Regulatory Guide 1.122

Development of Floor Design Response Spectra for Seismic Design of Floor-Supported Equipment or Components

ML13350A275
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Issue date:	09/30/1976
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RG-1.122
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U.S. NUCLEAR REGULATORY COMMISSIONREGULATORY GUIDEOFFICE OF STANDARDS DEVELOPMENTREGULATORY GUIDE 1.122DEVELOPMENT OF FLOOR DESIGN RESPONSE SPECTRAFOR SEISMIC DESIGN OF FLOOR-SUPPORTED EQUIPMENT ORCOMPONENTSSeptember 1976

A. INTRODUCTION

Criterion 2, "Design Bases for Protection AgainstNatural Phenomena," of Appendix A, "General DesignCriteria for Nuclear Power Plants," to 10 CFR Part 50,"Licensing of Production and Utilization Facilities," re-quires, in part, that nuclear power plant structures,systems, and components important to safety be de-signed to withstand the effects of earthquakes withoutloss of capability to perform their safety functions. Para-graph (a)(1) of Section VI, "Application to EngineeringDesign," of Appendix A, "Seismic and Geologic SitingCriteria for Nuclear Power Plants," to 10 CFR Part 100,"Reactor Site Criteria," requires, in part, that safety-related structures, systems, and components remainfunctional in the event of a Safe Shutdown Earthquake(SSE). It specifies the use of a suitable dynamic analysisas one method of ensuring that the structures, systems,and components can withstand the seismic loads.Similarly, paragraph (a)(2) of Section VI of the sarklappendix requires, in part, that the structures, sy.i.ms,and components necessary for continued opezation""th-out undue risk to the health and ie remain functional in the event of an t~qratinrnBasisEarthquake (OBE). Again, the use of suit .d> .amicanalysis is specified as one method of ensurt'iAiat thestructures, systems, and nents can withstand theseismic loads.* This guide cri e ds acceptable to the NRC* staff for 4 l g tw ori ontal and one vertical floordesign .o t various floors or other equip-ment-su locafs of interest from the time-historymotions Iting from the dynamic analysis of thesupporting cture. These floor design response spectraare needed for the dynamic analysis of the systems orequipment supported at various locations of the sup-porting structure.

B. DISCUSSION

Nuclear facility structures can be approximated bymathematical models to permit analysis of responses toearthquake motions. Because of the large number ofdegrees of freedom that would be necessaiy. and thepossible ill-conditioning of the resulting stfqiiess matrixif the complete plant were treated in a cal model, the plant is usually dvided intQ. severalseparate systems for analysis P.-Urs's-Thus itis usualthat there arc one or more matheail models of sup-.,t" I, .= M;porting structures. Each supporting structure normallysupports one or rte systems or of equipment.Also, different m"-.l structure may berequired For these reasons, themathen .Mo A used to generate the seismic excita-tion ..for s ects ent separate analyses of supported.syterZ,_.., -. eq~upment may not be suitable for the*t.Wed-Aoidized analyses of the supporting structure.I'1ost equipment having a sr'all mass relative to that:of the supporting structure will 'lave negligible interac-tion effects on the support structu, -and will need to beincluded only in the mass distributicin of the mathemati-cal model for that structure. For such equipment, aseparate analysis will be performed using the floor designresponse spectra or time-history excitations at the equip-ment-support locations derived from the analysis of thesupporting structure. This guide addresses the acceptabil-ity and development of floor design response spectraonly. Time-history motions that will give resultscomparable to the floor design response spectra are alsoacceptable.There are, however, other major equipment systemssuch as the reactor coolant system whose stiffness, mass,and resulting frequency range should be considered forinclusion in the model of the supporting structure toUSNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission. U.S NuclearRegulatory Commission. Washington. D.C. 2V566. Attention Docketing andRegulatory Guides ate issued to deoctibS and make available to the public Service Section.methods acceptable to the NRC sltal of implemenling specific parts of theCommission aeIgulations. to delineate techniques used by the Staff in Osalu The guides are issued in the following tIn broad divisionscling specific problems or postulated accidents, at to provide guidance to applicents. Regulatory Guides ate not substitutes for regulations, and compliance i. Power Aeacti's 6 Productswith them is tal required Malhods and solutions dilflatent from those sat out in 2. Research and Test Reactors 7 Transportationthe guides will be acceptable if they provide a basis for the findings requisite to I. Fuels and Materials Facilities 8. Occupational Healththe issuance Ot continuance ofe permit or license by the Commission 4 Environmental and Siting S. Antitrust ReviewComments and suggestions fo, improvements in these guides are encouraged 5 Materials end Plant Protection 10. Generalat alf times, and guides will be revised, as appropriate, to accommodate camments and to reflect new rnfotmation or esperiance However. comments on Copies of published guides may be obtained by written request indicating thethis guide. if received within about two months after its issuance, will be par divisions desited to the U S Nuclear Regulatory Commilsion. Washington. D.C.ticularle useful in evaluating the need foran early revision 20%6. Attention. Director, Office o0 Standards Development.

• account for possible dynamic interaction effects. Suchequipmlent can be analyzed by combining the completeequipment model with the model of the supportingstructure and applying the proper excitation to the base* of the supporting structure. With this method, no sepa-*rate equipment-support excitations need be generatedbecause the equipment will be excited directly throughthe structure. It should be noted that a combined modelof the building and equipment must be formulated toperform such an analysis.I. Floor Response SpectraThe two horizontal and the vertical response spectra* can be computed from the time-history motions of the* supporting structure at the various floors or other equip-ment-support locations of interest. The spectrum ordi-.nates should be computed at frequency intervalssufficiently small to produce accurate response spectra(see Table 1 for guidance). Spectrum peaks normallywould be expected to occur at the natural frequencies ofthe supporting structure.2. Smoothing Floor Response Spectra and BroadeningPeaksTo account for variations in the structural frequenciesowing to -uncertainties in the material properties of the* structure and soil and to approximations in the modelingtechniques used in seismic analysis, the computed floorresponse spectra should be smoothed, and peaks asso-ciated with each of the structural frequencies should bebroadened. One acceptable method for determining theamount of peak broadening associated with each of thestructural frequencies is described below.Let fj be the Jth mode structural frequency that isdetermined from the mathematical models. The varia-tion in each of the structural frequencies is determinedby evaluating the variation due to each significant para-meter such as the soil modulus, material density, etc.The total frequency variation, +/-.eAfj, (see Figure 1) isthen determined by taking the square root of the sum of* squares (SRSS) of a minimum variation of 0.05fj and theindividual frequency variations, A(Jn, as described inregulatory position 1.Figure 1 shows a sample of a smoothed floor responsespectrum curve. Note that the broadened peak isbounded on each side by lines that are parallel to thelines forming the original spectrum peak.3. Floor Design Response Spectra* Nuclear power plant facilities are designed for three-component earthquakes, as indicated in RegulatoryGuide 1.60, "De.sign Response Spectra for SeismicDesign of Nuclear Power Plants." When a structuralseismic analysis is performed separately for each direc-tion (two horizontal and one vertical), and in the case ofTABLE 1SUGGESTED FREQUENCY INTERVALS FORCALCULATION OF RESPONSE SPECTRAFrequencyRange Increment(hertz) (hertz)0.2- 3.0 0.103.0- 3.6 0.153.6- 5.0 0.205.0- 8.0 0.258.0- 15.0 0.5015.0- 18.0 1.018.0-22.0 2.022.0 -34.0 3.0unsymmetric structures, the structural motion in a givendirection at a given location will contain contributionsfrom the vertical and the two horizontal excitations. Insuch cases, the contribution from each Individualanalysis will generate a response spectrum at a givenlocation in each of the three directions. The ordinatesof these three smoothed response spectra (with peaksbroadened) for a given direction should be combinedaccording to the SRSS criterion to predict the floordesign response spectrum at the given location and forthe given direction. In the case of symmetric structures,there will be only one floor response spectrum in each ofthe three directions. The smoothed versions of thesefloor response spectra will be the floor design responsespectra. In those cases in which the mathematical modelis subjected simultaneously to the action of three spatialcomponents of an earthquake, the three computed andsmoothed floor response spectra at a given level will bethe floor design response spectra.

C. REGULATORY POSITION

The following procedures for smoothing the floorresponse spectra (with peaks broadened) and combiningthe smoothed floor response spectra to obtain the floordesign response spectra are acceptable to the NRC staff:1. To account for variations In the structural fre-quencies owing to uncertainties in such parameters asthe material properties of the structure and soil,damping values, soil-structure interaction techniques,and the approximations In the modeling techniques usedin seismic analysis, the computed floor response spectra1.122-2 z0L-t~J ILlVJ.2 .3 .4 .5 .6 .7.8.91. 2. 3. 4. 5. 6. 7. 8.9.10.COMPUTED FREQUENCY (CPS)20. 30. 40. 50.60.Figure 1 Response Spectrum Peak Broadening and Smoothing irnfroni the floor time-history motions should besmoothed, and peaks associated with each of tihe struc-tural frequencies should be broadened (see the sample inFigure 1) by a frequency, +Afj, whereXP (Afn) ]Afj .[)=(0.05J)2 + X Wild 2 1 4o0.ofJn=lwhere /fJn denotes the variation in Jth mode fre.quency, f., due to variation in parameter number n, andP is the number of significant parameters considered. Avalue of 0.10fj should be used if the actual computedvalue of Afj is less than 0.10fj. If the above procedureis not used, AfJ should be taken as 0.1 5fJ.2. When the seismic analysis is performed separatelyfor each 6f the three directions, and in the case of un-symmetric structures, the ordinates of the floor Oesignresponse spectrum for a given direction should beobtained by combining the ordinates of the threesmoothed floor response spectra for that directionaccording to the SRSS criterion. In the case of symme-tric structures, the floor design response spectrum for aUNITED STATESNUCLEAR REGULATORY COMMISSIONWASHINGTON, 0. C, 20555OFFICIAL BUSINESSPENALTY FOR PRIVATE USE. S300given direction will be the smoothed floor responsefor that direction.3. When the mathematical model of the supportingstructure is subjected simultaneously to the action ofthree spatial components of an earthquake, thecomputed and smoothed response spectrum in a givendirection will be the floor design response spectrum inthat directiotn.

D. IMPLEMENTATION

The purpose of this section is to piovide informationto applicants regarding the NRC staff's plans for usingthis regulatory guide.This guide reflects current NRC staff practice. There.fore, except in those cases in which tie applicantproposes an acceptable alternative method forcomplying with specified portions of the Commission'sregulations, the method described herein is being andwill continue to be used in the evaluation of submittalsfor construction permit applications until this guide isrevised as a result of suggestions from the public or addi-tional staff review.POSTAGE AND FEES PAIDU.S. NUCLEAR REGULATORY .rCOMMISSION -1.122-4