ML20083G587

Midland Independent Design & Const Verification Program: Structural Evaluation of Diesel Generator Bldg

ML20083G587
Person / Time
Site:	Midland
Issue date:	01/04/1984
From:	Levin H TERA CORP.
To:
Shared Package
ML20083G568	List: ML20083G564 ML20083G587
References
ISSUANCES-OL, ISSUANCES-OM, ISSURANCES-OL, ISSURANCES-OM, NUDOCS 8401060294
Download: ML20083G587 (46)

Text

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@nited States Nuclear Regulatory Commission Docket Numbers 50-329 & 50-320 x ,. '

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840t060294 840104 TERA CORPORATION PDR ADOCK 05000329 A PDR

L J January 4,198$4 Mr. James W. Cook Vice President Consumers Power Company 1945 West Pornoll Road Jackson, Michigan 49201 Mr. J. G. Keppler Administrator, Region ill Office of Inspection and Enforcement U.S. Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, IL 60137 Mr. D. G. Eisenhut Director, Division of Licensing Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washingten; D.C. 20555 Re: Docket Nos. 50-329 OM, OL and 50-330 CM, OL Midland Nuclear Plant - Units I and 2 Independent Design and Construction Verification (IDCV) Program Structural Evoluotion of the Diesel Generator Building -

Assessment of the Structural Performance Capability as Potentially Affected by Settlement Ir.duced Cracking Gentlemen:

Attached is our recently completed engineering evoluotion of the structural performance capability of the diesel generator building. This evoluotion was undertaken in accordance with the defined scope of the IDCVP as port of our broader assessment of the quality of the design and constructed product of the Midland plant Standby Electric Power system. We are fransmitting it to you because of its relevance to ongoing discussions concerning the potential effects of settlement induced cracking on the capability of the DGB to meet intended performance requirements over its service life.

We have concluded that the existing cracks, generally being of small size, are not indicative of a condition that would compromise the capability of the DGB in meeting its intended performance requirements. Furthermore, it is judged that significont future cracking is unanticipated and the DGB is expected to remain serviceable without further remedial action at this point in time. We have TERA CORPORATION BETHESDA. MARYLAND 20814 301c5H%0 7101 WISCONSIN AVENUE

Multiple Addressees reviewed Consumers Power Company's commitments to verify continued serviceability and have concluded that these are acceptable; however, we have

_ offered certain recornmeridations for consideration that are intended to improve available information and reduce operational constraints.

Should you desire further articulation of the bases for our conclusions, we would welcome the opportunity for discussion.

Sincerely, rI / .

% ,dtw Howard A. Levin Project Monoger Midland IDCV Program Enclosure cc: L. Gibson, CPC.

R. Erhardt, CPC J. Mooney, CPC D. Budzik, CPC D. Quommy, CPC (site)

R. Whitaker, CPC (site)

R. Burg, Bechtel J. Taylor, NRC, l&E HQ T. Ankrum, NRC, I&E HQ D. Hood, NRC, NRR Midland IDCVP Service List HAL/sl TERA CORPORATON

SERVICE LIST FOR MIDLMO ltOEPEPOENT DESIGN MO CONSTRUCTION VERIFICATION PROGRAM cc: Harold R. Denton, Director Ms. Borboro Stamiris Office of Nuclear Reactor Regulatim 5795 N. River U.S. Nuclear Regulatory Commissim Freeland, Michigan 48623 Mr. Wendell Marshall

. ' James G. Keppler, Regional Administrator Route 10 U.S. Nuclear Regulatory Commission, Midland, Michigan 48440 Region ill

- 799 Roosevelt Road Mr. Steve Godler Glen Ellyn, Illinois 60137 2120 Carter Avenue St. Paul, Minnesota S5108 U.S. Nuclear Regulatory Commission Resident inspectors Office Ms. Billie Pirner Garde Route 7 Director, Citizens Clinic Midland, Michigan 48640 for Accountable Government Government Accountability Project Mr. J. W. Cook Institute for Policy Studies Vice President 1901 Que Street, N.W.

Consumers Power Company Washington, D.C. 20009 1945 West Parnall Road Jackson, Michigan 49201 Charles Bechhoefer, Esq.

Atomic Safety & Licensing Board Michoel 1. Miller, Esq. U.S. Nuclear Regulatory Commission Isham, Lincoln & Beale Washington, D.C. 20555 Three First National Plazo, Sist floor Dr. Frederick P. Cowan Chicago, Illinois 60602 Apt. B-125 6125 N. Verde Trail James E. Brunner, Esq. Boca Roton, Florido 33433 Consumers Power Company 212 West Michigan Avenue Jerry Horbour, Esq.

Jackson, Michigan 49201 Atomic Sofety and Licensing Board U.S. Nuclear Regulatory Commission Ms. Mary Sinclair Washington, D.C. 20555 57II Summerset Drive Midland, Michigan 48640 Mr. Ron Collen Michigan Public Service Commission Cherry & Flym 6545 Mercontile Way Suite 3700 P.O. Box 30221 Three First National Plazo Lonsing, Michigan 48909 Chicago, Illinois 60602 Mr. Poul Rau Ms. Lynne Bernobei Midland Dolly News Government Accountability Project 124 Mcdonald Street 1901 Q Street, NW Midland, Michigan 48640 Washington, D.C. 20009 TERA CORPORATION

ATT ACHmENT A. Pl.320l 001. REV 2 ENGlEERING EVALUATION COVER Shi uctual haluadon of de Diesel Genuatu BMg. CONT. lD. NO. 3201-001. 031 TITLE _ 2 NO. OF 5 HTS.

PRO.IECT : CONSUMERS POWER COMPANY ElDLAto IDCV .

SUPERSEDES ENG. EVAL. NO. DATE APPROVED BY DATE ORIGINATOR DATE REVIEWED BY REV.NO, REVISION 1/4/84 12/30/83 Jg 12/30/f 3,p O Original $4 U #'"

D-d "

TOPIC NUMBER _ l l i . 5-2.b 5$'.#dE ICivil/ Structural Design Considerations TOPIC TITLE W.ETHOD/ EXTENT OF REVIEW 1.

Review of Midland project generated information including calculations, consultant reports, tcstimony, etc.

2. Independent calculations and evaluations by IDCVP Review Team.

PURPOSE intended Evaluation of settlement induced cracking as it may potentially affect performance requirements and serviceability of the diesel generator building.

N CONTENTS (SEE SECTION 2., Pl 3201 00l)

O ABSTRACT E OVERVIEW OF REVIEW PROCESS E BASES FOR SAMPLE SELECTION E SOURCES OF INFORW.ATION/ REFERENCES E BACKGROUND DATA (ItPUTS, ASSUMPTIONS, RELATED CALCULATIONS)

O ACCEPTANCE CRITERIA (CODES, ST AtOARDS, FSAR NRC GUIDANCE, REGULATIONS)

O EVALUATION (DOCUMENT ATION OF REVIEW AGAINST CHECK LIST,(ACCEPT ANCE CRITERIA)

E CONCLUSIONS TERACORPORATION

f STRUCTURAL EVALUATION OF TE DIESEL GEERATOR BUILDING -

ASSESSMENT OF TE STRUCTURAL PERFORMANCE CAPABILITY APO SERVICEABILITY AS POTENTIALLY AFFECTED BY SETTLEMENT ltOUCED CRACKING TERA CORPORATION

TABLE OF CONTENTS PAGE ABSTRACT l-l l.0 2-1 2.0 OVERVIEW OF REVIEW PROCESS 3-1

3.0 BACKGROUND

DATA AND REFERENCES 4-1 4.0 ACCEPTANCE CRITERIA 5-1 S.O BASES FOR SAMPLE SELECTION 6-1 6.0 ENGINEERING EVALUATION Building Performance Requirements 6-1 6.1 Acceptance Criterio 6-2 6.2 6.2.1 Structural Primary Loadings e-3 6.2.2 Secondary Loadings - Settlement Effects 6-4 Evoluotion of DGB Performance Capability 6-8 6.3 6.3.1 Avalloble Data G8 6.3.2 Midland Project Evoluotions 6-9 6.3.2.1 Evol. of DGB Based on Cracking 6-10 6.3.2.2 Evol. of DGB Based on Settlement 6-10 6-14 6.3.3 IDCVP Evoluotions Building Inspection 6-14 6.3.3.1 Settlement Dato 6-14 6.3.3.2 Gross Stress Estimation 6-17 6.3.3.3 6.3.4 IDCVP Assessment / Interpretation of Results 6-18 Serviceability Future Copobility and Monitoring 6-l?

6.4 6.4.1 Midiond Project Evoluotion and Commitment 6-19 6-21 6.4.2 IDCVP Arsessment 7-l

7.0 CONCLUSION

S i

TERA CORPORATION

l.0 ABSTRACT An engineering evoluotion has been completed to ossess the structural performance copobility and serviceability of the Midland plant diesel generator The building (DGB) as potentially offected by settlement induced cracking.

evoluotion was initiated by TERA Corporation as part of the Midland independent Design and Construction Verification Program (IDCVP). The performance requirements for the DGB were identified and the acceptance criteria for meeting these requirements were reviewed. Information generated by the Midland project as well as independent calculations and evoluotions by the IDCVP review team serve os input to the conclusions of the engineering evoluotion, it was concluded that the existing cracks, generally being of small size, are not indicative of a condition that would compromise the copobility of the DGB in meeting its intended performance requirements.

Furthermore, it was judged that significant future cracking is unonticipated and the DGB is expected to remain serviceable without further remedial action at this time. Consumers Power Company (CPC) commitments to verify continued serviceability were reviewed and found to be acceptoble. Certain recommendations have been offered for consideration that are intended to improve available information and reduce operational constroints.

1-1 TERA CORPORATION

2.0 OVERVIEW OF REVIEW PROCESS This engineering evoluotion was undertaken as part of a brooder ossessment of the quality of the design and constructed product of the Midland plant Standby Electric Power (SEP) system. The specific scope of review documented herein includes a structural evoluotion of the diesel generator building (DGB), the structure which houses four emergency diesel generators which are principal components of the SEP system. The main emphasis of the review is on the civil / structural design considerations for the DGB ond how se:tlement induced cracking may potentially offect the intended performance requirements.

Accordingly, this evoluotion oddresses the following topics within the Midland IDCVP:

e Topic ll1.5 Civil / Structural Design Considerations e Topic lil.6 Foundations, and e Topic 111.7 Concrete / Steel Design; therefore, representing partial fulfillment of the structural design review scope pertaining to SEP system. This evoluotion has required input from other ongoing topic reviews such as:

e Topic 111.1 Seismic Design / Input to Equipment, and e Topic 111.2 - 2 - Wind and Tornado Design / Missile Protection; however, these evoluotions are documented under separate covers. The DGB construction / installation documentation reviews and the associated physical verification have not been completed and are not documented in this evoluotion.

Accordingly, should the results of these evoluotions offect the conclusions drawn herein, the engineering evoluotion will be oppropriately revised.

The review concept includes o determination of the DGB performance requirements and important design inputs (i.e. engineering dato and assumptions);

on evoluotion of their occuracy, consistency, and adequacy; and on evoluotion of 2-1 TERA CORPORATION

Current licensing criteria are the implementation of these commitments.

utilized as a baseline as well as consideration of various other regulatory criteria which evolved during the licensing process. Given the unique circumstances associated with the DGB design and construction processes, the IDCVP assessment used the intent of today's licensing criteria and corresponding margins of safety and reliability.

The review draws upon two principal sources of information; that generated by the Midland project (e.g. Bechtel calculations, consultant reports, testimony, etc.) and by the IDCVP review team (e.g. Independent calculations and evoluotions, etc.). Pertinent background dato and references are documerned in Section 3.0. Conclusions are reached through on integrated ossessment of these data, discussions with Midland project personnel, os well as engineering Judgement.

The following individuals made technical contributions to this engineering evoluotion:

Structural Reviewer, Midfond IDCVP and Senior Dr. Jormo Arros -

Structural Engineer, TERA Corporation Member Senior Review Team, Midland IDCVP Dr. William J. Hall -

and Professor of Civil Engineering, University of Illinois Professor Myle J. Holley - Consultant, Midland IDCVP, Professor of Civil Engineering Emeritus, Massachusetts institute of Technology and President, Hansen, Holley and Biggs, Inc.

Mr. Howard Levin - Project Monoger, Midland IDCVP and Manager, Engineering, TERA Corporation Dr. Christion Mortgot - Leod Technical Reviewer, Standby Electric Power System Structural Review, Midiond IDCVP and Principal Structural Engineer, TERA Corporation 2-2 TERA CORPORATION

The following chronology of external interactions transpired as part of this review.

Activity Date August 24,1983 Review team members observe NRC task force meeting on structural rereview of DGB of Bechtel's Ann Arbor, Michigan offices.

November 17,1983 Review team members inspect diesel-generator building.

November 18,1983 Review team members discuss civil / structural design considerations for the DGB ond collect information at Bechtel's Ann Arbor, Michigan offices.

December 12-16, 19 83 Review team members review DGB finite element and seismic stick models at Bechtel's Ann Arbor, Michigan offices.

2-3 TERA CORPORATION

3.0 BACKGROUbD DATA Ato REFERENCES The following table identifies references and sources of information that were selected for review and served as input to this engineering evoluotion. The numbers in the left margin correspond to references made within the body of the engineering evoluotion.

3-1 TERA CORPORATION

ATT ACt#ALNT B.Pl.3201001.MV 2.

REFERENCES / SOURCES OF INFORMATION ie.7-2 I TOPIC NO. I I I . 5-2. I I I .6-2 PAGE OF 3 TOPIC TITLE C1vi1/ Structural Desion Considerations. Foundations. O 12/30/8; Concrete /Strpctural SteelEval . of the Diesel Generator Bl d<rONT. lD.NO.3201-001-031 REV DATE atructural ENGINEERING EVALUAllON DOCUMENT WHERE/HOW IDENTIFICATION / DATL TITLE LOCATED TYPE ORIGINATING ORG./ REV.

AUTHOR NUMBER

. File 0485 16/Bl: 3 48 5/83 Final Safety Analysis Report Ann Arbor FSAR

1. Bechtel Serial 22423 Report on the Review of the Diesel 10/21 Generator Building - Midland Docket Report 50-329/330 0

2. NRC 83 Ann Arbor, 11/18/8 3 8/24/ Midland Units 1 and 2 Diesel Gen. Bldg. Exec. Summary heeting 0 ga 3 Bechtel --

Testimony testimony at pp 9/8/ TgbtimonyofKarlWiednerforthe M: land Plant Diesel Gen. Bldg.

Docket

4. Wiedner 10804-11007 0 82 File 0485.16,l 6/1/ Technical Report-Structural CPC, Jackson Report CPC B3 0.3, Seria 0 82 Stresses Induced by Differential 5

17228 Settlement of the Diesel Generator B1da.

Docket Report 3 9/79 hyIf nse to NRC regarding Mant

6. CPC

" '"9 * "9"I'***" Library Standard 7 ACI ACI 318-77 Re!'forcedConcrete in cgggrSafet, Library Standard

8. ACI ACI 349-76 kSYat$8NnENN'S Project 7/15/ Engineering Program Plan IDCVP Proj. Files instruction 9 TERA PI-3201-009 3 83 Transcript at

-- 0 2/11/ Eval. of the Effect on Struc6 ural Strength of Cracks in the Walls of 10950 Report

10. Sozen 82 the Diesel Generator Building Transcript at Rgon Testimony at 12/6/ Testimony of Ralph Peck 10180

11. Peck p. 10180 0 82 Transcript at 4/19/ abii ty o 11204 Report

12. Corley, et. al. -- 0 82 EffStsofk'racksonServiM501and tructures at Plant R1 Tech. Spec. 16.3/4.13 Settlement Ann Arbor FSAR FSAR Ch. 16 45 9/82 Partial /

s 13 O

CPC Monttorino DGB Areas for Crack Width Monitor- Ann Arbor,II/18/83 Corres.

O Exhibit 29R 0 -

ing During Operation of the Plant g 14. CPC Meeting Diesel Gen. Bldg. Reanalysis Uring Inn Arbor Calc 15 Bechtel 0 % !.0(Q) 2 l/9/

3 Revi sed Set t l ement Load Cam Z

AT T ACHMENT B. Pl.3701-001. HEV 2.

REFERENCES / SOURCES OF INFORMATION 2

TOPIC NO. II IIi 5I.7-2111.6-2 PAf,E 2 OF 3 TOPIC TITLE Civil / Structural Desian Considerations. Foundations. 0 12/30/8:

REV DATE ENGINEERING ENLbNTY NbN abkN1. of the Di esel Genera t or Bl do -CONT. lD. NO.3201 -001 -031 ORIGl ORG./ IDEN F TlON/ REV. DATE TITLE Nf

~

8/2/ DGB Settlement Analysis - Load Calc

16. Bechtel DQ-52. l (Q) 1 82 Case 1A Ann Arbor BSy8tlement Analysis - Load Ann Arbor Calc 17 Bechtel DQ-52.2(Q) O hl2s e 28' Ann Arbor Calc

18. Bechtel DQ-52.3(Q) 1 DGB Surcharge Condition (2A) g28' DGB Settlement for 40 yr Life (28) Ann Arbor Calc Bechtel DQ-52.4 (Q) 0 19 DGB Analysis for Uniform Torsion Ann Arbor Calc

20. Bechtel DQ-52.6 (Q) I h7/

Anal. Imposing 40 yr displace- Ann Arbor Calc

21. Bechtel DQ-52.7(Q) I h7/

15/ B ags ding Ann Arbor Calc

22. Bechtel DQ-12(Q) 1 8;/18 Optcon ACI-349 - Nonseismic Load Ann Arbor Calc Bechtel DQ-52.0-C7(Q) 0 82 Cases 7 Diesel Gen. Bldg.

23 Settlement Analysis (partial)

DGB Load Combination (partial) Ann Arbor Calc

24. Bechtel DQ-52.0-C2(Q) 0 5/12; DGB Settlement Analysis - Load Calc DQ-52.2-C5(Q) 0 82 Case IB - Fre Body Analysis Ann Arbor 25 Bechtel of Trial #3 (partial) 9/28/ DGB - Settlement Case 2A - Free Calc

26. Bechtel DQ-52.3-C7(Q) 0 Body Analysis on Best Fit (Sur- Ann Arbor 83 charge) (Partial) 5/12/ DGB Analysis - Free Body Analysis Ann Arbor Calc

4 27 Bechtel DQ-52.4-C4 (Q) 0 82 of Best Fit 40-Year Case Ann Arbor Calc

28. Bechtel DQ-23-C4(Q) 0 h/II' DGB Roller Support (FSAR Criteria) 11/11 Static f, Dynamic Spring Constant Calc O 29 Bechtel S-110 1 of DGB for Structural Stress Anal. Ann Arbor 82 2/22/ Update of Settlement Prediction Ann Arbor Calc

( l30 . Bechtel S-175 3 82 DGB - After Surcharoe Removal DGB Between 9/14/79 O 31. Bechtel S-238 0 gl5/ gtggj Ann Arbor Calc L_______ ..

ATT ACHnAENT B, Pt.3301.001, EV 2.

REFERENCES / SOURCES OF lif0RMATION Civil / Structural Design Considerations, Foundations, TOPIC NO.

PAGE 3 OF 3 TOPIC TITLE 0 12/30/8:

Concrete /5tructural Steel 3201-001-031 REV DATE ENGINEERING EVALUATION % t rur t i,ral Fva1. nf tha Diece1 Coneeatnr R 1 d ar'ONT. ID. NO.

WHERE/HOW DOCtWENT ORIGINATING ORG./ IDENTIFICATION / REV. DATE TITLE LOCA1ED TYPE AUTHOR NUMBER I 2/ c Analysis of DGB and DG

32. Bechtel SQ-147(Q) h3 pg f Settlement 33 Bechtel DQ-52.ll(Q) 0

$9/ $8j (gggn Ann Arbor Drawing Settlement Data for DGB

34. Bechtel SK-C-2343-1/24 F h/5 Att. 2 to Testimon)

File 0485.16 8/2/ Midland Concrete Wall Repair of Corley @ p.ll206 Letter 35* CPC 0 82 Program Serial 18371 Trip Report - Midland DGB Struc- Report 0 $/29/ tural nesign Audit Docket 36- NRC 50-329/330 1 Properties of Concrete Library Text Book 37 Neville' IN h

• 7$[lho8f **' 1 1975 Structurd Analysis of Diesel FgVPProj.

es

38. TERA 3201-003-007 0 kf30/ Generatne- c. i i t a ,. n n 8/12/ Response to NRC Question 26 Re: Ann Arbor Calc 39 Bechtel DQ-14 (Q) 1 83 Diesel Generator Buildino

40. Bechtel DQ-23(Q) 1 8f is Db!ES u

!$tehtyupgtof Ann Arbor Calc 1

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l 4.0 ACCEPTANCE CRITERIA t

4.1 LOAD COMBINATION 5

> / '

The loods and Icod combinations employed for the original design and analysis

}

were provided in ty.e FSAR iutisection 3.8.6.3 (revision 0, dated November 1977).

' > These original design critetto did not contain settlement ef fects. Four additional r icoding combinations were established 'and committed for consideration as a result 6f Questior; 15 of'the NRC Requests Regarding Plant Fill of September

1. 1979. These fooding combinations comb il r ed differential settlement w ti h long-term operating loods and either wind or the operating basis earthquake (ObE).

As '#iedner (reference 4) and CPC (reference 5) point out these expressions are more stringent than the requirements of ACi 318 0 eference 7), but leis stringent than ACI 349 (reference 8). In the lotted cose the fooding combinations combine f? differentia! settlement with extreme loods such as tornadoes and the scfe 3

.Ir shutdown earthquake (SSE). Subsequently, in response to Question 26 of the NRC Requests Regard:ng Plant Fill, o commitment was made to underteke o separate structural recnslysis o'f ihe DGB in accordance with ACl-349 as supplementec' by I NRC Reguhtory} Guide I.I'42 for cccnparison purpose only.

<t The following (dads were considered in the reonalysis

(a) deod loods (D) j #

(b) effects of setf(ement combined with creep, shrinkoge and temperature (T)'

(c) live foods (L)

(d) wind foods (W)

(e) tornado foods (W')

(f) OBE loods (E) ,

(g) SSE loods (E') . .

(h) thermal effect.i(Top I

i 3 1 4-t

' TERA CORPORATION

It is to be noted that thermal effects appear twice by virtue of the manner in which the loading combinations were developed. The load combination established and committed to in response to NRC Requests Regarding Plant Fill, Question 15 are os follows:

a. l.05 D + l.28 L + 1.05 T

b. l.4 D + 1.4 T

c. l.0 D + 1.0 L + 1.0 W + 1.0 T

d. 1.0 D + 1.0 L + 1.0 E + 1.0 T A number of lood cases appearing in the load combinations for Seismic Category I structures listed in FSAR Subsection 3.8.6.3 do not occur in the diesel generator building cnd other food combinations con be eliminated from the analysis af ter comparison with more severe loads or lood equations (reference 5).

As a result the remaining load combinations to be considered are:

e. l.4 D + l.7 L

f. l.25 (D + L + W) + 1.0 To

g. 1.4 (D + L + E) + 1.0 To

h. 0.9 D + l.25 E + 1.0 To I. l.0 (D + L + E') + 1.0 To J. l.0 (D + L + W') + 1.0 To 4.2 ALLOWABLE MATERIAL LIMITS In occordance with regulatory requirements, the maximum rebor tensile stress allowed in the diesel generator building rebor should not exceed 0.90 yf (where fy equals yleid strength) for computation of section capacities. Because the diesel

generator building rebar hos on fy value of 60 ksi, the maximum allowable tensile rebor stress due to flexural and oxial !oods is 54.0 ksi. Accordingly, reinforced concrete section capacities for the diesel generator building were based on this 4-2 TERA CORPORATION

l maximum allowable rebor stress volve (54 ksi), o design concrete compressive strength of 4000 psi and a maximum allowable concrete compressive strain level of 0.003 in./in.

s 1

C 4-3 TERA CORPORAT'ON

' 5.0 BASES FOR SAMPLE SELECTION The diesel generator building (DGB) was selected for review because it serves on important support function in providing protection against external hozords for the diesel generators which are integral components of the Standby Electric Power (SEP) System. The DGB falls within the sample selection boundaries defined in the Engineering Program Plan (reference 9). Commitments were made in this reference to review civil / structural design considerations for the DGB including foundations and concrete / steel design. Based on programmatic commitments, emphasis is to be placed on structural performance and not detailed soll mechanics aspects which are not within the scope of the Midland Independent Design and Construction Verification Program (IDCVP).

This engineering evoluotion addresses the potential effects of settlement induced cracking on the ability of the DGB to meet its intended performance requirements. Accordingly, verification of the Midiond project treatment of the settlement / cracking issues which have offected several structures of the Midland site is addressed herein. While a structural review of the auxiliary building is also within the IDCVP scope os part of the Auxiliary Feedwater (AFW) system review, the specific settlement / cracking issue os it may offect the auxiliary building is not being treated directly by the IDCVP. Thus, this evoluotion of the DGB represents the IDCVP sompte addressing the settlement / cracking issues.

It is es!!moted that opproximately one third of the project's calculations cnd evoluotions addressing the structural design of the DGB were selected for review. Emphcsis was placed on the selection of portions of the project's evoluotions that address controlling design conditions (e.g. important locd cambinations producing the highest predicted stresses or strains, as oppropriate).

Principal project consultant reports were reviewed as well as other docketed information that documents CPC commitments to the NRC (see section 3.0).

5-1 e

TERA CORPORATION

6.0 ENGitEERitO EVALUATION 6.1 BUILDING PERFORMANCE REQUIREMENTS The diesel generator building (DGB)is a two story reinforced concrete box type building partitioned into four boys, each boy containing one diesel powered electric generator (see Figure 6-l). The purpose of the diesel generators is to supply standby electrical power to operate the Midland plant during power outoges and to provide the necessary power to ensure safe shutdown of the plant in the event of a design basis event. Accordingly, the diesel generators and the DGB are classified as Seismic Category 1, and as a result must maintain functionability during external events such as earthquakes and tornadoes.

The DGB provides protection for the diesel generators and associated supply ono service lines, instruments and equipment, assuring ready ovallobility of this supplementary power source. This protective function includes not only the normal sheltering of building contents from rain, snow, wind, and ice, but in addition, resistance to the effects of earthquakes and tornadoes including tornado generated missiles. It is these latter effects which are of principal structural interest, and which dictate a more massive type of construction than normo!!y would be employed for shelter from the commonly considered weather extremes.

The DGB was founded on plant fill and constructed between the Fall of 1977 and the Spring of 1979. During that period it was discovered ihot the building was experiencing on unusual rate of unequal settlement, and duct banks had made contact with the footings which led to building distortion and reinforced concrete cracking. The details of the settlement monitoring, duct structural modifications, and surcharge consolidation program are described in detail in references 3 and 5.

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6.2 ACCEPTANCE CRITERIA in response to applied loadings (dead, live, earthquake-induced, wind, tornado, tornado missiles) and certain secondary effects such as settlement, local internal forces are developed throughout the structure. These local forces consist of in-plane forces, sometimes termed membrane forces, and out-of-plane forces, i.e.,

transverse shear forces, and bending moments. In design it is customary for the internal forces associated with a particular loading to be multiplied by o specified "lood factor" and these food factored sets must be combined for the several specified loadings to obtain what may be colled a local internal demand.

This demand must not exceed the local " strength", i.e., capacity of the structure.

The acceptance criterio consists of the following:

e Statements of the several different lood combinations that must be satisfied, and the load factors to be opplied to each of the loadings (dead, live, tornado, etc.) within that combination.

e Specific expressions, or procedures, for determining the local strength which must not be exceeded.

It may be noted that certain of the specified load combinations focus on serviceability of the structure. These do not include the infrequent extreme loadings, but incorporate relatively large load factors to assure o modest demand /copacity ratio for (unfoctored) loadings experienced in normal operating conditions. For the combinations which incluce extreme and rare loadings, safety in the sense of protecting personnel and equipment, yet retaining functionability, is the primary consideration rather than serviceability. Thus crack widths, including those widths which may reflect yielding of the tension rebars, are not a consideration provided that they do not imply o reduction in the local strengths. Accordingly, such specified factored load combinations typically incorporate smaller specified food factors. In effect a larger demand /copocity ratio for these unfactored load combinations is occeptable for these rare conditions.

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1 It should be noted that the specified expressions, or procedures, for determining t',e local internal strength do not typically include any direct limitation on rebor tensile strain, or on crack widths which accompany such stroin, although there are indirect limitations for certain conditions. (Note that the limiting condition specified by various ACI codes (references 7 and 8) are related to maximum allowable concrete compressive strains where o value of 0.003 in./in. is specified). This strain reflects the fact that certain components of local strength are not sensitive to rebor strain but only to the tensile yield strength of the rebars. As on example, full development of the local out-of-plane bending strength of a slob, or beam, with a modest rebor ratio may imply tensile rebor strain into the yield range. Indeed this is specifically recognized by codes which specify that, for rebor strains in excess of the elastic strain at yield stress the stress must be assumed to be constant at the yield stress value. This approach often is overlooked because, for the majority of local conditions of interest it is computationally much more convenient to evoluote local sections on the assumption that the steel strains remain within the elastic rcnge, and to compute rebor stresses associated with the porticular factored load combination demand rather than to compute the local section strength, per se. In some cases this approach is slightly conservative, but often there is no difference whatever.

However, the fact that there are circumstances, where small tensile rebor strains into the yield range occur, yet are acceptable, and do not degrade the required local strength, may be unrecognized because of the focus on elastic behavior inherent in the computation process. Margins of strength, os reflected in codes, are implicitly based on the ductile behavior of structural systems os just noted.

6.2.1 Structural Primary Loadings The DGB must resist the folicwing principal primary loadings:

e Gravity- Induced dead and live loads e Earthquake- induced loods e Tornado- induced differential air pressure e Tornado- borne missiles 6-3 TERA CORPORATION

Gravity- Induced loads produce out-of-plane shear forces and bending moments in the floor and roof systems and in portions of the walls immediately adjacent thereto. These loads also produce in-plane forces in the walls and, of course, bending moments and shear forces in the strip footings.

Earthquake- induced loads produce in-plane forces in the walls which are substantial, and more modest in-plane forces in floor and roof sicbs. They also produce out-of-plane shear forces in floor and roof slabs and walls.

Tornadic winds produce in-plane and out-of-plane forces in walls and roofs.

Tornado- induced differential air pressures are the principal source of out-of-plane shear forces and bending moments in floor systems and walls, and they also produce in-plane forces.

Tornado- borne missiles produce highly localized out-of-plane loading of the walls. The capacity of the wall to resist such missiles is evoluoted independently of all other loadings.

6.2.2 Secondary Loadings Restrained non-lood-induced volume changes (e,g., due to concrete shrinkage and or temperature strains) may produce internal forces. It has long been recognized that these forces rarely have any significant effect on the local strengths, and in most cases they are neglected. The reasons relate directly to the ductility of the tension rebars. If the local strength is mobilized, by on imposed set of local demor.d forces,it typically will be the some whether or not the forces ossociated with the non-lood induced effects are included. The difference will be that the tensile rebor strain, including some yield strain, will be larger when these secondary forces are included. This yielding has the effect of decreasing, ond sometimes completely eliminating, the local forces which were initicly introduced by the non-load effect. It is for this reason that the forces associated with such non-load induced effects often are termed "self-relieving" or secondary.

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in the design of most reinforced concrete buildings the local in9ernal forces crising from restrained shrinkoge and thermal strains os well as that induced by settlement are not included in the application of the strength criterio. In the design of nuclear safety related concrete structures it is the accepted practice to account for through-the-woll thermal gradients, although shrinkage ef fects are not typically included. Even accounting for the thermal gradients is a conservative requirement the justification for which is at least debotable.

However they were accounted for in the DGB design as required by the acceptance criterio. It may be noted that underlying codes, from which the acceptance criteria were developed, typically colled for inclusion of these non-lood-Induced forces with the lood-induced forces only where their structural effects may be significant. In the cose of the DGB it may reasonably be debated whether such effects are indeed "significant", as envisioned by the code.

In the initial design of the DGB it would not reasonbly have been assumed that the forces ossociated with foundation settlement could be significant nor, that they should be included with the lood-induced forces in the factored load combinations. Clearly, the building was designed for continuous support on what was intended to be o relatively hornogeneous soil medium. Thus the designer could justifiably assume that there would be little if any redistribution of the upward soll reactions on the strip footings due to major point-to-point variations in local stiffness of the supporting medium. When the building was only partly completed it become evident that such stiffness variations did, in fact, exist i.e.,

a very stiff support at the iocation of footiig contact with ducts, together with poorly consolidated soil (low in stiffness, and non-uniform) elsewhere. These conditions caused on extreme example of non-uniform settlement which did indeed induce internal forces sufficient to cause cracks in the walls of the then portially completed Structure.

Upon noting that the settlement had led to interference between the foundation and buried ducts, the unintended footing-to-duct connections were physically disengaged and the unsatisfactory foundation condition was corrected by a surcharge loading procedure. It is to be noted (reference 36) that the surcharge loading procedure began on January 26, 1979, incrementally, and that 6-5 TERA CORPORATION

1 construction of the DGB continued thereafter. The final surcharge placement took place between March 22,1979 and April 7,1979, just as the roof and paropet construction was completed. The subsequently completed DGB structure has been in place, in its completed condition for more than four years with no indications of odditional distress in any way comparable to that associated with the footing-to-duct contact and the poorly consolidated soil. It may be argued that the structure now is supported as was intended at the time of aesign, that the effects of any future differential settlement will not be significant, and that the effects of such cracking as developed in the partially completed structure also are not significant to local internal strengths relied upon to resist the forces associated with opplied load combinations. From all this it would naturally follow that the internal forces induced by differential settlements need not necessarily be included with the lood-induced forces in the combinations specified by the acceptance criteria. These arguments may be justified but, in fact, there is a licensing commitment to include the settlement-induced forces in the relevant load combinations.

Since the internal forces induced by a specific non-uniform settlement are self-relieving (as was described earlier, for thermolly induced forces), why must they be included; i.e., when may their effects be "significant". In some structures the magnitude of possible future settlement may be uncertain, and there may be little or no prospect for monitoring of the settlement or the state of the structure during its service life. Accordingly, inclusion of settlement-induced forces in the design would be oppropriate to limit the possible develcpment of structural distress which would be costly to repair, or which in some special cases, like o containment structure, may offect functionability by creation of large liner strains. For other structures these forces might prudently be included to avoid excessive yield strains in the tension rebars (and the ossociated large crack widths) which might degrade the local internal strength under some set of 1

the local internal forces ossociated with applied foods, particularly if no monitoring of the structure for such effects could be anticipated.

For the DGB structure the principal structural elements are relatively accessible, and a monitoring program is planned. Nevertheless it is required to 6-6 TERA CORPORATION

demonstrate by application of the relevant acceptance criterio, including the effects of differential settlement, that the local internal strengths are not presently degraded and are unlikely to be degraded by any probcble future differential settlements. The acceptance criterio do not include any specification of the method by which the associated internal forces are to be determined. This is on important consideration in any effort to apply the acceptance criterio. There are essentialy three alternatives:

a) One may assume o magnitude and distribution of differential settlement and impose this displacement pattern upon the structure, in contrast to the situation of the design stage the onolyst for the DGB has settlement measurements to consider in arriving at the postulated differential settlements to be used.

b) One may postulate one or more perturbations of the distribution of upward soil reactions associated with dead load which may be associated with differential settlement, and determine the local internal forces for each. It will be apparent that this approach produces the forces due to dead loads plus differential settlement.

This is not on unreasonable opproach, if sufficient attention is given to parametric variations, particularly if the analyst lacks data on differential settlement which he considers sufficiently precise to use directly in method (a).

c) One may postulate the local internal forces directly from the observed condition of on (existing) structure; i.e., the crack widths in the DGB. This is on option clearly not ovalloble at the time of design.

The method of imposed differential settlements may lead to unrealisticoliy large internal forces unless the analysis con account for cracking, and time-dependent concrete properties. The cost-benefit of such on analysis may not be justified, particularly if other suitable options (b or c) exist.

The method of analyzing the dead load condition for several pc bloted

]

distributions of soil reaction is suitable, but it may be difficult to choose sets of distributiorts whicts cover the possible differential settlements but which are not unjustifiably extreme.

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For the DGB, which hos been observed in its completed state for more than four years, inference of the internal local forces from the condition of the existing structure (c) seems to be the most attractive approach. It is the most direct. It is particularly attractive since any significant changes in the condition of the structure will be observable during its service life. Observations related to this approach follow.

i 6.3 EVALUATION OF BUILDING PERFORMANCE CAPABILITY The performance copobility of the structure is to be assessed in two steps: the first one considerlag the building in its present state and the other addressing its structural integrity and serviceability over the next 40 years. Inputs to the evoluotion are keyed to a number of elements such as: ovoilable physical dato, analytical studies, understanding of concrete behavior and engineering judgement.

6.3.1 Availcble Do;o The most important dato available to estimate the present state of stress in the DGB consists of:

1. Observations of the building as it exists today.

2. The record of the crack monitoring program.

3. The settlement history of the building.

The cracks have been surveyed on several occasions (Reference 3). The maximum crack width recorded during the monitoring program prior to isolation of the duct banks was 28 mils. After the isolation of the duct banks, the cracks decreased in size (testimony Peck and Weidner references 11 and 4 respectively) implying a stress decrease in the higher stressed creas. Presently the largest cracks are of the order of 20 mils. An evoluotion of the existing cracks has been performed by two Sechtel consultants, Dr. Mete Sozen (reference 10) of the University of Illinois and Dr. W. Gene Corley (reference 12) of the Portiond Cement Association.

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I The building settlements have been monitored at close intervals during the construction period and thereafter. Figure 6-2 presents the location of the settlement markers indicating where survey measurements are taken. The dato spans over o period of 5 years with measurements token opproximately every other week. This large amount of dato allows one to follow the settlement history through the stages of construction, duct bank isolation, surcharge period, dewatering, and up through the present. It also provides o means of assessing The Midland potential random and systematic errors in the measurements.

project has concluded that significant errors exist in the measurements due to o variety of circumstances. A study of these data is presented in the fe!!owing section.

6.3.2 Midland Project Evoluctions The Midland project followed two separate approaches to estimate the state of stress in the building:

e study of the cracking history e study of the settlement history.

The future state of stres- due to settlement was estimated based upon prediend settlements.

6.3.2.1 Evoluotion of DGB Based on Observed Cracking in its present condition the DGB has cracks which appear to be settlement-induced or settlement-intensified, generally crising during the early construction phases. Maximum present crack widths are reported to be about 20 mils, and Dr.

Sozen (reference 10) has shown that the associated rebor stress as estimated in c region of numerous. cracks, adjacent to o duct bank penetration of the center wall, may be judged to be between 20 and 30 ksi. We find his evolvat!on to be reasonobie incorporating techniques that are state of the ort, widely accepted and supported by laborotory tests. Dr. Sozen also has argued that the presence of initial cracks does not degrade the capacity of a reinforced concrete element 6-9 lERA CORPORATION

in any of the important structural modes; i.e., direct tension force, direct compression force, in-plane shear force, and out-of plane bending. Again, we agree with Dr. Sozen that precracks of the width thus for evidenced in the walls of the Midlo'nd DGB would not significantly degrade capacities in the several modes developed by the principal loadings, and in their required factored combinations.

Dr. Sozen did not specifically address the possible influence of on initial rebor stress which is associated with a self-relieving internal force, that is, o force caused by foundation settlement. He does not indicate his opinion whether or not the self-relieving internal force implied by the initici rebor stress should be included with the internal forces due to applied loadings or con be neglected because it is self-relieving, it is our understanding that the Bechtel evoluotions of the DGB for the effects of dead load plus foundation settlement did not utilize the initial rebor stress magnitude estimated by Dr. Sozen but rather d

computed it based on the settlement history of the building.

6.3.2.2 Evoluotion of DGB Based on Settlement History The settlement effects were modeled by Bechtel into the structure considering four distinct time periods. Measured or estimated settlement values corresponding to each of the time periods were und:

o Case IA: 3/28/78 to 8/15/78 (Structure partially completed to elevation 656.5') - A long hond calculation was used to determine the stresses due to early settlements. The structure was assumed fully cracked and the stresses in the reinforcing steel were ossessed based upon local strains corresponding to on imposed differential settlement (reference i6).

e Case IB: 8/15/78 to 1/5/79 (Structure portially completed to elevation 662.'O.)- The duct banks were seperated from the structure which caused the north wall to settle rapidly. (reference 17) 6-10 TERA CORPORATION

e Case 2A: 1/5/79 to 8/3/79 (Structure in process of completion.)-

Surcharge period. (reference 18) e Case 2B: Forty year settlement composed of:

o measured settlements from 8/3/79 to 12/31/81, and e predicted secondary consolidation settlement from 12/31/81to 12/31/2025. (reference 19)

The lost three analyses used a finite element model having stiffness corresponding to on uncracked condition, lo these analyses the foundation stiffnesses have been varied, in on iterative process, to achieve final settlements approximating a set of target settlements. These target settlements were based upon a linear best fit through the measured settlement dato. The analyses have been criticized (reference 2) because the analytically predicted settlements do not match variations in the measured settlements. It is appropriate to ask whether the iterated non-linear foundation stiffnesses are realistic since the target settlements were not the measured settlements but a linear best fit, essentially assuming rigid motion of flie North and South walls. The best fit data were utilized in on attempt to deal with scatter in the measured data. Such scatter potentially due to either random or systematic errors was estimated to be of the order of plus or minus 0.125 inches.

In our opinion the described method of occounting for foundation stiffnesses utilizing the linear best fit dato may not be satisfactory for correlation with observed cracking in relation to differential settlement. We concur that settlement measurements may not be of sufficient occuracy to permit a precision computatien of set!!ement-induced internal forces. Furthermore, the marker locations are spaced at wider intervals than would be desirable os input to analyses of building stroins. Nevertheless, the general level of stress implied by the magnitude of cracking is not in contradiction to that which may be derived from the measured settlement dato, realistically accounting for flexibility including consideration of phenomeno such as creep (see section 6-11 277"

6.3.3 for o more detailed discussion). As discussed in Section 6.2.2, on exact determination of secondary stress levels is of lesser importance given the nature of the loading and the fact that copocity is not adversely offected, in separate sensitivity studies Bechte engineers considered among others, the two following cases:

e The zero spring condition analysis (reference 3) which investigated the structure's ability to span any soft soil condition. A zero soil spring value was used at the junction of the south woil and east linearly boek to their center wall. Soit values were increased original value within a distance of approximately 15 feet from the zero spring. The stresses in the building underwent moderate increase in the area of the bridging. In our judgement this is a reasonable opproach, but one may ask whether the size and locations of such postulated " soft" zones were bounding.

e The imposed 40 year settlement analysis (reference 21) which forced the building to match the predicted settlement values of 10 points along the foundation. This analysis led to very large reaction forces at the points of imposed settlements, and some of these octed downward on the structure, i.e., implying tensions in the soil, which is not possible. Moreover, the analysis indicated very large rebor tensile stresses, where at several points a multiple of the yield strength was indicated. Of course the structure does not display the For very wide cracks which would accompany such high stresses.

these reasons Bechtel engineers concluded that the settlement measurements cannot be on occurate representation of the octual settlement nonuniformities.

We have noted that the settlement dato may not be on adequate basis for However, we believe the described analysis computing settlement effects.

exaggerates the effects of the displacement input data which was questioned by the project. Our reasons are that the analysis assumed uncracked concrete and 6-12 TERA CORPORATION

used the short-term concrete modulus of elasticity. Appropriate reduction of the concrete modulus, to reflect creep under sustained loading, would have lead to reactions and internal forces perhaps 50 percent less than were obtained.

Decreases in stiffness associated with concrete cracking could result in additional large reductions. An excellent discussion of the physical and engineering significance of creep is found in chapter 6 of reference 37.

Perhaps more important, rebor stresses appear to have been computed on the assumption that the local internal tensile forces developed in the uncracked concrete are unreduced by cracking, i.e., this unreduced force is imposed on the rebars. In our judgment this is not the best physical representation. The rebor stresses are expected to be more nearly indicated by the local strains in the concrete (uncracked) tFon by the forces in the concrete (uncracked). Thus, the rebor stresses are better opproximated by the product of steel modulus and concrete strain (uncrocked); i.e., by the product of modular ratio, n, (Youngs modulus of the steel / Youngs modulus of the concrete) or.d concrete stress.

fse n fc in contrast we believe that the following expression was used is 21fc P

where p is the reinforcement ratio (rebor crea/section crea). This later expression greatly overestimates rebor stress. To illustrate, for p = 0.0043 and n

= 8, the suggested opproach gives rebor stress about 1/30 of the Bechtel computed value. While reality is likely in between, and the former expression is approximate, we believe that it is a closer representation of the existing situation.

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6.3.3 IDCVP EVALUATIONS in addition to reviewing the information generated by the project and the studies performed by others, the IDCVP concentrated attention on two major elements in the review process:

e Observations of the building and its present state of cracking, and e The settlement history of the building.

- Settlement data

- Gross stress estimation 6.3.3.1 Building inspection A careful inspection of the building u , erformed together with a review of the cru e mapping dato. As it exists at present, many cracks of small size are evident in the building but there is no evidence to support that these cracks are indicative of a high state cf stress in the building and degraded capacity. Post experience and laboratory tests indicate that concrete elements in a state of distress -particularly stiff shear walls of the type in the DGB - exhibit large deformations and cracks, much greater than present in the DGB. This would probably be accompanied by scobbing and other phenomeno which are not apparent in the DGB.

Our conclusion from visuo! inspection of the building is that its state of stress is low and would not impair its performance and functionability. A body of relevant information developed in industry, university and government programs and structural experience supports this conclusion.

6.3.3.2 Settlement Data A study of the settlement data recorded between 11/24/78 and 8/28/80 is presented in reference 5. We reproduced and expanded this onalysis to include the most recent dato (reference 38). The two time periods covered were from 6-14

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5/12/78 to 9/14/79 (reference 33) cnd 9/!4/79 to 8/23/83 (reference 34). Our goal was two fold: (1) assess the overall deformation of the building with time and (2) estimate the rondom error pres,ent in any one set of measurements. We studied the following dato.

1. Cumulative settlement recorded over time.

2. Incremental settlement between successive rec dings.

3. A measure of the curvature between any three consecutive markers along the foundation as it varies with time. The curvature d"i of merker i is defined as:

d"i = 0.5 (di .l +d+l)-di i

where di is the total settlement.

The quantity d" equals zero when the three points are on a straight line; it remains constant in time if the three points move us o rigid body.

4. A measure of the deformation of the building with respect to its rigid body motion. The rigid body motion i.,

" removed" by computing the vertical position of all markers with rer.pect to the plane defined by three corner markers. This analysis was done both for each incremental reading cod cumulatively.

An upper limit of the random error in any set of readings is given by the maximum difference of incrementc' settlement between any two markers from one reading time to the next. When the building hos not experienced any settlement between twu readings, this quantity is the random error; it bounds it otherwise. At the beginning of the record, this quantity is large where the building was undergoing large differential settlements and reading occuracy might have been reduced by marker transfer necessitated by the placement of surcharge. However, this quantity decreases rapidly and ofter June 1979 is never greater than 0.150". After the removal of the surcharge for the readings starting 9/19/79 which we will refer to os the recent readings, the rondc+n error is smaller than 0.125", 95 percent of the time which would give o random error of about 1 1/lf of on inch. This implies that a higher level of confidence con be given to the recent measurements.

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Jumps in readings from one period it ' . next are sometimes lorge implying that the building would rapidly move up or down by a uniform omount. These jumps are attributed to systematic errors in locating the reference elevation.

Figure 6-3 shows the incremental settlement for 6 time periods between July 1978 and August 1979 for the south wall of the DGB. The first three measurements show large differential deformations and introduction of curvature in the wall. The latter ones show stabilization of differential settlements implying that the wall is still settling but as a rigid unit, introducing little additional in-plane bending. For more recent recordings the stabilizing trend is even more noticeable. Study of the foundation curvature variation and deformation of the building viith respect to its rigid body motion point toward the some trend. This is supported by on evoluotion discussed in reference 4, where it was noted that the settlements occurring during the time periods represented by lines e and d (reference 4, figure DGB-7), were those that cre expected of a rigid body, in figure DGB-7, line c represents settlement during the surcharging period (1/79 - 8/79) and line d represents estimated settlement during the post-surcharge period (9/79 - 12/2025). The point here is that the early cracking occurred when the building was only partially completed. Upon completion, the five sided (four walls and a roof) structure is now responding as a stiffer, essentially rigid body as would be expected.

Hence during its construction stoge, the building underwent substantial differential settlement that introduced in-plane curvature in the walls with As resulting stress and cracking compounded with normal shrinkoge crackir;g.

the building was completed and the concrete aged, its tended to behave more cr d more os a rigid unit, the whole foundation (or building) moving as a plane (or a unit). The recent dato indicates that for ine lost four years the building has generally settled as a rigid body introducing relatively little additional distortion in the structure. We expect this behavio- to persist in time.

One may speculate on the magnitude of the obsolute settlements over the service life; however, these are of lesser structural concern to the building itself, and would only offect clearance to obstructions and connected items.

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These latter elements con accommodate some degree of distortion and con be modified in the future if worronted.

6.3.3.3 Gross Stress Estimation Even though we have noted that settlement data may not provide on acceptable basis for computing settlement effects, it is our opinion that if credit had been taken to account for:

- creep and stress relaxation in young concrete,

- reduced stiffness associated with the gecmetry of the uncompleted structure

- stiffness reduction due to cracking the exact recorded settlement could have been imposed on the structure without generating stresses in gross contradiction to that observed vio crack patterns in the DGB. This woeld have qualitative value to on overall understanding of building behavior, in order to improve our understanding of building behovlor and to generally qualify the influence of these effects, we modeled the north and south walls of the building using a simplified finite element model (reference 38). As a first order check of our partial model, we reproduced the 40 year imposed settlement onalysis performed by Bechtel on the uncracked structure. We obtained stresses within 25 percent of Bechtel's which is reasonable considering the simplified model we used.

We imposed the recorded settlements on the incomplete wall for Case lA and IB l ond on the complete wall for Case 28. For crocked corerete, the stresses were computed es described in Section 6.3.2.2.

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The following approximate maximum values of stress were obtained:

LOADING STEEL (ksi)

CASE lA l 1.3 CASE IB 3.5 CASE 2A 4.6 This leads to a total stress of 19.4 ksi which is in good agreement with Dr.

Sozen's independent onelysis (see section 6.3.2.1 and reference 10).

We recognize that the above anotysis represents a simplified approximation of the very complicated effects of creep and cracking but it provides a qualitative estimate of the state of stress of the building.

We believe the results of our analyses, properly interpreted are both useful and positive, specifically.

e When modified for the effect of concrete creep and concrete cracking the foundation reactions when combined with reactions due to dead load, would not imply a physically impossible state of tension stress in the soil, e When the rebor tension stresses are properly determined, that is on the basis of strain in the uncracked concrete rather than on the basis of stress in the uncrocked than concrete, they are quite modest rather unrealistically lorge.

6.3.4 IDCVP Assessment / Interpretation of Results in our opinion the settlement-induced internal forces impliet by the ossociated rebor stresses, as they presently exist in the IAidland DGB wiil not degrade the capacities to resist the internal forces and moments caused by the factored food 6-18 TERA CORPORATION

1 combinations and therefore the DGB is expected to meet its intended performance requirements. There is reason to believe os supported by recent observations, that the completed building is settling as a rigid unit based upon the stabilized foundation properties. In this mode, the DGB capacity is not expected to be compromised over time. We believe that the settlement-induced, self-relieving, internal forces implied by the present crack widths and associated rebor stresses could safely be ignored in evoluoting the building. However, licensing criteria include certain load combinations in which it is specifically required to include the settlement-induced internal forces. Based upon our knowledge of ovailable margins associated with controlling load combinations, we believe that compliance with these criteria con generally be demonstrated, oppropriately accounting for creep, relaxation and other phenomeno; however, we do not endorse such on endeavor because of the secondary nature of the settlement induced loads and the fact that capacity is unaffected.

6.4 SERVICEABILITY, FUTURE CAPABILITY, AND MONITORING The previous sections address the significance of settlement induced cracking on the performance copobility of the DGB in its current condition. It is important that the DGB continue to meet specified performance requirements over its service life; hence, this section oddresses serviceability of the DGB and any actions that may be necessary to identify and mitigate potential future conditions which could compromise the DGB performance.

6.4.1 Midland Project Evoluotions and Commitments The effects of cracks on the serviceability of Midland plant structures were addressed in reference 12. Three principal issues were evoluoted:

e Freezing and thawing resistance, e Chemical attack, and e Corrosion of reinforcement 6-19 TERA CORPORATION

lt was concluded in reference 12 that observed cracks are not expected to have o significant influence on the durability of the DGB. Accordingly, remedial measures such as epoxy ir.jection were considered unnecessary to ensure long term performance capability. Nevertheless, CPC committed (reference 35) to repair existing cracks which are 20 mils and larger (up to o point in length where the crack remains 10 mils or larger) by epoxy injection and application of a concrete seolont to accessible surfaces.

A Technical Specification (TS) 16.3/4.13 (reference 13) has been proposed to monitor settlement over the service life of the DGB. The specification requires that the total settlement be measured (to nearest 0.01 foot) of least once every 90 days for the first year of operation. The frequency for subsequent years hos been left for future determination. The total allowable settlement corresponding to predictions for the service life (12/31/81 thru 12/31/2025) has been specified at 12 markers. Engineering evoluotions are required if total settlement reaches 80% of the allowable values (Alert Limit). Additionally, the inspection frequency is to be increased to once every 60 days if the 80% level has been reached, if the DGB exceeds total allowable settlements, the plant must initiate actions to be in cold shutdown within 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br /> (Action Limit).

CPC has also committed to conduct a crack width monitoring program (reference 14) which includes individual crock width and cumulative crock width measurements at 3 locations over o 10 foot gage length. This program will be conducted once every year for the first five years of operation and at five year intervals thereafter. The following criteria apply:

Alert Limit Action Limit single crack 50 mils 60 mils cumulative cracks 150 mils 200 mills (over 10' gage length) 6-20 TERA CORPORATION

Identical actions os defined in T.S. 16.3/4.13 are required if these limits are reached.

I 6.4.2 IDCVP Assessment We concur with the conclusions drown in reference 12 relative to the influence of existing cracks on the performance capability of the DGB and its continued serviceability. While significant future cracking is unonticipated, it would only be in these circumstances that we would recommend remedial actions such as epoxy or scolont opplication to insure continued durability. Furthermore, should such procedures continue to be contemplated for purposes of potential increased protection, we urge that opplications of any compounds not be made in such a manner os to mosk surfaces so that cracks are not visually accessible.

Notwithstanding the potential future inconvenience of removing compounds from selected surfaces, there is a potentici that these compounds may influence behavior and modify surface expression of cracks, making future engineering evoluotions more difficult.

We recommend that consideration be given to modifying T.S. 16.3/4.13. The following points summarize our evoluotion and our recommendations.

e Visual inspection - The building should be examined visual!y twice a year in concert with on evoluotion of settlement data to identify any unusual deviations in crack potterns and gross changes in dimentions. This may represent on additional commitment.

e Total allowable settlement - These limits should be based upon structurol/ mechanical performance requirements considering items such as the physical clearances to obstructions (e.g. dcct banks) and permissible deflections incoming fuel lines).

for ottoched items (e.g.considerations, absolute Notwithstanding these settlements and corresponding rigid body motion of the building is of minor concern to building performance copobility other than as it might offect The clearances existing limits to obstructions and connected items.

may trigger potentially unnecessary evoluotions. A 90-day survey interval appears reasonable for the first year of operation. This approach may represent a redefinition of certain total allowable settlement limits.

6-21 TERA CORPORATION

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l e Differential settlement

- Diesel Generator Building Forces induced by differential motion within the DGB ore of interest, but generally only at a time of which crack width levels opproach on order of magnitude greater than hos been observed.

Copacity is not expected to be degraded' for settlement induced cracks with sizes up to this general level. Even at this point, the residual state of secondary stress in the DGB may be low due to factors discussed in Section 6.3; however, one must evoluote shear transfer mechanics across crack boundaries of dimensions of the some order os the fracture surface roughness, it is recommended for consideration that limits for differential motion between points within the DGB (discounting all rigid body components of motion) be specified such that these motions are correlated with potential future crack widths up to on order of magnitude greater than has been observed to date; thus providing functionally defined limits for differential movements. Remedial effort to protect external surfaces may be considered at opproximately half these values. The program may include development of on initial set of dotc which would provide a baseline for potential future reference.

Additional survey data would be collected in the future if indicated by the visual inspection program and absolute settlement measurement surveys. If adopted ibis approach may represent a redefiniton of allowable settlement limits and a restructuring of the proposed tech specs.

- Diesel Generator Pedestals Although, relatively o' lesser concern, of such a t time os the diesel tenerators are run for on extended period, potential differential movement of the isolated diesel generator pedestals is of interest as such movement may offect connected lines.

Accordingly, we endorse continued monitoring of pedestal settlement and comparison to functionally defined differential movements.

We conclude that the committed crack monitoring program will produce results which are of engineering interest but not necessarily of safety significance.

Accordingly, we do not see o need to specify ofert and action limits based upon 6-22 TERA CORPORATION

this program. We base this conclusion primarily on the limited number of locations to be monitored and the fact that oppropriate locations are difficult to determine o priori, not knowing how the building will behave in the future. One could specify locottons based upon predictions of future response, but if the building responds os predicted, this will be of less interest then if it does not, in which cose alternate locations would be more desireable. This is related to our recommendation not to mask surfaces through opplication of new compounds.

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' In summary, we concluda that the performance chorocteristics of the DGB ore not likely to be compromised over its service life. Various commitmenis have been made by CPC to verify continued serviceability. While we conclude that several of these commitments may not be totally necessary, we do not view that safety will be compromised by the specified actions. Certain improvements may be made which may produce valuable information and reduce operational constraints.

6-23

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7.0 CONCLUSION

S As the diesel generator building exists today it is quite capable of performing its intended design functions. Many cracks of small size are evident in the existing building but there is no evidence to suggest that these cracks - in spite of the various possible mechanisms of origin - generally of small size, would be indicative of a condition that would suggest the DGB is incapable of performing its function. It is our belief that in its present condition this building is fully functional in all respects. Although we believe it is improbable, if excessive i

localized differential settlement is observed, remedial corrective measures could l

be undertaken to improve serviceability.

The committed monitoring program clearly will reveal any potential distress. It is suggested that a comprehensive visual inspection of DGB be corried out bionnually (twice a year) in concert with the settlement measurement program.

In Section 6.4 we have offered certain recommendations for consideration that are intended to improve information collected and reduce operational constraints.

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7-1 TERA CORPORATION

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