are

• Cables inside the conduit

• Threaded couplings between conduit sections

• Support details

• Excitation level

Conduit systems exhibit lower damping than heavily loaded cable trays in tests of "comparable"

configurations (Ref. 21). Trends identified from the test data are

• The damping effect of the cables inside conduits is much less than the damping

effect of loose cables in cable trays.

• Conduit damping increases with increased cable fill up to typical design fill levels

(which are dictated by electrical criteria rather than structural criteria).

• Overfilling of conduits tends to reduce the measured damping.
19

• The available test results of damping versus cable fill are not comprehensive and

exhibit considerable scatter.

• There is a definite trend toward higher damping as the excitation level is

increased.

• Based on test results for empty conduit on rigid supports, the threaded couplings

would appear to contribute to system damping.

• Conduit system damping is affected by support and clamp details; however, no

clear-cut quantitative trends suitable for regulatory guidance can be readily

extracted from the available test data.

The following can be determined from the available test data and regulatory precedence:

• Bolted steel structure damping (4% OBE, 7% SSE) has been approved by the

NRC for welded conduit supports at Watts Bar (Ref. 24).

• TVA test data that justified these values were based on "rigidly" mounted test

configurations; i.e., the conduit was clamped directly to the test fixture (Ref. 22).

• The data reduction performed by TVA, to satisfy NRC requirements, would also

support 5% damping at OBE response levels (Ref. 24).

• 7% damping has been approved by the NRC for Train C conduits supported by

one-hole "C" (or "finger") clamps at Comanche Peak (Ref. 25); TU Electric’s

technical justification is documented in Reference 23.

• The test results presented in Reference 21 provide the basis for BECHTEL’s

recommendation of 7% damping for conduit systems, as reported in

Reference 17.

The proposed damping values for conduits with maximum design fill are below most of the

available pertinent test data, regardless of support and clamp details. They do not represent a

lower bound to all data. However, considering that use of these damping values will be in

conjunction with all other NRC requirements and guidance for seismic analysis, the proposed

damping values provide an appropriate level of conservatism.

The proposed damping values for empty conduit essentially provide a lower bound to the

available test data for conduit with and without cables. The intent is to provide some regulatory

guidance for cases where design calculations assume less than the maximum design cable fill.

Note 4 to Table 4 would allow case-by-case justification of damping for intermediate cable fill

levels.
20

It is desirable from a regulatory perspective that the maximum design cable fill be used in all

design calculations, regardless of the actual cable fill level. This proposal is intended to promote

this course of action by future licensees.
21

6. HEATING VENTILATION AND AIR CONDITIONING (HVAC) DUCT DAMPING

This section discusses damping values for use in the analysis of heating, ventilation, and air

conditioning (HVAC) ducting systems.

6.1 Proposed HVAC Duct Damping Values

Table 5 presents the constant damping values specified for SSE and OBE (when required)

analyses of HVAC duct systems. They are applicable to response spectrum and equivalent

static analysis procedures for structural qualification.

The analysis methodology must consider the flexibility of supports in the determination of

system response to seismic excitation.

Table 5 Damping values for HVAC duct systems

Type of Duct Construction

Damping Value

SSE OBE > SSE/3

Pocket Lock 10% 7%

Companion Angle 7% 5%

Welded 4% 3%

6.2 Justification for Proposed HVAC Duct Damping

The proposed damping values are consistent with the recommendation made for steel

structures with the exception that higher damping has been determined experimentally by TVA

for "pocket lock" duct construction (see Refs. 26 and 27). The TVA test results for "companion

angle" duct construction support the proposed damping values. No tests of welded duct

construction have been identified; therefore, the technical basis is the same as for welded steel

structures.

For "pocket lock" duct construction, Reference 27 recommends 10% damping for use with

Regulatory Guide 1.60 type ground spectra. Damping data were obtained from five (5) tests at a

low level of input (0.05 g s) and a stiff support configuration. The mean value obtained was

9.7%, with a standard deviation of 0.9%. Based on this limited test data, 7% OBE and 10% SSE

damping are proposed as reasonably conservative values for design calculations.

For "companion angle" duct construction, Reference 27 provides damping estimates at five

levels of input (0.05 g s, 0.10 g s, 0.15 g s, 0.20 g s, 0.25 g s), with a stiff support configuration.

Based on this data, Reference 27 recommends 6% OBE and 7% SSE damping for use with

Regulatory Guide 1.60 type ground spectra.

At the time Reference 27 was prepared, the OBE was specified as SSE/2. With recent changes

in the role of the OBE in seismic qualification, it is possible that an OBE as low as SSE/3 may
22

be used in analysis. To provide some conservatism, 5% OBE damping is recommended in this

current proposal. This makes the damping values consistent with the proposed bolted steel

structure damping.
23

7. MECHANICAL AND ELECTRICAL COMPONENT DAMPING

This section discusses damping values for use in the analysis of mechanical and electrical

components.

7.1 Proposed Component Damping Values

In the seismic qualification of mechanical and electrical components, it is important to

distinguish between active sub-components (functionality requirements) and passive

sub-components (containment, protection, structural support, pressure boundary requirements).

Active sub-components do not readily lend themselves to seismic qualification by analysis, and

require seismic qualification by test. Seismic qualification by test is addressed in SRP Section

3.10.

The damping values specified herein for mechanical and electrical components pertain only to

passive sub-components that are amenable to seismic qualification by analysis. Table 6

presents the damping values for mechanical and electrical components.

Table 6 Damping values for mechanical and electrical components

Component Type

Damping Value

SSE OBE > SSE/3

Motor, Fan, and Compressor Housings 

(protection, structural support)

3% 2%

Pressure Vessels, Heat Exchangers, Pump and

Valve Bodies (pressure boundary) 3% 2%

Welded Instrument Racks (structural support ) 3% 2%

Electrical Cabinets, Panels, Motor Control

Centers (protection, structural support)

3% 2%

Metal Atmospheric Storage Tanks

(containment, protection)

- Impulsive Mode

3% 2%

Metal Atmospheric Storage Tanks

(containment, protection)

- Sloshing Mode

0.5% 0.5%
24

7.2 Justification for Proposed Component Damping

The proposed damping values for mechanical and electrical components are based on review of

References 2, 3, 4, 16, and 28. For mechanical and electrical components, the limited

information provided in References 2 and 4, and the more extensive information provided in

References 3 and 28 are reasonably consistent. In Reference 16, the staff accepted the

damping values specified in Reference 28 for AP1000. The NRC Damping Task Force has also

informally reviewed the damping values in Reference 3, and has found them to be acceptable,

with minor exceptions.

7.2.1 Motor, Fan, and Compressor Housings

Reference 3 addresses these sub-components under the category "massive, low-stressed

mechanical components", which includes pumps, motors, fans, compressors, etc. The

Response Level 1 (2%) and Level 2 (3%) damping values in Reference 3 are proposed for OBE

and SSE damping, respectively.

7.2.2 Pressure Vessels, Heat Exchangers, Pump and Valve Bodies

Reference 16 accepted 4% SSE damping for AP1000 welded equipment. Because of their

similarity to piping (i.e., highly stressed by operating pressure loads), and for compatibility with

damping specified for attached piping, BNL initially proposed 4% SSE damping and 3% OBE

damping for this category.

However, Reference 3 includes pumps under the category "massive, low-stressed mechanical

components", discussed in 7.2.1 above. 

The NRC Damping Task Force recommended that 2% OBE damping and 3% SSE damping be

specified for this category, unless there is test data to support the higher values that BNL

recommended. Of particular concern to the staff are vertical RHR heat exchangers and vertical

pumps with motors (e.g., Deep-Well pumps). In the absence of definitive test data, Table 6

incorporates the staff’s recommendations.

7.2.3 Welded Instrument Racks

Both References 3 and 28 address welded instrument racks. The proposed damping values are

consistent with each other. Reference 16 accepted the Reference 28 value for SSE (3%). The

Response Level 1 (2%) and Level 2 (3%) damping values in Reference 3 are proposed for OBE

and SSE damping, respectively.

7.2.4 Electrical Cabinets, Panels, and Motor Control Centers (MCCs)

Both References 3 and 28 address enclosures for electrical devices. Reference 16 accepted 5%

SSE damping specified in Reference 28 for "cabinets and panels for electrical equipment." In

Reference 3, Response Level 1 damping of 3% and Response Level 2 damping of 4% are

specified for "electrical cabinets and other equipment." On this basis, BNL initially proposed 3%

OBE damping and 4% SSE damping for this category.
25

The NRC Damping Task Force recommended that 2% OBE damping and 3% SSE damping be

specified for this category, unless there is test data to support the higher values that BNL

recommended. Of particular concern to the staff are sensitive electrical equipment with welded

support. In the absence of definitive test data, Table 6 incorporates the staff’s

recommendations.

7.2.5 Metal Atmospheric Storage Tanks

Reference 3 addresses "liquid containing metal tanks", and specifies recommended damping

values for both the impulsive mode and the sloshing mode. Response Level 1 (2%) and Level 2

(3%) damping values are specified for the impulsive mode; 0.5% damping is specified for the

sloshing mode at all response levels. This is consistent with previous staff guidance for seismic

analysis of metal atmospheric storage tanks. The Reference 3 damping values are proposed.
26

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27

8. CONCLUSIONS AND RECOMMENDATIONS

The damping values proposed in Sections 3 through 7 of this report are based on the

recommendations submitted by BNL in 1995 (Ref. 1), additional work conducted by BNL under

the current project, and the recommendations of the NRC Damping Task Force. The

recommendations proposed herein do not include any radical changes from the damping values

specified in Regulatory Guide 1.61, Rev. 0. The primary improvements are (1) explicit guidance

for damping of components and non-piping distribution systems; (2) explicit guidance for

structural damping at low response levels; and (3) consideration of experimental data,

significant staff licensing actions related to damping, and revisions to codes and standards

related to damping, developed since Revision 0 was issued in 1973. 

Issuance of Revision 1 to Regulatory Guide 1.61, incorporating the recommendations contained

herein, will help to streamline the application and staff review process by minimizing the number

of case-by-case assessments required. 
28

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29

9. REFERENCES

1. Morante, R., "Damping Values for the Seismic Design of Nuclear Power Plants,"

Brookhaven National Laboratory, Technical Report L-1106-11/95, Prepared for

NRC/RES, November 1995.

2. U.S. Nuclear Regulatory Commission, Regulatory Guide 1.61, “Damping Values for

Seismic Design of Nuclear Power Plants,” Rev. 0, NRC: Washington, D.C. October

1973.

3. American Society of Civil Engineers, ASCE Standard 43-05, "Seismic Design Criteria

For Structures, Systems, and Components in Nuclear Facilities," Reston, VA, 2005.

4. American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, 2004

Edition, Section III, Division 1, Non-Mandatory Appendix N, "Dynamic Analysis Methods,"

New York, NY.

5. U.S. Nuclear Regulatory Commission, NUREG-0800, “Standard Review Plan for the

Review of Safety Analysis Reports for Nuclear Power Plants,” NRC: Washington, D.C.

6. U.S. Nuclear Regulatory Commission, NUREG/CR-6011, "Review of Structure Damping

Values for Elastic Seismic Analysis of Nuclear Power Plants," EQE Engineering

Consultants, March 1993.

7. American Society of Civil Engineers, ASCE Standard 4-98, "Seismic Analysis of

Safety-Related Nuclear Structures and Commentary", Subsection 2.2.1(b) and

Subsection 3.4.2.4, Reston, VA, 1998.

8. Ware, AG., "The History of Allowable Damping Values for U.S. Nuclear Plant Piping,"

Journal of Pressure Vessel Technology, Vol. 113, May 1991.

9. American Society of Mechanical Engineers, "ASME Boiler and Pressure Vessel Code,

Code Case N-411-1, Alternative Damping Values for Response Spectra Analysis of

Class 1, 2 and 3 Piping - Section III, Division 1," New York, NY. Approved February 20,

1986, Annulled May 5, 2000.

10. U.S. Nuclear Regulatory Commission, Regulatory Guide 1.84, “Design, Fabrication, and

Materials Code Case Acceptability, ASME Section III," Rev. 33, NRC: Washington, D.C.,

August 2005.

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30

13. Welding Research Council Bulletin 300, "Technical Position on Damping Values for

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Analysis/Test Correlation," prepared for TU Electric, February 1987. (Proprietary

information. Not publicly available.)

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Plant Seismic Margin (Revision 1)," August 1991. (Proprietary information. Not publicly

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23. Impell Corporation Report 01-0210-1527, Rev. 1, "Comanche Peak Steam Electric

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January 1992.
31

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32

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