During the first quarter of 1988, the Nuclear Regulatory Commission (NRC) prepared a proposed Revision 2 to the NUREG-0800 Standard Review Plan (SRP) Sections 2.5.2 (Vibratory Ground Motion), 3.7.1 (Seismic Design Parameters), 3.7.2 (Seismic Systems Analysis) and 3.7.3 (Seismic Subsystem Analysis). The proposed Revision 2 to the SRP was a result of many years' work carried out by the NRC and the nuclear industry on the Unresolved Safety Issue (USI) A-40: ''Seismic Design Criteria.'' The background material related to NRC's efforts for resolving the A-40 issue is described in NUREG-1233. In June 1988, the proposed Revision 2 of the SRP was issued by NRC for public review and comments. Comments were received from Sargent and Lundy Engineers, Westinghouse Electric Corporation, Stevenson and Associates, Duke Power Company, General Electric Company and Electric Power Research Institute. In September 1988, Brookhaven National Laboratory (BNL) and its consultants (C.J. Costantino, R.P. Kennedy, J. Stevenson, M. Shinozuka and A.S. Veletsos) were requested to carry out a review of the comments received from the above six organizations. The objective of this review was to assist the NRC staff with the evaluation and resolution of the public comments. This review was initiated during October 1988 and it was completed on January 1989. As a result of this review, a set of modifications to the above mentioned sections of the SRP were recommended by BNL and its consultants. This paper summarizes the recommended modifications. 4 refs

Bagchi, G.

Chokshi, N. C.

Philippacopoulos, A. J.

Shaukat, S. K.

UNT Digital Library

r^/ BNL-NUREG-42788REVIEW OF PUBLIC COMMENTS ON PROPOSED SEISMICDESIGN CRITERIA*BNL-NUREG—42788by* T «.,,« ! D E 8 9 014191A.J. PhilippacopoulosBrookhaven National LaboratoryUpton, New York 11973" S.K. ShaukatEngineering Issues Branch, Office of Nuclear Regulatory ResearchU.S. Nuclear Regulatory Commission, Washington, D.C. 20555N.C. ChokshiProbabilistic Bisk Analysis Branch, Office of Nuclear Regulatory ResearchU.S. Nuclear Regulatory Commission, Washington, D.C. 20555G. BagchiStructural & Geosciences Branch, Office of Nuclear Reactor RegulationU.S. Nuclear Regulatory Commission, Washington, D.C. 20555DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United Slates Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-ence herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The viewsand opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.DISTRIBUTE OF THIS DOCUMENT IS UNLIMITEDINTRODUCTIONDuring the first quarter of 1988, the Nuclear Regulatory Commission (NRC)prepared a proposed Revision 2 to the NUREG-0800 Standard Review Plan (SRP)Sections 2.5.2 (Vibratory Ground Motion), 3.7.1 (Seismic Design Parameters),3.7.2 (Seismic Systems Analysis) and 3.7.3 (Seismic Subsystem Analysis). Theproposed Revision 2 to the SRP was a result of many years work carried out bythe NRG and the nuclear industry on the Unresolved Safety Issue (USI) A-40:"Seismic Design Criteria." The background material related to NRC's efforts forresolving the A-40 issue is described in NUREG-1233 (Shaukat et al. , 1988).Details pertaining to SSI issues are given by (Chokshi et al. , 1989).In June 1988, the proposed Revision 2 of the SRP was issued by NRC forpublic review and comments. Comments were received from Sargent and LundyEngineers, Westinghouse Electric Corporation, Stevenson and Associates, DukePower Company, General Electric Company and Electric Power Research Institute.In September 1988, Brookhaven National Laboratory (BNL) and its consultants(C.J. Costantino, R.P. Kennedy, J. Stevenson, M. Shinozuka and A.S. Veletsos)were requested to carry out a review of the comments received from the abovesix organizations. The objective of this review was to assist the NRC staff withthe evaluation and resolution of the public comments. This review was initiatedduring October 1988 and it was completed on January 1989. As a result of thisreview, a set of modifications to the above mentioned sections of the SRP wererecommended by BNL and its consultants (Philippacopoulos, 1989). This papersummarizes the recommended modifications.GROUND MOTIONThe main concerns expressed in the public comments with respect to theproposed Revision 2 to SRP requirements on ground motion are related to thefollowing items:a) Power Spectral Density (PSD) criteria;b) Duration of input design time history;c) Number of time histories for multiple time history analysis; andd) Ratio of vertical to horizontal ground design spectra.*The USNRC has neither approved nor disapproved the contents of this paper. Theviews and conclusions contained in this paper are those of authors and shouldnot be interpreted as necessarily representing the official position orrecommendations of the USNRC.From the above items, the public comments were particularly focused on theproposed PSD requirements. The basic objection was that it could forceunnecessary conservatism in the design time history. It appears that the mainsource of the public reaction to the proposed PSD requirement is that the latteris based on a target PSD which produces response spectra which are much higherthan the corresponding Regulatory Guide 1.60 spectra (especially at higherfraquencies, i.e., greater than 10 cps) . This causes the following problem:A design time history which matches closely the Regulatory Guide 1.60 spectrahas to be subsequently modified so that its PSD meets a target PSD which in turnproduces response spectra which lie much above the Regulatory Guide 1.60 spectra.This problem can be resolved by refining the PSD requirements of the proposedRevision 2 to the SRP so that they correspond to a minimum type PSD requirements.Minimum simply implies that they preserve (within reasonable limits) the levelof conservatism associated with the design response spectra. By doing so, theprimary role of design spectra is maintained in the design process, whereas thePSD requirement is of a complementary nature. As such, any time history whichsatisfies the design spectra should be able to pass the PSD check, unless thereis a lack of ground motion energy at specific frequency windows.Several questions were also raised in the public comments with respect tothe minimum requirement of five time histories for the multiple time historyanalysis option. This requirement was found to be too high. In addition, itwas suggested that specific guidelines be given in the SRP for acceptance of timedurations for the design time histories used in seismic analysis. Finally,public comments suggested that the 2/3 rule for defining the vertical componentof the design ground motion be included in the SRP acceptance criteria.The following recommendations are made for resolution of the publiccomments on the proposed Revision 2 to the SRP areas dealing with ground motionrequirements:a) First, the PSD criteria (target PSD function as well as requirements tomeet the target PSD) of the proposed SRP Revision 2 should be replaced withthe minimum PSD criteria given in NUREG/CR-5347. The SRP should alsoclarify that the design response spectra are the primary acceptancecriteria while the PSD requirement is a secondary one. Secondly, thefollowing items are suggested to be further investigated:o PSD requirements for other types of generic broad-banded designspectra be developed.o PSD requirements for both horizontal and vertical cases be specified.o The purpose of PSD functions in seismic analysis be clarified, e.g.,should PSD representations of input motion be also used inconjunction with other aspects of seismic analysis?o The case of implementing power requirements directly at the levelof the response spectrum be further investigated.b) The following provisions regarding the duration of ground design timehistories are recommended:o Strong motion duration: Minimum: 6 secondsMaximum: 15 secondso Total duration: 10-25 secondsShorter or longer durations should also be accepted on a case-by casebasis.c) The minimum number of time histories required to perform a multiple timehistory seismic analysis should,be reduced from five to four. At thistime, sufficient basis does not exist for further reduction.d) The vertical ground design spectrum should be taken as 2/3 of thehorizontal over the entire frequency range of interest provided that theepicentral distance of the design earthquake is more than 10 Km. Forsmaller epicentral distances the definition of the vertical ground designspectrum should be reviewed and accepted on a case-by-case basis. Inaddition, the 2/3 scaling rule should not be permitted for cases in whichthe horizontal ground design spectrum is generated using the site-specificapproach described in Item 1 of SRP Subsection 2.5.2.6. In such cases,the same procedure should be followed for generating both the horizontalas well as the corresponding vertical ground design spectra.SOIL-STRUCTURE INTERACTIONThe majority of the public comments on the soil-structure interaction (SSI)requirements of the proposed SRP Revision 2 were primarily focused on thefollowing items:1) Acceptance criteria for justification of fixed-base analysis.2) Enveloping requirement of alternate 1 SSI analysis.3) Requirements for variation of soil properties in SSI.4) Limit of soil hysteretic damping to 5% of critical.5) Limit on reduction of ground motion with embedment.6) Acceptance criteria for composite modal damping in SSI.7) Requirements for alternate 1 and 2 procedures of SSI.These items are briefly discussed next.A fixed-base analysis is usually justified for the case in which theunderlying foundation material is of the rock type. The provisions of the ASCEStandard 4-86 on this issue seem to be appropriate and it is recommended thatthey should be accepted by the SRP for fixed-base frequencies of lOcps or less.On the other hand, it is appropriate to maintain in the SRP acceptance criteriathe justification of fixed-base analysis in terms of fixed-base and interactionfrequencies. The latter approach is more sound and should be the primaryacceptance criteria in the SRP.In the public comments it has been unanimously suggested that therequirement for enveloping the results of different analysis methods which areimposed by alternate 1 procedures of SSI, should be deleted. This subject hasbeen carefully examined during the last ten years and at this time it is veryimportant that the enveloping requirement be deleted since it is no longernecessary. No loss of conservatism is implied by such deletion. In fact, itwill allow the design process to focus on the more important sources ofconservatism which are usually hidden behind the enveloping requirement.Public comments suggested that some clarifications be given for the SRPacceptance criteria related to soil properties variations employed in the SSIanalysis (Section 3.7.2). BNL and its consultants also recommend that this areaneeds to be improved. First of all, it should be spelled out what is meant bylow shear strain soil properties. As such, the 10'* percent shear strain shouldbe defined as the low shear strain for the purpose of SSI analysis. Second, theSRP should clarify that the soil shear modulus and the damping (hysteretic) usedin the SSI analysis are not necessarily those corresponding to low strains.Specifically, it should be made clear that these values should be consistent withthe strains obtained in the soil profile through the free-field analysis of thedesign earthquake. Finally, certain changes should be made in this section ofthe SRP so that the best estimate, lower and upper values for soil variationscases be better defined especially in terms of soil shear strain. Specifically,the following clarifications are -recommended:o The upper bound shear modulus at low strain can be taken as twice thebest-estimate value while the lower bound shear modulus can be defined asone-half this value, provided that this range of variability suitablyencompasses the scatter typically found in the site investigation program.The average shear modulus degradation (G/G max vs effective shear strain)and hysteretic damping ratio (D vs effective shear strain) curves, asdefined in ASCE Standard 4-86 can be determined form the laboratory testingprogram, together with typical data available for similar soil. Thesecurves can then be used in the iterative pseudo-liner analyses to determineshear moduli and hysteretic damping ratios compatible with the effectiveshear strains computed in the free-field for the input seismic motions forall soil layers for each of the three cases of interest. These propertiescan then be used directly in the SSI computational model.o The lower bound shear moduli should not be less than the moduli requiredfor an acceptable foundation design, that is, lead to static settlementsmuch greater than considered acceptable for normal foundation design.Secondly, the upper bound shear moduli should not be less than the bestestimate shear moduli defined at low strain (.Gmax defined at 10"* percenteffective shear strain) for all soils.Public comments suggested that the requirement of 5% maximum hystereticsoil damping is too low and it should be raised to 15% which was also recommendedin NUREG/CR-1161. BNL and its consultants concur with this recommendation sincesuch arbitrary limit on soil damping is not consistent with experience from soildata. Furthermore, the SRP should also provide a definition of the hystereticdamping since different definitions currently exist in the SSI literature. TheASCE Standard 4-86 gives a clear definition of the hysteretic soil damping andSRP should refer to it.The subject of reduction of motion with embedment received considerableattention by the public comments. The positions taken ranged from thosequestioning the advisability of adopting a limit on the reduction to thosesuggesting that the SRP should adopt the ASCE Standard 4-86 provisions on thissubject. It is realized that this issue is particularly difficult and cannotbe completely resolved at this time. A sufficiently rational approach forestablishing acceptance criteria for the reduction of motion with depth isdescribed below:o Reduction of the translational components of the ground motion withembedment should be permitted in SSI analyses provided that the relevantrotational components are accounted for. This is supported by physicalconsiderations of the problem as well as by recorded data.o At this time, it is appropriate to impose a limit on the reduction of theground motion with embedment. This will guard against the uncertaintiesassociated with the spatial variation of seismic motion.o The reduction should refer to the difference between the surfacetranslational motion and the corresponding motion in the free-field at thefoundation level. This form of reduction has primarily two advantages:a) the reduction can be conveniently applied to botn the direct as wellas the substructure method of SSI analysis and b) the reduction can bedirectly measured with field data.o The amount of reduction should be reasonably taken in the range of 30-40%,with the 30% limit being considered as very conservative.o The reduction should refer t> the envelope (best estimate, lower bound andupper bound cases) of the free-field spectra at the foundation level. Thiswill allow the analyst to meet the reduction criteria by enveloping theresults of the three soil property variation cases.In view of the public comments, BNL and its consultants recommend that theSRP should place a limit on the composite modal damping used in SSI analysesbased on modal superposition. This limit should be reasonably set to 20%.Finally, they also recommend that the current distinction of alternate 1 and 2procedures of SSI analysis should be deleted from Section 3.7.2 of the proposedSRP Revision 2. In summary, the following are recommended by BNL and itsconsultants with respect to soil-structure interaction requirements:o It is recommended that the definition of rock-like materials per ASCEStandard 4-86 be adopted in the SRP.o It is recommended that the ASCE Standard 4-86 provision for fixed-base assumption be accepted in the SRP for fixed-base frequenciesof 10 cps or less.o It is recommended that the enveloping requirement of results fromdifferent SSI methods be deleted from the SRP.o It is recommended that the following clarifications/criteria be givenin the SRP with regard to soil property variations:o The shear modulus and the soil damping of hysteretic type usedin the SSI analysis should be compatible with the effectiveshear strains (65% of corresponding peak values) associatedwith the free-field analysis of the design ground motion.o The low strain best estimate shear modulus should be definedat 10"* percent shear strain. The lower and upper bound shearmoduli at low strain should be defied as half and twice thisvalue respectively.o The lower bound shear moduli should not be less than thoserequired for an acceptable foundation design.o The upper bound shear moduli should not drop at any shearstrain below the value of the best estimate at low strain.o It is recommended that the current 5% limit on soil damping ofhysteretic type be changed to 15%. Furthermore, the ASCE Standard4-86 definition of hysteretic damping or other equivalent bespecified in the SRP.o The translational components of the free-field motion at thefoundation level should not be less than 60% of the correspondingsurface motion. This provision should be:a) allowed only when the associated rotational components areaccounted for; andb) applied in terms of the envelope of the best estimate, lowerand upper bound soil property variation cases.o It is recommended that when modal superposition is used in the SSIanalysis, the modal damping be limited to 20%. If the compositemodal damping is higher than 20%, then acceptable methods should be:a) time domain analysis based on solution of complex eigenvalueproblem;b) frequency domain analysis; orc) direct integration.o It is recommended that alternate 1 and 2 procedures of SSI analysisbe deleted from SRP Section 3.7.2 and their design philosophy beassociated with the specification of the vibratory ground motion(Section 2.5.2).OTHER ISSUESAdditional issues which were included in the public comments are:1) Requirements for modal combination.2) Correlation of damping with stress levels.3) Greater use of professional standards by the SRP.The following recommendations were made by BNL and its consultants on theseissues:o It is recommended that the algebraic sum method in modal combinationbe accepted in the SRP by appropriately revising Regulatory Guide1.92 to that effect.o It is recommended that the provisions of the ASCE Standard 4-86 oncorrelation of damping with stress levels be an acceptable procedurein the SRP.o It is recommended that the SRP make greater use of availablestandards of professional societies and other organizations.CONCLUSIONSAs a result of the review of the public comments, BNL and its consultantsrecommended major changes on the proposed SRP Revision 2 which primarily affectthe particular SRP areas dealing with a) ground motion requirements and; b) soil-structure interaction requirements. These changes will advance the licensingprocess in view of the developments over the last two decades in the seismic areaand on the other hand they will provide ar. improved accountability ofconservatism in the seismic design review process.REFERENCESPhilippacopoulos, A.J. (1989), Recommendation of Resolution of Public Commentson USI A-40, "Seismic Design Criteria," NUREG/CR-5347.Coats, D.W. (1980), Recommended Revisions of Nuclear Regulatory CommissionSeismic Design Criteria, NUREG/CR-1161.Shaukat, S.K., Chokshi, N.C., Anderson, N.R., (1988) Regulatory Analysis for USIA-40, "Seismic Design Criteria," NUREG-1233, Draft Report for Comment.Chokshi, N.C., Shaukat, S.K., Anderson, N.R., (1989), The Proposed Soil-StructureInteraction Licensing Criteria-Resolution of USI A-40, "Seismic DesignCriteria," SMiRT 10.

Review of public comments on proposed seismic design criteria

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Review of public comments on proposed seismic design criteria

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