ML20083G874
| ML20083G874 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 01/04/1984 |
| From: | TERA CORP. |
| To: | |
| Shared Package | |
| ML20083G872 | List: |
| References | |
| NUDOCS 8401120351 | |
| Download: ML20083G874 (47) | |
Text
{{#Wiki_filter:. . . . . . . United States Nuclear Regulatory Commission 1 Docket Numbers 50-329 & 50-330 _ , . . . ^
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p.y 3 --_ . r ----~-.a _ g hpIkooko!$000$9 TERA CORPORATION E A PDR - M
t - L J hl January 4,1983
. - 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 L 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 Washington, D.C. 20555 Re: Docket Nos. 50-329 OM, OL and 50-330 OM, 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 induced 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 part of our broader ossessment of the quality of the design and constructed product of the Midland plant Standby Electric Power system. We are transmitting it to you ', because of its relevance to ongoing discussions concerning the potential effects of settlement induced cracking on the copobility cf 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 significant future cracking is unonticipated and the DGB is expected to remain serviceable without further remedial action at this po!nt in time. We have TERA CORPORATION 7101 WISCONSIN AVENUE BETHESDA. MAFNLAND 208u 301 654 8960
Multiple Addressees reviewed Consumers Power Company's commitments to verify continued serviceability and have concluded that these ore acceptable; however, we have offered certain recommendations for consideration that are intended to improve
~
ovalloble information and reduce operational constraints. Should you desire further articulation of the bases for our ccnclusions, we would welcome the opportunity for discussion. Sincerely, f f M ,
,cNe Howord A. Levin Project Manager Midland IDCV Program Enclosure cc: L. Gibson, CPC.
R. Erhardt, CPC J. Mooney, CPC D. Budzik, CPC D. Quammy, CPC (site) R. Whitaker, CPC (site) R. Burg, Bechtel J. Taylor, NRC, I&E HQ T. Ankrum, NRC, I&E HQ D. Hood, NRC, NRR Midland IDCVP Service List HAL/sl 1 TERA CORPORATION
=
SERVICE LIST FOR MIDLAPO INDEPEPOENT DESIGN AND CONSTRUCTION VERIFICATION PROGRAM cc: Hor,old R. Denton, Director Ms. Borboro Stamiris Office of Nuclear Reactor Regulat. ion 5795 N. River ' U.S. Nuclear Regulatory Commission Freeland, Michigan 48623 Washington, D.C. 20555 Mr. Wendell Marshall
. . James G. Keppler, Regional Administrator Route 10 U.S. Nuclear Regulatory Commission, Midland, Michigan 48440 Region til 799 Roosevelt Rood Mr. Steve Godler Glen Ellyn, Illinois 60137 2120 Corter Avenue 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 Pres,ident 1901 Ove Street, N.W. Consumers Power Company Washington, D.C. 20009 1945 West Parnall Road Jockson, Michigan 49201 Charles Bechhoefer, Esq. Atomic Safety & Licensing Board Michoel I. Miller, Esq. U.S. Nuclear Regulatory Commission Isham, Lmcoln p Beale Washington, D.C. 20555 J Three First National Plaza, 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 Harbour, Esq. Jackson, Michigan 49201 Atomic Sofety and Licensing Board U.S. Nuclear Regulatory Commission Ms. Mary Sincla.ir Washington, D.C. 20555 5711 Summerset Dr.ive .. Midiond, Michigan 48640 Mr. Ron Collen Michigan Public Service Commission Cherry & Flym 6545 Mercontile Way
^
Suite 3700 P.O. Box 30221 Three First National Plaz Lansing, Michigan 48909 Chicago,Ill%is 60602 - ' Mr. Poul Rou Ms. Lynne Bernobe.' Midland Daily News = Government Accountability Proj.ect 124 Mcdonald Street 1901 Q Street, NW Midiond, Michigan 48640
- Washington, D.C. 20009 i-TERA CORPORATION ,
( ATT ACHMENT A.Pl.3208 001.REV 2 ENGNEERING EVALUATION COVER SEET Structural Evaluation of the Diesel Generator Bldg. CONT. lD. NO. 3201-001 031 TI M b2 NO. OF 5 HTS. PROKCT: CONSUMERS POWER COMPANY MICtAto IDCV . SUPERSEDES ENG. EVAL. NO. DATE APPROVED BY DATE ORIGINATOR DATE REVIEWED BY REV. NO, O REVISION Original f[fA 12/30/83 Jg 12/30/l 3, p 1/4/84 D SWORM ENATION TOPIC NUMBER . I I I . 5-2,S 1
$Y'.#dE Design Eg,Tj0N Foundations, Concrete / Steel Design Considerations, TOPIC TITLE Civil / Structural NETHOD/ EXTENT OF REVIEW
- 1. Review of Midland project generated information including calculations, consultant reports, testimony, 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. CONTENTS (SEE SECTION 2., Pi-3201-001) E ABSTRACT E OVERVIEW OF REVIEW PROCESS E BASES FOR SAMPLE SELECTION E SOURCES OF INFORMATION/ REFERENCES E BACKGROUND DATA (ItPUTS, ASSUMPTIONS, RELATED CALCULATIONS) E ACCEPT ANCE CRITERI A (CODES, ST AtOARDS, FSAR, NRC GUIDANCE, REGULATIONS) E EVALUATION (DOCUMENTATION OF REVIEW ACAlNST CHECK LIST,(ACCEPT ANCE CRITERIA) E CONCLUS10td TERACORPORATION
STRUCTURAL EVALUATION OF Tif DIESEL GENERATOR BUILDING - ASSESSMENT OF TFE STRUCTURAL PERFORMANCE CAPABILITY Ato SERVICEABILITY AS POTENTIALLY AFFECTED BY SETTLEMENT lfOUCED CRACKING I TERA CORPORATION
TABLE OF CONTENTS PAGE l-1 I.0 ABSTRACT 2-1 2.0 ~ OVERVIEW OF REVIEW PROCESS 3-1
3.0 BACKGROUND
DATA AND REFERENCES 4-1 4.0 ACCEPTANCE CRITERIA 5.0 BASES FOR SAMPLE SELECTION 5-l 6-1 6.0 ENGINEERING EVALUATION Building Performance Requirements 6-1 6.1 6.2 Acceptance Criterio 6-2 6.2.1 Structural Primary Loadings 6-3 6.2.2 Secondary Loadings - Settlement Effects 6-4 6.3 Evaluation of DGB Performance Capability 6-8 6.3.1 Available Dato 6-8 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 6.3.3 IDCVP Evoluotions 6-14 6.3.3.1 Building Inspection 6-14 6.3.3.2 Settlement Dato 6-14 6.3.3.3 Gross Stress Estimation 6-17 6.3.4 IDCVP Assessment / Interpretation of Results 6-18 6.4 Serviceability Future Capability and Monitoring 6-19 6.4.1 Midland Project Evoluotion and Commitment 6-19 6.4.2 IDCVP Assessment 6-21 CONCLUSIONS 7-1 7.0 i TERA CORPORATION
4 1.0 ABSTRACT An engineering evoluotion has been completed to assess the structural performance copobility or.d serviceability of the Midland plant diesel generator building (DGB) as p ,tentially offected by settlement induced cracking. The
- 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 capability of the DGB in meeting its intended performance requirements.
Furthermore, it v.as ,udged i that significant future cracking is unanticipated and the DGB is expected to remain serviceable without further remedial action at this time. Censumers Power Company (CPC) commitments to verify continued . serviceability were reviewed and found to be acceptable. Certain recommendations have been offered for consideration that are intended to improve available information and reduce operational constraints. ad' l-l TERA CORPORATION
2.0 OVERVIEW OF REVIEW PROCESS This engineering evoluotion was undertaken as part of a broader assessment 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 settlement induced crccking may potentially offect the intended performance requirements. : Accordingly, this evoluotion addresses the following topics within the Midland IDCVP: e Topic 111.5 Civil / Structural Design Considerations e Topic 111.6 Foundations, and e Topic 111.7 Concrete / Steel Design; 2 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 ll1.2 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 a determination of the DGB performance requirements and important design inputs (i.e. engineering dato and assumptions); on evoluot;on of their occuracy, consistency, and adequacy; and on evolvation of , 2-1 TERA CORPORATION ;
I the implementation of these commitments. Current licensing criteria are utilized as a baseline os well as consideration of various other regulatory criteria which evolved during the licensing process. Given the unique circumstances ossociated with the DGB design and construction processes, the IDCVP ossessment 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 documented in Section 3.0. Conclusions are reached through cn integrated assessment of these dato, discussions with Midiond project personnel, as well as engineering judgement. The following individuals made technical contributions to this engineering evoluotion: Dr. Jormo Arros - Structural Reviewer, Midland IDCVP and Senior Structural Engineer, TERA Corporation Dr. William J. Hall - ' Member Senior Review Team, Midland IDCVP 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 1 and Biggs, Inc. Mr. Howard Levin - Project Manager, Midland IDCVP and Manager, Engineering, TERA Corporation Dr. Christion Mortgot - Lead Technical Reviewer, Standby Electric Power System Structural Review, Midland IDCVP and Principal Structural Engineer, TERA Corporation 2-2 TERA CORPORATION
The following chronology of external interactions transpired as part of this review. Date Activity 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 and 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 BACKGROUtO DATA APO 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
AT T ACHMENT B. Pi-3201 -00i. FE V 2. REFERENCES / SOURCES OF INFORMATION --
.iii.7-2 TOPIC TITLE Civil / Structural Desian Considerations. Foundations. TOPIC NO. 111. 5-2.111.6-2, P4GE I cf 3 Concrete / Structural Steel REV 0 DATE 12/30/8 ,
ENGINEERING EVALUATION Structural Eval. of the Diesel Generator BldTONT. lD. NO.3201 -001 -031 WHERE/HOW DOCUMENT ORIGINATING ORG./ IDENTIFICATION / REV. DATE TITLE LOCATED TYPE AUTHOR NUMBER
.. File 0485.16/81 3 Final Safety Analysis Report Ann A'rbor FSAR
- 1. Bechtel Serial 22423 48 5/83 10/21 Report on the Review of the Diesel
- 2. NRC 50-329/330 0 83 Generator Building - Midland Docket Report 8/24/ Midland Units 1 and 2 Ann Arbor, 11/18/8 3 Bechtel -- O ga Diesel Gen. Bldg. Exec. Summary Meeting 3
testimony at pp 9/8/ Midtimony Tg land Plantof Karl Wiedner Diesel Gen. for the Bldg. Docket Test.imony
- 4. Wiedner 10804-11007 0 82 File 0485.16,l 6/1/ Technical Report-Structural CPC, Jackson Report 5 CPC B3.0.3, Seria 0 82 Stresses Induced by Differential 17228 Settlement of the Diesel Generator Blda.
NRC regarding Mant Docket Report
- 6. CPC 3 9/79 hiIYnset Bujiding Code Requirements for Library Standard 7 ACI ACI 318-77 Reinforced Concrete cM$rSafeti Library Standard
- 8. ACI ACI 349-76 R$$t0 $ eses Project 7/15/ Engineering Program Plan IDCVP Proj. Files Instruction 9 TERA PI-3201-009 3 83 2/11/ Eval. of the Effect on Structural Transcript at
- 10. Sozen -- 0 82 Strength of Cracks in the Walls of 10950 Report the Diesel Generator Building Testimony at 12/6/ Testimony of Ralph Peck Transcript at Report II. Peck p. 10180 0 82 10180 Transcript at 4/19/ abiitsoEffityo kracksonServitructuresatMi011204 Report
- 12. Corley, et. al. -- 0 82 land Plant
~
rd FSAR Ch. 16 9/82 Teq. Spec. 16.3/4.13 Settlement Ann Arbor FSAR i
$ 13
() CPC 45 Monitorina Partial / O DGB Areas for Crack Width Monitor- Ann Arbor,lI/18/83 Corres.
- o 14. CPC Exhibit 29R 0 --
inq During Operation of the Plant Meeting Diesel Gen. Bldg. Reanalysis Using Ann Arbor Calc O 15 Bechtel DQ-52.0(Q) 2 g/9/ 3 Revi sed Set t len?nt load Caso z 1
ATT Act#AENT B, Pl.3201-001, f<EV 2. REFERENCES / SOURCES OF INFORMATION 2 topic TITLE Civil / Structural DesIan Considerations. Foundations. TOPIC NO. II i-2111.6-2 i OGE 2 III 5I OF 3 abN1_ of the Diesel Gene ra to r B l dg 40NT. lD. NO. 3201 -001 -0 31 REv 0 DATE 12/30/8; ENGINEERING EkL T WHERE/HOW DOCWENT ORIGINATING ORG./ IDENTIFICATION / REV. DATE TITLE LOCATED TYPE AUTHOR NUMBER 3/2/ DGB Settlement Analysis - Load Calc
- 16. Bechtel DQ-52.l(Q) 1 82 Case IA Ann Arbor 5/12s DGB Settlement Analysis - Load Ann Arbor Calc 17 Bechtel DQ-52. 2 (Q) 0 82 Case 18 Ann Arber Calc
- 18. Bechtel DQ-52.3(Q) I h28' DGB Surcharge Condition (2A)
Calc 19 Bechtel DQ-52.4 (Q) O h28' DGB Settlement for 40 yr Life (2B) Ann Arbor DGB Analysis for Uniform Torsion Ann Arbor Calc
- 20. Bechtel DQ-52.6(Q) I h7/
I Anal. Imposing 40 yr displace- Ann Arbor Calc
- 21. Bechtel DQ-52.7(Q) h7/
Ann Arbor Calc
- 22. Bechtel DQ-12(Q) I c c on c;/18 Optcon ACl-349 - Nonseismic Load Calc Bechtel DQ-52.0-C7(Q) 0 82 Cases 7 Diesel Gen. Bldg. Ann Arbor 23 Settlement Analysis (partial)
DGB Load Combination (partial) Ann Arbor 8 Calc
- 24. Bechtel DQ-52.0-C2(Q) 0 5/12s DGB Settlement Analysis - Load. Calc 25 Bechtel DQ-52.2-C5(Q) 0 82 Case IB - Free Body Analysis Ann Arbor of Trial #3 (partial) 9/28/ DGB - Settlement Case 2A - Free Ca,ic
- 26. Bechtel DQ-52.3-C7(Q) 0 83 Body Analysis on Best Fit (Sur- Ann Arbor charge) (Partial)
S/12/ DGB Analysis - Free Body Analysis i Bechtel DQ-52. 4-C4 (Q) 0 82 of Best Fit 40-Year Case Ann Arbor Calc g 27 DGB Roller Support (FSAR Criteria) Ann Arbor Calc
- 28. Bechtel DQ-23-C4(Q) 0 h/II' Static E, Dynamic Spring Constant O Bechtel S-Il0 1 11/11 Ann Arbor Calc
" 29 82 of DGB for Structural Stress Anal. i b 2/22/ Update of Settlement Prediction Ann Arbor Calc
- 30. Bechtel S-175 3 82 DGB - After Surcharge Removal 0 gl5/ gtgg3 DGB Between 9/14/79 Ann Arbor Calc O 31. Bechtel S-238 l
l
ATT ACHMENT B. PI-320141. REV 2. REFERENCES / SOURCES OF llFORMATION Civil / Structural Design Considerations, Foundations, TOPIC NO, - 4GE 3 OF 3 TOPIC TITLE Concrete / Structural Steel 3201-001-031 REV 0 DATE 12/30/8: ENGIKERING EVALUATIOc4 9tructural Fval- nf the ninen1 Conorntne R I rf arONT. lD. NO. WHERE/HOW DOCUMENT ORIGINATING ORG./ IDENTIFICATION / REV. DATE TITLE LOCATED TYPE AUTHOR NUMBER
* /2/ Seismic Analysis of DGB and DG Calc 32* Bechtel SQ-147(Q) I g3 Pedestal Ann Arbor 33 Bechtel DQ-52. l l (Q) 0
[/ $$ (Re MnofSettlement Settlement Data for DGB Ann Arbor Drawing
- 34. Bechtel SK-C-2343-1/24 F g/5 Att. 2 to Testimony File 0485.16 8/2/ Midland Concrete Wall Repair of Corley @ p.ll206 Letter 35* CPC Serial 18371 0 82 Program Trip Report - Midland DGB Struc-
- 36. NRC 5D-329/330 0 $(29/ tural Design Audit Docket Report Properties of Concrete Library Text Book 37 Neville k*7flho88 rest 1 1975 Stmctural Analysis of Diesel piyVPProj. Ca k
- 38. TERA 3201-003-007 0 g2/30/
3 Generatne- nutia,.nn es 8/12/ Response to NRC Qu'estion 26 Re: Ann Arbor Calc 39 Bechtel DQ-14(Q) 1 83 Diesel Generator Buildina Calc
- 40. Bechtel DQ-23(Q) I k# jNa a at t Nbt of Ann Arbor o
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I
.A 4.0 ACCEPTANCE CRITERIA 4.1 LOAD COMBINATIONS The loads and load combinations employed for the original design and analysis were provided in the FSAR subsection 3.8.6.3 (revision 0, dated November 1977).
These original design criterio did not contain settlement effects. Four additional loading combinations were established and committed for consideration as a result of Question 15 of the NRC Requests Regarding Plant Fill of September 1979. These loading combinations combined differential settlement with long-term operating loods and either wind or the operating basis earthquake (ObE). As Wiedner (reference 4) and CPC (reference 5) point out these expressions are more stringent than the requirements of ACI 318 (reference 7), but less stringent than ACI 349 (reference 8). In the latter case the loading combinatione combine differential settlement with extreme loads such as tornadoes and the safe shutdown earthquake (SSE). Subsequently, in response to Question 26 of the NRC Requests Regarding Plant Fill, o commitment was made to undertake a separate . structural reonalysis of the DGB in accordance with ACl-349 as supplemented by NRC Regulatory Guide 1.142 for comparison purpose only. The following loads were considered in the reonalysis: (a) dead loads (D) (b) effects of settlement combined with creep, shrinkage and temperature (T) (c) live loads (L) (d) wind loads (W) (e) tornado loads (W') (f) OBE loads (E) (g) SSE loads (E') (h) thermal effects (To) 4-1 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. 1.05 D + l.28 L + 1.05 T
- b. l.4 D + 1.4 T
- c. 1.0 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 load 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 and other load combinations can be eliminated from the analysis offer comparison with more severe loads or load 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. l.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 + I. + W') + 1.0 To 4.2 ALLOWABLE MATERIAL LIMITS In accordance with regulatory requirements, the maximum rebor tensile stress allowed in the diesel generator building rebar should not exceed 0.90yf (where fy equals yield strength) for computation of section capacities. Because the diesel generator building rebar has on fy value of 60 ksi, the maximum allowable tensile rebar stress due to flexural and exial loads is 54.0 ksi. Accordingly, reinforced concrete section capacities for the diesel generator building were based on this 4-2 TERA CORPORATION
maximum allowable rebor stress value (54 ksi), a design concrete compressive strength of 4000 psi and a maximum allowable concrete compressive strain level of 0.003 in./in. 4-3 TERA CORPORATION
5.0 BASES FOR SAMPLE SELECTION The diesel generator building (DGB) was selected (or review because it serves on Important support function in providing protection against external hozords for the die el 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 soil mechanics aspects which are not within the scope of the Midland independent Design and Construction Verification Program (IDCVP). This engineering evoluotion addresses the potential ef fects of settlement induced cracking on the ability of the DGB to meet its intended performance requirements. Accordingly, verification of the Midland project treatment of the settlement / cracking issues which have offected several structures at the Midland site is addressed herein. While o 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 sample addressing the settlement / cracking issues. It is estimated that opproximately one third of the project's calculations and evoluotions addressing the structural design of the DGB were selected for review. Emphasis was placed on the selection of portions of the project's evoluotions that address controlling design conditions (e.g. important load combinations producing the highest predicted stresses or strains, as appropriate). - Principal project consultant reports were reviewed as well as other docketed information that documents CPC commitments to the NRC (see section 3.0). J" 5-1 TERA CORPORATION
6.0 ENGIEERING 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 6-1). The purpose of the diesel generators is to supply standby electrical power to operate the Midland plant during power outages and to provide the necessary power to ensure safe shutdown of the piant 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 and service lines, instruments and equipment, assuring ready availability of this supplementary power source. This protective function includes not only the normal sheluring 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 normally 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 that 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 ore described in detail in references 3 and 5. 6-1 TERA CORPORATION
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 membrone 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 load 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 loco! " strength", i.e., capacity of the structure. .. The acceptance criterio consists of the following:
. Statements of the several different load combinations that must be satisfied, and the load factors to be opplied to each of the loadings I
(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 (unfactored) Ioadings experienced in normal operating conditions. For the combinations which include extreme and rare loadings, safety in the sense of protecting personnel and equipment, yet retaining y 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 load factors. In effect o larger demand /copacity ratio for these unfactored load combinations is _ occeptable for these rare conditions. , 6-2 TERA CORPORATION
It should be noted that the specified expressions, or procedures, for determining the local internal strength do not typically include any direct limitation on rebor tensile strain, or on crack widths which accompany such strain, 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 a value of 0.003 in./in. is specified). This strain reflects the fact that certain components of local strength cre 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 range, and to compute rebor stresses associated with the particular 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. Howc /er, 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, as reflected in codes, are implicitly based on the ductile behavior of structural systems as just noted. 6.2.1 Structural Primary Loadings The DGB must resist the following principal primary loadings: e Gravity- induced dead and live loads e Earthquake- induced loads e Tornado- induced differential air pressure e Tornado- borne missiles 6-3 TERA CORPORATION
- - - -- - - -a, w. . . .. ...m. m #
2 2 Gravity- Induced loads produce out-of-plane shear forces and bending moments in the floor and roof systems and in portions of the walls immediutely adjacent l. thereto. These loads also produce in-plane forces in the walls and, of course, d bending moments and shear forces in the strip footings. Earthquake- induced loads produce in-plane forces in the walls which are k. m I substantial, and more modest in-plane forces in floor and roof slabs. They also { ' produce out-of-plane shear forces in floor and roof slabs and walls. Tornadic wirds produce in-plane and out-of-plane forces in walls and roofs. Tornado- induced differential air pressures are the principal source of out-of- f plane shear forces and bending moments in floor systems and walls, and they also { produce in-plane forces. E' _ a
- Tornado- borne missiles produce highly localized out-of-plane loading of the f walls. The capacity of the wall to resist such missiles is evoluoted f '
independently of all other loadings. ,-- e 6.2.2 Secondary Loadings ss Restrained non-lood-induced volume changes (e,g., due to concrete shrinkoge 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 E most cases they are neglected. The reasons relate directly to the ductility of g the tension rebars. If the local strength is mobilized, by on imposed set of local f4 - demand forces,it typically will be the some whether or not the forces associated -- with the non-load induced effects are included. The difference will be that the tensile rebor strain, including some yield stroin, will be larger when these 'f secondary forces are included. This yielding has the effect of decreasing, and sometimes completely eliminating, the local forces which were initicly 4 introduced by the non-load effect. It is for this reason that the forces associated y with such non-load induced effects often are termed "self-relieving" or secondary. _ a a C
.5 6-4
% e, TERA CORPORATION ]
l
u In the design of most reinforced concrete buildings the local internal forces S crising from restrained shrinkoge and thermal strains os well as that induced by settlement are not included in the applicotton of the strength criterio. In the design of nuclear safety related concrete structures it is the accepted practice to account for through-the-woll thermol gradients, although shrinkoge effects are not typically included. Even accounting for the thermal gradients is a conservative requirement the justification for which is at least debatable. - However they were accounted for in the DGB design as required by the acceptance criteria. It may be noted that underlying codes, from which the acceptance criteria were developed, typically coiled for inclusion of these non-lood-induced forces with the lood-induced #orces only where their structural _ effects may be significant. In the case of the DGB it may reasonably be debateo 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 associated with foundation settlement could be significant nor, that they should be included with the lood-induced forces in the factored Icad combinations. Clearly, the building was designed for continuou: support on what was intended to be o relatively homogeneous soil medium. Thus the designer ./ could justifiably assume that there would be little if any redistribution of the g upward soil reactions on the strip footings due to major poir.t-to-point variations f 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 location of footing contact with ducts, together with poorly consolidated soil (Iow 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.
,. J hlhh.
av Upon noting that the settlement had led to interference between the foundation ^{.( and buried ducts, the unintended footing-to-dset connections were physically ~ d7 disengaged and the unsatisfactory foundation condition was corrected by o surcharge loading procedure. It is to be noted (reference 36) that the surcharge loading procedure began on .lonuary 26, 1979, incrementally, and that
' 6-5 TERA CORPORATION ,
x
' MM -
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 parapet 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 additional 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 design, 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 signifunt to local inte noi strengths relied upon to resist the forces associated with applied load combinations. From all ' Sis it .ould naturally follow that the internal forces induced by differential settlements need not necessarily be included with the load-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. V^ a. {l 3,pC Since the internal forces induced by a specific non-uniform settlement are self-hj i' rel; ving (as was described earlier, for thermally induced forccs), why must they be included; i.e., when may their effects be "significant", in some structures the [.: , moanitude of possible future settlement may be uncertain, and there may be f.y.. . , . i little or no prespect for monitoring of the settlement or the state of the g;p.. Ui structure during its service life. Accordingly, inclusion of settlement-induced forces in the design would be appropriate to limit the possible development of $y@g
- g. -
structural distress which would be costly to repair, or which in some special $, 2 , cases, like a containment structure, may affect functionability by creation of D 4
- s. .
large liner strains. For other structures these forces might prudently be included isw.. T] to avmd excessive yield sh ains in the tension rebars (and the associated large mA i:. m. crock widths) which might degrade the local internal strength under some set of 6 4: the local internal forces associated with applied loads, particularly if no y).
;n, monitoring of the structure for such effects could be anticipated. j_ '( %
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's .' :. h For the DGB structure the principal structural elements are relatively [.M <
accessible, and a monitoring program is planned. Nevertheless it is required to 9 5 ,.; 2.y. 4
- k isi . ," a p L.
- e. _p 6-6 W vfl* ?'b
demonstrate by application of the relevant acceptance criteria, including the effects of differential settlement, that the local internal strengths are not l' . presently degraded and are unlikely to be degraded by any probable future 1 differential set tlements. The acceptance criterio do not include any .. spe-ificotton of the method by which the associated internal forces are to be
=
deteimined. This is on important consideration in any effort to apply the occeptance criteria. There ore essentialy three cliernatives: l a) One may assume o magnitude and distribution of _
.l differential settlement and impose this displacement ,.; y K
";s w.^/.i pattern upon the structure. In contrast to the situation at the design stage the analyst for the DGB has settlement t measurements to consider in arriving of the postulated [p.}, v differential settlements to be used. dbQ
- &. f r b) One may postulate one or more perturbations of the TT,4 3.g.
distribution of upward soil reactions associated with dead 72 load which may be associated with differential fn? 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.
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Mij This is not on unreasonable opproach, if sufficient p63 ottention is giv(, to parametric variations, particularly if Q the analyst lacks dato on differential settlement which he y i considers sufficiently precise to use directly in method j@;g f; yn' (a). . +:.-. c) One may postulate the local internal forces directly from YII
., h4 .'
the observed condition of on (existing) structure; i.e., the crack widths in the DGB. This is on option clearly not ?.+
,j.?7l'M avalloble at the time of design.
dhM The method of imposed differential settlements may lead to unrealistically large g%f; , internal forces unless the analysis con account for cracking, and time-dependent ji$ concrete properties. The cost-benefit of such on analysis may not be justified, , particularly if other suitable options (b or c) exist. y j:
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The method of onalyzing the dead load condition for several postulated W* distributions of soil reaction is suitable, but it may be difficult to chocie :=u, of e?Drg) distributions which cover the possible differential settlements but which are not h t' unjustifiably extreme. %j 33g M.:'
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For the DGD, which has 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 opproach. it is the most direct, it is porticularly attractive since any significant changes in the condition of the structure wil! be observable during its service life. Observations related to this approach follow. 6.3 EVALUATION OF BUILDING PERFORMANCE CAPABILITY The performance capability of the structure is to be assessed in two steps: the first one considering 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 o number of elements such as: available physical data, analytical studies, understanding of concrete behavior and engineering judgement. 6.3.1 Available Data 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 crcck 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 Bechtel coasultants, Dr. Mete Sozen (reference 10) of the University of Illinois and Dr. W. Gene Corley (reference 12) of the Portiond Cement Assoclotion. 6-8 TERA CORPORATION
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 meesurements are token. The dato spans over o period of 5 years with measurements taken 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, dowatering, and up through the preseni. It also provides a means of assessing potential rondom and systematic errors in the measurements. The Midland 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 following section. 6.3.2 Midland Project Evoluotions The Midland project followed two separate opproaches to estimate the state of stress in the building: o study of the cracking history e study of the settlement history. The future state of stress due to settlement was estimated based upon predicted settlements. 6.3.2.1 Evoluotion of DGB Based on Observed Cracking in its present condition the DGB has cracks which cppear to be settlement-induced or settlement-intensified, generally arising 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 ossociated rebor stress os estimated in a 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 evoluotion to be reasonable incorporating techniquei that are state of the art, widely accepted and supported by laboratory tests. Dr. Sozen also has argued that the presence of initial cracks does not degrade the capacity of a reinforced concrete element 6-9 TERA 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 Midland DGB would not significantly degrade capacities in the several . Z., u m. modes developed by the principal loadings, and in their required factored T." 1.WA J.g - 7. ,, comb,mations. 3
' s.%= z g
' Ogb 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, a force y.
] j caused by foundation settlement. He does not indicate his opinion whether or -e s , #
. ;' ,t.-g-not the self-relieving internal force implied by the initial rebor stress should be included with the internal forces due to applied loadings or con be neglected N. ( . y.@ s
- 1. g g .?
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 fh4 fN utilize the initial rebor stress magnitude estimated by Dr. Sozen but rather gr; w, computed it based on the settlement history of the building.
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Nfhsf Evoluotion of DGB Based on Settlement History 3,j.N.i 6.3.2.2 gw
.y The settlement effects were modeled by Bechtel into the structure considering Oh_
t h.;]. four distinct time periods. Measured or estimated settlement values
&.%..,[f corresponding to each of the time periods were used-e Case IA: 3/28/78 to 8/15/78 (Structure partially completed to ).((
jd elevation 656.5')- A long hand calculation was used to determine the ~*
,. . .. ; j
.J stresses due to early settlements. The structure was assumed fully ep.[A.
6 cracked and the stresses in the reinforcing steel were assessed based upon local strains corresponding to on imposed differential i dyb settlement (reference 16). kw'm % 4."
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- i. V yi e Case 18: 8/15/78 to 1/5/79 (Structure partially completed to elevation 662.'0.)- The duct banks were seperated from the structure which caused the north wall to settle rapidly. (reference 17) t. Q. /
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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 12/31/81 to 12/31/2025. (reference 19) The lost three analyses used a finite element model having stiffness corresponding to on uncracked condition. in 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 (refe.ence 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 the North and South walls. The best fit data were utilized in on ottempt 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 accounting for foundation stiffnesses utilizing the linear best fit data may not be satisfcciory for correlation with .y observed cracking in relation to differential settlement. We concur that . i settlement measurements may not be of sufficient occuracy to permit o I ) precision computation of settlement-induced internal forces. Furthermore, the N. marker locations are spaced at wider intervals than would be desirable os input h5
- to analyses of building strains. Nevertheless, the general level of stress implied M1 by the magnitude of cracking is not in contradiction to that which may be derived from the measured settlement data, realistically accounting for flexibility including consideration of phenomeno such as creep (see section 6-11 TERA CORPORATION
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 s of the loading and the fact that capacity is not adversely offected. In separate sensitivity studies Bechtel engineers considered among others, the two following cases: i The zero spring condition analysis (reference 3) which investigated e k the structure's ability to span any soft soil condition. A zero soil spring value was used at the junction of the south wall and east ! center wall. Soil values were increased linearly back to their E original value within a distance of approximately 15 feet from the a zero spring. The stresses in the building underwent moderate increase in the creo of the bridging, in our judgement this is a reasonable approach, but one may ask whether the size and locations ' ' 5 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 at 10 points h - along the foundation. This analysis led to very large reaction forces - E of the points of imposed settlements, and some of these acted = downward on the structure, i.e., implying tensions in the soil, which is not possible. Moreover, the analysis indicated very large rebor -.
<f.t"T, ' >
t E k tensile stresses, where at several points a multiple of the yield ff; strength was indicated. Of course the structure does not display the 'h f;.( . [ [:,f i very wide cracks which would accompany such high stresses. For .** $$ w these reasons Bechtel engineers concluded that the settlement %% F w !c measurements connot be on occurate representation of the actual pg . . ;- r setitement nonuniformities. jf y _- .: .) <- g l We have noted that the settlement data may not be on adequate basis for mf
, M .i f computing settlement effects. However, we believe the described analysis * " , " . , . ,
C R- %..., E exaggerates the effects of the displacement input data which was questioned by .. I the project. Our reasons are that the analysis assumed uncracked concrete and hj;,m. e L
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6-12 P.,n,
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. _ _ . 3
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. Parhaps 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) than by the forces in the concrete (uncrocked). Thus, the rebor stresses are better approximated by the product of steel modulus and concrete strain (uncracked); i.e., by the product of modular ratio, n, (Youngs modulus of the steel / Youngs modulus of the concrete) and concrete stress. fsa n fc in contrast we believe that the following expression was used fs E Ifc P where p is the reinforcement ratio (rebor oreo/section area). This later expression greatly overestimates rebor stress. To illustrate, for p = 0.0043 and n
= 8, the sugoested approach 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.
6-13 TERA CORPORATION
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 dato
- Cross stress estimation 1
6.3.3.1 Buildi ig Inspection A careful inspection of the building was performed together with a review of the crack mapping dato. As it exists at present, many cracks of small size are evident in the building but there is no evidtnce to support that these cracks are indicative of a high state of 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 scabbing and other phenomeno which are not apparent in the DGB. Our conclusion from visual 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 dato recorded between 11/24/78 and 8/28/80 is presented in reference S. We reproduced and expanded this analysis to include the most recent dato (reference 38). The two time periods covered were from 6-14 TERA CORPORATION
5/12/78 to 9/14/79 (reference 33) and 9/14/79 to 8/23/83 (reference 34). Our goal was two fold: (1) assess the overoll deformation of the building with time and (2) estimate the rondom error present in any one set of measurements. We studied # the following data. Cumulative settlement recorded overtime, s_-
- l. -
- 2. Incremental settlement between successive readings. _
- 3. A measure of the curvature between any three consecutive markers along the foundation as it varies with time. The curvature d"; at marker i is defined as:
=
d"i = 0.5 (di _l +d;+l)-di 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 os a rigid body. Si
- 4. A measure of the deformation of the building with respect to its rigid body motion. The rigid body motion is e
" removed" by computing the vertical position of o!!
markers with respect to the plane defined by three corner 'e markers. This analysis was done both for each incremental reading and cumulatively. An upper limit of the random error in any set of readings is given by the maximum difference of incremental settlement between any two markers from one reading time to the next. When the building has not experienced any settlement between two 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 m might have been reduced by marker transfer necessitated by the placement of _ surcharge. However, this quantity decreases rapidly and offer June 1979 is never greater than 0.150". After the removal of the surcharge for the readings storting 9/19/79 which we will refer to os the recent readings, the random error is smaller than 0.I25",95 percent of the time which would give a rondom error of about t I/16 of an inch. This implies that a higher level of confidence con be given to the ' recent measurements. 6-15
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TERA CORPORATION
Jumps in readings from one period to the next are sometimes large implying that i the building would rapidiy move up or down by a uniform amount. These jumps are attributed to systematic errors in locating the reference elevation. 1 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 differenti6. deformations and introduction of c.tvoture in the wall. The latter ones show stabilization of differential settlements implying that the wall is still settling but as o 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 with respect to its rigid body motion point toward the some trend. This is supported by on evaluation discussed in reference 4, where it was noted that the settlements occurring during the time periods represented by lines c and d (reference 4, figure DGB-7), were those that are 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 stage, the building underwent substantial differential settlement that introduced in-plane curvature in the walls with resulting stress and cracking compounded with normal shrinkage cracking. As the building was completed and the concrete aged, its tended to behave more and more os a rigid unit, the who'e foundation (or building) moving as a plane (or a unit). The recent data indicates that for the lost four years the building has generally settled as a rigid body introducing relatively little additional distortion in the structure. We expect this behavior 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. 6-16 CL TERA CORPORATION V
- 4.l_
These latter elements con accommodate some degree of distortion and con be rrodified in the future if warranted. 6.3.3.3 Gross Stress Estimation Even though we have noted that settlement dato may not provide on acceptable basis for computing settlement effects, it is our opinion that if credit had been token to account for:
- creep and stress relaxation in young concrete,
- reduced stiffness associated with the geometry of the uncompleted structure
- stiffness reduction due to cracking the exact recorded settlement could have been imposed on the structure without generati::g stresses in gross contradiction to that observed via crack patterns in the DGB. This would have qualitative value to on overall understanding of building behavior.
In order to improve our understanding of building behavior 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 j within 25 percent of Bechtel's which is reasonoble considering the simplified model we used. We imposed the recorded settlements on the incomplete wall for Case lA and IB ond on the complete wall for Case 28. For crccked concrete, the stresses were computed as described in Section 6.3.2.2. 6-17 TERA CORPORATION
r The following opproximate maximum values of stress were obtained: LOADING STEEL ; (ksi) CASE lA I l .3 CASE IB 3.5 CASE 2A 4.6 This leads to o total stress of 19.4 ksi which is in good agreement with Dr. Sozen's independent onalysis (see section 6.3.2.l and reference 10). We recognize that the above analysis 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 rebar 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 uncracked concrete, they are quite modest rather than unrealistically lorge. 6.3.4 IDCVP Assessment / Interpretation of Results in our opinion the settlement-induced internal forces implied by the associated rebar stresses, as they presently exist in the Midland DGB will not degroue the capacities to resist the internal forces and moments caused by the factored load 6-18 TERA CORPORATION
a - r E, S F = 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 i expected to be compromised over time. We believe that the settlement-induced, f 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 available margins ossociated with controlling load combinations, we believe that compliance with these criteria con generally be demonstrated, { E 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 loods and the fact that capacity is unaffected. i E 6.4 SERVICEABILITY, FUTURE CAPABILITY, AND MONITORING c M .. E r The previous sections address the significance of settlement induced cracking on i 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 addresses serviceability of the DGB ond any g
; actions that may be necessary to identify and mitigate potential future 2 conditions which could compromise the DGB performance.
E. 6.4.I Midland Project Evoluotions and Commitments E The effects of cracks on the serviceability of Midland plant structures were f addressed in reference 12. Three principal issues were evoluoted: e Freezing and thawing resistance, [ [ e Chemical attack, and
- e Corrosion of reinforcement e
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= .. lt was concluded in reference 12 that observed cracks are not expected to have a Accordingly, remedial significant influence on the durability of the DGB. measures such as epoxy injection 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 e 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 has { 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 -f 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 hours (Action Limit). CPC hat also committed to conduct a crack width monitoring program (reference 14) which includes individual crack width and cumulative crack width measurements at 3 locations over a 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: q Alert Limit Action Limit __ single crack 50 mils 60 mils 200 mills .e cumulative cracks 150 mils - (over 10' gage length)
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Identical actions os defined in T.S. 16.3/4.13 are required if these limits are . r reached. . . 6.4.2 IDCVP Assecsment }
- i We concur with the conclusions drawn 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 unanticipated, it would only be in these circumstances that we would recommend remedial actions such as epoxy or seolont application to insure continued durability. Furthermore, should .
such procedures continue to be contemplated for purposes of potential increased - protection, we urge that applications of any compounds not be made in such o manner os to mask surfaces so that cracks are not visually accessible. Notwithstanding the potential future inconvenience of removing compounds from selected surfaces, there is a potential 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
-1 following points summarize our evoluotion and our recommendations.
e Visual inspection - The building should be examined visually twice a year in concert with on evoluotion of - settlement dato to identify any unusual deviations in - crack patterns and gross changes in dimensions. This may - represent on additional commitment. o Total allowable settlement - These limits should be based upon structural / mechanical performance requirement:, _- considering items such as the physical clecronces to
- obstructions (e.g. duct banks) and permissible deflections , .-
for ottoched items (e.g. incominc fuel obsolute lines). Notwithstanding these considerations, - settlements and corresponding rigid body motion of the - building is of minor concern to building performance ' capability other than as it might offect clecronces to obstructions and connected items. The existing limits 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. m _n 6-21 TERA CORPORATION -
e Differential settlement
- Diesel Generator Building
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Forces induced by differential motion within the DGB ore of interest, but generally only at a time at which crack width levels approach on order of magnitude greater than has been observed. Capacity 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 si'ecified 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 data which would provide o baseline for potential future reference. Additional survey data would be collected in the future if indicated by the visual inspection program and obsolute settlement measurement surveys, if adopted this approach may represent a redefiniton of allowable settlement limits and a restructuring of the proposed tech specs.
- Diesel Generator Pedestals Although, relatively of lesser concern, of such a time os the diesel generators are run for on extended period, potential differential movement of ..
the isolated diesel generator pedestols is of interest as such movement may offect connected lines. Accordingly, we endorse continued monitoring of pedestal settlement and comparison to functior.olly 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 appropriate locations are difficult to determine a priori, not knowing how the building will behave in the future. One could specify locations based upon predictions of future response, but if the building responds as predicted, this will be of less interest then if it '_'oes not, in which case alternate locations would be more desireable. This is related to our recommendation not to mask surfaces through application of new compounds, in summary, we conclude that the performance chorocteristics of the DGB are 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 TERA CORPORATION
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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 incopoble 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 localized differential settlement is observed, remedial corrective measures could 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 biannuclly (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 constroints. i 7-l TERA CORPORATION
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