ML20080T422

Draft Rept, Review of Diesel Generator Bldg at Midland Plant

ML20080T422
Person / Time
Site:	Midland
Issue date:	09/23/1983
From:	Costantino C, Chris Miller BROOKHAVEN NATIONAL LABORATORY
To:
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FOIA-83-707 NUDOCS 8403010217
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Text

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23 September, 1983 DRAFT REPORT Review of Diesel Generator Building

[

At~ Midland Plant by C.A.

Miller

.C. J. Costantino 1.O~ INTRODUCTION p-

.This report describes a

study undertaken by Brookhaven i

. National. Laboratory (BNL) to evaluate the e:: tent to which

. settlement cracks observed in the Diesel Generatc-Building (DGB)

'at the: Midland Nuclear Power Plant impact on the ability of the building to satisfy design requirements. Dr.

R.

B.

Landsman, of

Region-'III, has raised questions regarding this safety issue (Ref.

1 ) ~.

'The specific objective. of this stud 7 is to asses the

~

Jsignificance of his comments and to prepare a written response.

~This objective was achieved by reviewing the axisting pertinent.

r

. work (

published

reports, testimony ano analytical

' studies), and ay interviewing key personnel so that a

correct

. interpretation of the work ~ performed could be made.

Additionel calculations were specifically ommitted f rom the scope of this

~

" study. All of the conclusions drawn in this report are' based on an assesment of calculations and studies performed by'others.

The. study-described herein was carried out during the period b{_.

tof-'Auguct'thruugh September 1983.

On August 4, a meeting was held lat.NRCito. discuss the problem and to obtaan some of the pertinent 8403010217 831214 PDR FOIA

-GARDES3-707 PDR 2

~

~~1-J

p l:

.}

.- ( -

' literature. Some of this literature was carried back to BNL while other' documents were mailed to NRC during the following week.

I Appendix A contains a listing of all reports used during the program. On August 24, a meeting was held at Bechtel Corportion of fices in Ann Arbor, Michigan. Presentations were made by Bec.htel and Consumers Power staff summarizing the work nerformed by project personnel to demonstrate the adequacy of the DGB.

Their f

- ccr.sul tant 's (Dr.M. So:en of t'he Univdrsity of Illinois and Dr.

G.

I Technology Laboratories) also discussed Lorley;of Construction' their work. An inspection of the DGB was held on the evening of 6;

August'24 and during the morning of August 25. At thi s insoection, the cracks were observed although no new detailed crack maps were

'made.

Di scussi ons were held with construction personnel to p

' determine'the sequence of concrete placement.

Furtheriinterviews were held at NRC on September 8.

Individual interviews were held.with Dr. Harry Singh (soils consultant for NRC f rom the Army Cc eps of Engineers)

Joseph Kane (NRC staff) and Lyman Heller (NRC-. staff).

A combired interview was also conducted with Frank Rinaldi (NRC staff)

John Matra (structural consultant for NRC from Naval Special Weapons Center)

,and Dr.

Gunnar-Haarstead ( m*mr uctural consultant for NRC). The purpose of W

theshcanterviews wa+ to explore the role each played in the design b

and analysis of the DGB and to learn of their concerns regarding

~

the. adequacy of the D3B.

An audit of the DGB calculations was held at Bechtel's Ann 1.,

Arbor ~ offices on September 12 and 13. Dr.

Sozen was present on

-t:

Septemberf 13. - The f ollowing items were reviewed in detail during 6:.

Y L.'

C L

'l this audit: numerical models used by Bechtel to calculate stresses

.in the DGB due to settlement I the magnitude of stresses due to the various load cases i the method,of determining stresses from

.c crack data 1 the accuracy of the survey methods used to monitor settlements I and the concrete pour. data. A meeting was held with Dr. Landsman of Region III on September.

13, at which time his specific concerns ~ raise ~d in Ref.1 were' discussed.

This report'is organized as follows.

Ar.

evaluation of the literature is presented in Section 2 of the report.

Section 3

contains BNL's assesstnent of the adequacy of the

DGB, while specific reponses to Dr. Landsman's concerns are given in l'

4.

Section Conclusions are listed in Section 5.

2.0 EVALUATIDi' OF PERTINENT WORK 6

The material on the DGB which was reviewed during

]

the course of this study is divided 'into si:. categories; 1

namely, hi stori cal description of the structure and its settlement behavior; I

j devel oped crack patterns; structural analyses to evaluate settlement stresses; treatment of other loads and stresses; and survey data. The. material in each category is described and E

).)

evaluated in this section of the report.

.2'1 HISTORY OF STRUCTURE The DGB' -i s a

reinforced concrete shear wall building consisting of five cross walls connecting a north and south

[

wall.

The. interior walls are 18" thich while the exterior wall I-thick.

s are 30" f.

The structure is 155' by 70' in plan and is 51' high with h

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e

/

an intermediate floor slab located 35' above the foundation.

Wail footings are located under each of the walls, the footings being 10' wide end 30" deep. The building is founded on about 30' of various fi)1s overlying the natural glacial till.

The fill was placed from 1975 through 1977 with construction of the DGB begun in October 1977. Concrete was placed in 6

lifts 4

as follows:

October 1977 to elev. 630.5 (f oundati on) l.

December 1977 to elev. 635.0 March 1978 to elev. 654.0 August 1978 to elev. 662.0 December 1978 to elev, 664.0 February 1079 to elev. 678.3 Within each-' lift th6 pours were generally made from east to west.

Construction joints occur in the middlo of the cross walls and at the west end of each bay for the north and south walls.

i Large settlements ar.d cracks in the concrete were noticed while the lif t going to Elev.662 was being poured.

Construction was halted while the problem was being studied. It was concluded that the.large settlement was due-to poor compaction of the fill material. This settlement caused the structure to " hang up" on the

~

-duct banks which penetrate the footings on the cross walls.

The duct banks were cut loose from the DGB foundation in November 1978 k

and construction of the building restarted.

In January 1979 20

' feet of' sand surcharge was placed on the site to consolidate the

. fill._This remained in place until August 1979. In September 1980 a permanent dewatering system was installed to maintain the water

'(1 table below Elev. 610.

F;,

):

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c[

2.2' SETTLEMENT HISTORY The DGB is 'f ounded on epproximately 30 f eet of fill

material, underlain-by a very stiff glacial till about 190 feet thick.

A dense sand 1.ayer about 140-feet thick lies belcw the

till, which is in turn underlain by bedrock. The-maj ori ty of the fill was

.placed at the site between 1975 and 1977, with actual foundation construction completed - b.y..,J anuar.y 1978.

During July

1978, e

settlements of the order of 3.5' inches' (Ref. 7) were noted which

'were greater.than the-original

-40 year predicted settlements.

4 App ar,entl y, consolidation of the' fill was taking place as h.

structural dead loads-were applied.

In

addition, the four electrical duct banljs un_ der the structural crosswalls were acting as hard points to the foundation since they were in turn being supported-by the stiff natural soils below the fill.

This caused rotation of the building about the duct banks.

Construction'was halted during August

1978, a

soil boring program undertaken to determine the problem with the fill and Drs.

R.B.

Peck and A.J.

Hendron retained to advise on the r emedi al action. The exploratory program consisted of 32 borings (with no

-undisturbed sampling) and 14 Dutch cone.penetrometers.

These i:onfirmed that the fill' had been improperly placed (in an

-extremely variable density state) and consisted of varying amounts E

"of. cohesive as-well as granular backfill. Lean concrete was also encountered in the backfill. The thickness of silty clay backfill I

was found to be greater under the. south-east side of the building l

' leading to,the generally larger settlements on this side.

A surcharge program was implemented to attempt to consolidate

~

t m.

'the fill more_ uniformly. In eddition, the duct banks were cut loose from the

' foundation in November 1978 to eliminate the

' foundation hard points. Surcharging began in January 1979 and remained in place until August 1979, when it was determined that primary consolidation had been completed.

Instrumentation (primarily. settlement plates and Borros anchors) placed in the (fill was used to. arrive at.th.is' con.clusion-It should be noted that the consolidation test results,' obtained f?om undisturbed san'.ples taken af ter completion of the surcharge program, did not k

confirm this conclusion.

Data was sufficiently scattered to indicate that the fill may not be uniformly consolidated.

At'the_ completion of.the surcharge program, it was decided i

that since loose sands still existed in the

fill, a

permanent I

dewatering system would be installed to preclude the potential for soil liquefaction during a seismic event. This dewatering caused additional settlements to be developed at the site, but apparently r

-these were'related to deep seated consol i dati on of the natural

. soils under the

fill, and would be more uniform than the settlements caused by the fill consolidation.

It is questionable whether the piezometer data was of any significance in analyzing the excess pore pressure condition developed in the fill during the consolidation process.

The readings indicate generally very low pore pressures, about 1/20 the magnitude of the applied surcharge pressures. It is not clear i

i in fact whether the fill was ever fully saturated at the time of

the surcharge program. Unfortunately, the boring program conducted

(

after the surcharge program was completed, did not include cone _

~

4 1

S

(

. penetrometer soundings for comparison with the readings taken before the surcharge was applied.

Peak settlements anticipated at the end of 2025 (actual b

settlement's to date plus secondary settlements from now till then) are specified in Ref. 7 to vary from 4.79 inches (under the NW corner) to 9.33-anches (under the SE corner). However, it should be mentioned that the,exac.t,,, settle. ment history at the various settlement markers at the DGB is open'to question. For example, it

.L is mentioned in Ref. 7 that the maximum settlements in August 78 (l

were about 3.5 inches. Yet the data used in the stress analyses for.the presurcharge period (Figure ES of Ref.

7) indicates k

peak settlements of only 1.99 inches. It was stated at une of the h

Bechtel presentations that prior to cutting the duct banks loose 4

.from.the footings, footings along the North wall actually lifted I

off from the' soil, with the DGB rotating about the duct ban' c.

There is no indication of this behavior in any of the settlement data used in the computations.

Ref.

8 lists the settlement

-increment from 8/79 to 12/2025 to be 2.36 inches under the SE

. corner of the building. For-the same period, Ref.

7 lists this data as 1.89 inches. Thus some inconsistencies appear to exist in the various documents.

f 2.3 CRACK PATTERNS

-After it was determined that settlement was a problem, Bechtel

~ initiated a program to monitor cracks in the structure. In general cracks were visually observed and an optical comparator used tu

)

Crack widths greater than 10 mils were of determine crack width.

4-9

'?

' specific interest as this corresponds to reinforcing stresses of about_10 ksi.. Crack maps were prepared based on surveys conducted during December 1978, September ~1979 February 1980,

and July

1981. Dr. Corley observed the cracking in January 1982.(Ref.6) and confirmed that the general pattern of cracks agreed with the July 1981-Bechtel. crack maps. He prepared a detailed crack map for the center interior, wall.. A. co,mpari s.on of this center wall map (Fig.4.21 of-Ref.6) with that prepared by Bechtel in July 1981 (Fig.4.17 ) indicates that more cracking had occured although the widths of the cracks appear to be about the same.

Cracks were observed during.the BNL inspection of the plant on August 25,1983 and some photographs taken. In general the pattern of cracks appears to be similar to the previously mapped cracks.

However cracks, which had not been shown on any of the Bechtel

' crack maps, were noted in both the north and south walls.

These k

additional cracks are in the lower level (up to El ev. 664) and

.run at-.45 degree angles to the horizontal up to the cross walls.

The first crack maps prepared from the December 1978 survey indicate vertical cracks in the cross walls which begin near the

' bottom of the wall and run up to Elev.664 (this was the top of the concrete. pour at the time the settlement problem was first

,. ; r.

noticed). The pattern of cracking is more severs in the east side

. nf the. building. This crack pattern is compatible with the model L

that assumes the cracks result from flexural stresses caused by the building " hanging up'on the duct banks". No crack maps were

' prepared for the north or south walls.

(

The second set of crack maps were prepared from the September i

. jsLW=

t, c

~

1 3-ff 1L979 survey. In general many of the cracks which occurred in the

. east':wal'1 prior to placingi the surcharge do not appear on these Jmaps.1Th'e east: center and center walls show the same type of crack patterns-as shown on,the first crack maps except for the

! appearance of; additional crack's. These maps also show cracks in f

fthe uppercleveloof the-b'uilding.'These cracks. occur near the south 2

side of the building in the.c,ross walls. The cracks tend to be vertical with some inclination of theJcracks near the south wall.

Some; cracks--are indicated in.these. maps for the south wall.

"I

~ Primary cracking occurs in the east side of the wall and are i

The north wall is

[

concentrated'in the. upper' portion of the wall.

i.-

shown,to-be more severely cracked than the south wall _and contains mostly vertical cracks in the upper part of the wall.

The cracks appear to,be centered aboutlthe three interior walls.

-The third set of crack' maps.were prepared from the July 1981 asurvey. These maps indicate the same type of cracking as before

although the cross walls now contain more cracking near the north

side of thejbuilding than was. evident-before.

The west wall contains many more' cracks than were shown previously. These cracks fru'n from"the Elev.664 level: down to the base of the structure.

.Itfappuars that many of the cracks:which have occurred may be attributed.to the building resting on the auct banks. Other cracks hnve-occurred-however 'which' were

.most likely caused by dif f erential : settlement of the wall

. footings.

Comparison of v

tsuccessive crack observations' generally indicates that more cracks s

-are occurring but that.the maximum size of the cracks is still

(

Labout'20 mils.'

p 94

!i-we

2.4 STRUCTURAL ANALYSES The various analyses which have been used to evaluate stresses in the DGB are dl~scussed in this section.

The first analysis described is the method used by Bechtel to estimate L-stresses due tc settlements for use in its load combination study.

This analysis mahes use nf tpe stra.ight line approximations to the profiles of the settlements of the North and South wal l s.

The

^

- second and' third analyses described are the Bechtel and Matra

. studies, which attempt to use the actual measured settlements to estimate settlement stresses. These analyses, though different in detai).. lead to the similar conclusion that the settlement measurements were (and continue to be) in significant error.

The fourth analysis describes a

cruder model which attempts to approximate an upper bound to r.ettlement stresses by looking at crack measurements.-The first three analyses are based on detailed finite element model s, while the fourth is based on crack patterns and crack widths.

12.4.1 Bechtel's Computation of Settlement Stresses (Ref.2)

Since the building settlemente occurred when the structure was

. in various stages of construction the settlement stresses were evaluated *for four different time periods. The first period. spans from the beginning of construr. tion through August 1978 at which

- time construction ~was halted. The second time period extends from 1

August 1978 to January-1979 during which the duct banks were cut loose from the structure and construction resumed. The third time --

E tt -

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period entends f rom January 1979 to August 1979 during wh2ch time the surcharge.was placed. The last time period entends to the year 2025-and includes ' measured settlements from August 1979 to December 1981 ac well' as the predicted settlen.ents over the forty year life of the structure.

The actual measured settlements were used to calculate T

~

-str esses f or - the. f irst per.i od.. Stresses-were calculated in each of 1

the vall's by determing 'the; arc of a circle which fit any three adjacent measured displacements. The radius of'the arc was then usedLto find the resulting bending moment in the

wall, and the moment us=d to calculate stress..The maximum stress in each of the walls was assumed ' to exist over the entire wall. The stress in the south wa'll was 11 ksi; the east wall 6 ksi i and all other walls 2 a

to

'ksi..

The increments in stress which occurred during each of the

.other.three time periods were evaluated using a

finite element model of the DGB. This model was constructed and run on the Bechtel. version of SAPE(BSAM. The building was defined with 853 nodal ~ points. Plate elements were used to model the

walls, and

. beam elements used f or the f ootings.

Eighty four (84) boundary

~

elements were used to model the vertical soil stiffness

(equivalent to the coef ficient of subgrade reaction). An iterative

,n process was then used.to determine the stiffness of these boundary

' elements.'A best' fit straight.line -was first fit through the

. measured settlements for the north wall and another straight line fit to the data for_the south wall. It was shown that the measured

'S

~ displacements - departure f rom the best fit straight lines is within

~

-w w

~~

u O

x

~

k o

W' m:

[

n; '

[

n,f the toIntlarice,,of ' t'h>stIrve y.deia.

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Dead Icad reactions were ne::t

,-tj a

c gg N 4 estimated at each of thu 84 boundary elements.

The stiffness of g

^

'y

~

anyQoi1 element.was then determined as the ratio of the desd 1oed

.y s j eactioh-to;,the displscement of the best fit straight 1ine.

The u-

.rg e,,

DDSAP..proprjam was ru, n, and the reaction-found at each of these fb k

ibouridary el aments. ' A new. sti f f n'ess was then calculated as the n.-

gg. W@

,-N rJtio7af the reactiors?td the.di'splacement of the best fit straight a

c

-- id i n e.'

1

.g This ',pr ocess was< continued 4f or s'ever al iterations.

,It.is cui cipinion that thir. 'model will yi el d unconservative

.n

,w 7 estimates'of' stresses ~.' If the iteFation process were successfully s

,q ; dompl eted,- the'de' formation of the north and south walls will be N

straighti1ine9./Thri eniy stresses that would be computed would y.+e tne structure caused by the

$then ' occur. duef to racking

'of-

./A w

. difference in the. north and south wall straight lines.

It should n

-.t.

s be cle'ar).that if a best fit' plane could be passed through all the

~, -

- s

,. ~.

' ' Vet tl ement ip oi n,t s - under both'Ethe North and South

walls, no c

c.

,i ssresses would be computed "anywhere in the building. The stresses w.

e.1

-computed *by this' approach a o'fa functYon of which iterative cycle s

is used-ito'< define to. soi l spring parameters, and bears no presemblance-to the actual soil conditions at the site. There is no Y

+

. reason.2to expect that the soil stif f ness should vary f rom point to s

7poirit; as.-shown by. the - anal yse. We therefore conclude-that this 3

/: -

a

-approacheto-compute settlement stresses is inapprcpriate.

(

~..

.2L4.2.Bechtel's Analysis Using Heasured Settlements (Ref.3)

,e e

J, s

s,

[This.analysisfEastoerformed usingS the same finite element c -

'model described above.

This t i c '-

however, the known survey

.j:

displacement data.was input to the. proc, m at the ten (10) wall X

. 12_

s fs p.

fy'w..,-

-.-+,---r

-v

V..

p intersection points, lhe settlements used were the disp 1ecament ph(

increments-sceasured f or the f ourth time period described above. At

.the remaining 74 boundary element

points, the structure was f

allowed to deform as required to'~ maintain equilibrium

. forces

(

equal zero). It was found that computed stresses were very high in

'those elements adjtcent to the wall intersections, but. fall off rapidly away'from these po.ints. Th i.s indicates that the analysis overly-penalizes the ' structure by i'mposing large concentrated forces ~et the wall intersections. In fact, at some

points, the soil:is required to pull-the structure downward to match these 3

known displacements.

A modified analysis was. performed by Bechtel at the suggestion of-the -audit team.

Rather than input only the ten known

' displacements, a smoothed curve was. generated which matched the known settlement' data but eliminated the sharp profile changes

-developed ir the analysis described above. A best fit polynomial

-was passed th' rough both the North and South wall settlements, and displacements' computed at all boundary elament points of the finite element model. Comparative plots cf wall profiles indicate th'et-this approach would still yield high stresses.

However, the audit team has to date not received the results of this analysis.

/

,2.4.3 Matra's Analysis Using Measured Settlements (Ref. 4)

The-analysis performed by Matra is similar in intent to that

' described above.1 Differences between the two are as follows.

First, this finite. element analysis was performed for all four time periods described in Section 2.4.1.

Three separate finite

- 3 3_.

'F e

--'wwc-s-

T----

r%--

9 m

s+

v i

ww-

~

q--

.qs,

3 x

+

y g

g

3

.M

.g

~ >

.M 4> y.1 y

-s

~

' jN 2 n.[-

' l f,.y =f ie1ement' moders;were used to d$ fin'e the DGB st various; stages.'of

,,3 5;

^

construction.: For;.each problem analy::ed, the Niewn iettlement data q.

?

~ _ -

_,~s

. * < er

~

~1at the' wall 5i_ntersdctionypo1nts

) i npldt to-the models.

The-was 3.j 1

)

~.

report ? does : notGspeci f i tial ly sthte 7what* i,nput[was' used at.the

..s.

~

remaibing;bo'undary; element.poin$s.between the' wall intersection.

However,f atithe interview 7 Matra. stated' that a linear displacensnt n

s

^-

9 g.

profile was4assumedybetweentthese.-pcints.' The-stress results of

. <r ctheianalysesfare similar to those desc'ribed.above.for the Bechtel

~

i study', : wi th;.si mil ar conclusions.. reached.

In' fact,

.it-can ' be

+

6

'enticipated 3 hat the Matra stress ~ calculations woLil d be even w

~ higher 1than;the~ corresponding Bechtel results due to ~ the linear Jessumptionfbetween'dataLpoints.- If in fact this was

done, the rz conclusions 1 reached in that : report would be of little 've.lue. since y",

isuchl highj' bending ~-stresses

.would generated, at these

be

~

^

discontinuitles.

v

/

2. 4. 41 Estimation of. Stresses!.From Crack Data dRef. M w ;

1

Soren cpnsidered. the' problem of-predicting reinforcement-

. t.

^

J g

_ stressesufrom a. knowledge of the crack patterns. He observed that

~

the' usual ~; problem is-to predict crack width based upon a'

given Treinf orcement. stress. J When thesei' methods are applied to' the-DGB s

^

(center wall,ya:201ksi'~ steel stress is consistent with

~

crack ax

-' a -

. He also adds.the crack widths for a,

serie's of e

width:of 20. mils.

3 i

' " cracks incthe center wallian'd equates th~is to the total elongation x

.in7thel reinforcement. Using ansestimated gage - length over which

[

fthisielong'ationCoccurred he obtains an estimated stress of.24 ksi, Iandlindicates a-probable range o'f ~ 20-30 ksi considering the 4

s t

r

)

O

,._,,.i e-

_a k._I '

4, _ u.

/, y

w C

uncertainties,of the method.'It is likely that these stress values would be-reduced with time. - A major cause of cracking was the hard

" points provided by the' duct banks. Nhen these-were cut

free, one would expect.the stresses-induced by the ur.even 1 support to be p

relieveu. Creep in-the concrete ~ would also tend to relieve the

^

L

.settl ement-i n'duced ' str esses.

Rinsidi (pg..

11086

.c f.,

the.,t e sti mony) reported that he ctaluculated stresses using So:en's met' hod in each of the 5

cross wall s,: as well as the. north 'and south walls. He then added these stresses to.the. maximum stress reported in each of the walls by

~

'Bechtel. The resultant ma::imum reinf orcement stress was found to b'e.less'than 54 ksi (the allowable limit). It _was noted that the Bechtel stresses.already-included settlement stresses (to an unknown degree.however) from the analyses described in 2.4.1.

The crack-based' estimates of settlement stresses were added to the

maximum of._the1Bechtel stresses without regard to where they occurred.'While this is a

conservative

approach, there is no

. cocumentation of uthe computations. It should be noted that there

~

would be some question in'the' application of this method on those g

' walls where relatively'few cracks occurred.

-255 STRESS TOTALS The' finite element model described in 2.4.1 was used to calculate 1 wall: forces from all loadings except for the seismic loading.

lA lumped mass model was used to ' determine forces resulting. :from the -seismic loading.

These forces were then i

combined according to the ' load combinations requried in ACI 318

[

y

• ^

\\

e.

Xi2,

,3

y.

s t 6

J/

-an,dLACli349.- Critical elemerts were then identified in each of the 4 ~

[x; iwalls and Bechtel P s' program' OPTCON used to evaluate reinforcement

,......a m.

stresses..OPTCON determines-the reinforcement stress resulting f roin out-of @l ane-bending moment ' plus in-plane sheer ~ 1cading.The J

~

p

!shed capacity :of the' concrete is deducted 'f rom the total' shear T1oadi with':the ' difference assumed. to be carried by the t

p' reinforement.,,.The ' fallowing, areJ' peak reinforcement stresses ireported by[Bechtellfor-the critical l'oad cases: north wall 22 1

fksi 8,isouth, wall'- 34 ksi- ; - west ? wall ' -- 29 ksi '; east wall 23 y,

ksiil land:interiorz walls - 20 'ksi. ~ The ' allowable steel. stress is 3:

54 ! k si '.

i, >

2. 6MSURVEY : DATA

)

!Bechtel repo-ts-~that the' accuracy.of the survey data

-1 until. the surcharge was

. describing 1the DGB(settlements.is-1/8" j

I

r;emoved sand,1/16"~ since that' time. ' Standard survey.techni ques -and equi pment.' wer e "used.

j b--

t

,.3.OjASSESSMENT.OF THE-DIESEL GENERATOR' BUILDING.

The,DGB) - has':~ undergone -very

'large ' settlements which have d

s

!undouotedly'eaused1 serious-structural distress. This distress is

manif ested 'in ~the

cracks whi'ch have occurred in the building.. The

~

g-W~

j

'A q

7 purpose of;this sectionLof the' report is.to give an opinion as to

,y

- R: ~?

((1)4whether'the building'is structurally sound and (2) whether the E uilding'still; meets.thel criteria-as stated in the FSAR.

b

-Antimportant issue is whether the major part of the settlement

///

A

'h'as' occurred.7-The: settlement data indicate' that settlements are a4; F

4 9

7

- c

4 f

4

L (_-

_. well' into' the secondary--consolidation phase co that large additional settlements would not be anticipated.

This leads to confidence that predictions of the adequacy of the_ structure based on' settlements which-have taken place to date should hold for the life of the structure. Certainly, settlements should be monitored an'd-the problem reconsidered should more than the anticipated additional settlements. occur. Relative settlements of points on

'the st'ructure of

.005"I are significant.

The accuracy of the

~

settlement-measurements should be-refined to reflect this s

requirement.

While significant cracking has. occurred in the structure, it would appear that.there.'is little evidence to indicate that the H

' structure is unsound. The structure is very. massive and is not subj ected' to 'large l oadings. Even the torr. ado and seismic l oadings do'not~ introduce large' stresses and usually.these stresses occur at locations that are not critical locations for the settlemnet E

stresses.

[;

It is difficult to show that the stresses in the DGB meet the criteria of the FSAR. Bechtel's straight..line analysi s (see 2.4.1) is' based on-the claim that the-settlement survey data is not sufficiently accurate to calculate structural stresses.

The Jadjustment-they-make to account for this inaccuracy gives results c

that are :likely unconservative. If conservative assumptions are r

' made then.the-calculated stresses are too large to satify the criteria-cnd'not consistent with the crack-patterns observed in s

the structure (see 2.4.2).

It is dcubtful whether any analysis could now be developed which' would provide more realistic t

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estimates of settlement stresses with the required degree of

~ confidence.

The most likely source f or obtaining reasonable estimates of

" settlement stresses are the crack studies (see 2.4.4).

However,

'these studies must be documented much more completely than has been done to date. It is imperative that significantly better methods'be used to monitor, crack growth than is currently being

. considered.-Whitemore. strain gages sh'ould be used extensively.

' Plugs are attached to the concrete on a 2" gage. An instrument is

.then used to measure-the distance between the plugs. Accuracies of L'

.0001" is routine. Such gages would give a good picture of the

.overall behavior-of-ths: cracks. It should be noted that the repair of. cracks would not interfere with the use of these instruments.

LNo special " windows'" need to be maintained during the crack repair program.EThis program of ' crack monitoring is also important because-there. is some indication that cracks in the DGB have not stabilized.and that-the number of cracks may in fact be increasing.

J4.0 RESPONSE TO' CONCERNS OF R.B.

LANDSMAN

~

.The Region III inspector has raised four concerns (Ref.1) rega'rding the adequacy of the DGB. Each of these is addressed in the following.

Concern l':~ FINITE ELEMENT ANALYSIS The first concern deals with the Bechtel finite element models i

and 2.4.2) of the DGB used to evaluate stresses due to (see 2.4.1

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,f" (settlement'. ~ There Are f our -' objections made to the models.

W %,'p Concern ~is raised with regard to.the use of untracked section properties ~while the concrete is known to be cracked. All concrete

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.stbuctureslare cracked and'it:is' standard practice (specifically fpermitted in" the'.ACI -code)-

to -determine forces in concrete structures: based on gross.section properties (i. e.,

neglect the

cracks :i nithe. concrete.and the r einf orcement). If cracked section e

then the stres'ses calculated by Bechtel

properties were'used

(2.4.1) would have'been smaller. Therefore neglecting cracks in 4

thi slanalysisiis a conservative ' approximation. On the other

hand, OG'

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- the-analysis-reported in'.2.4.2 was used to show that the measur ed settlements result:in'str, esses whichcare so high that much more

~ severe cracking :would(be e::pected than was observed. It was then i argued ! th'ats the' ? measured val ues - must be in' error.

If cracked i

a' sections'were.' assumed for this analysis the. calculated-stresses

'wouldj have' been smaller and perhaps more. consistent.with the

. observed crack patterns.

1The. straight ' li n'e representation of the settl ements al ong the

. north) and.; south _ wall for the analysis ~ reported in 2.4.1 is said'to f tie in j errori-As - i~ndicate.d in? that section of this report,.

it-is

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S' (ouri csp i n'i on.' t hat ' this, analysis'~will result in unconservative

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s l predi ctions: - of -stresses :due-to' settlements.

As such 2t is Q "'

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• considered !to;be an inappropriate -analysis.

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+Th'e third part'offthis concern-raises questions regarding U

g M time ~ effects;of"the settlements.- Bechtel does calculate stresses y+

iforddifferen.t phases of the settlement. The structure was changing w

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during.theisignificant settlement period.. Construction was still q

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-i n progress during the largest' settlements.

Therefore the p

structural geometry. changed as did the concreta-properties (while maturing). ~ The~Bechtel models did not account for these changes.

This would have beer, conservative f or - the calculation of stresses

-but unconservative for the analyses performed to show that the measured ' settlements' were in-error.

'The f ou-th ' objection d.eals with, the claim that the NRC staff

did not ' approve of 'the Bechtel ' analysi's. It appears that this is the case and the intention of the staff was to use settlement stress data based on an analysis of the cracks rather than the finite element analyses.

Concern 2 : RELIABILITY OF MEASURED. SETTLEMENT VALUES

~

The analyses reported in 2.4.2 and 2.4.3 were used to show that_ stresses computed from structural models subjected to the measured ~ settlements are very high and would indicate cracking in (the structure where no cracks are observed.

The objection is

.raisedLthat a-linear model was used and that a

nonlinear model accounting [f ot-plastic effects.wowld result in a redistribution of

_ stresses and the same conclusion may not apply.

This observation is true but if all other factors remain the same the general conclusion drawn from these analyses would probably not change.

As stated above, however, there are other factors which when coupled with this objection may result in a dif f erent ennelusion.

'The other important factors are:

the assumed shape of the settlement between the. measured. points and the differing geometry of the DGD when the various phases of settlement

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Q.4 e Concern ~> : 'S1RESSES DETERMINED FROM CRACK SIZES 7

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.'.If the finite element-analyses are..not reliable then one g.,-

rV talternativefapproach is.to; find settlement. stresses from a-study sfithe. crack. sines.IThe objection raised is that this approach is

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notfconsistentiwith. normal;;e.ngineering practice and that there are n'o equations lavailable ko evaluate stresses from crack data r

%fV iwhen.the stress fields' are as complex as occur in the DGB.

It is

- true: thatL-thi s ' would not be ~ standard ; practice, but "non-standard" i

I analyses-may be'used provided they are sufficiently documented and

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'sh'ownfto'givefresults that-are conservative.

m, DGB

.An' approach that'could predict' settlement stresses-in the-Jwithin~1cose tolerances could.probably be used to demonstrate its

[ adequacy.'Thislis-true for two. reasons.

First, stresses in the

,e j struc ure -dUe[to!.other !1oadings are -rather - low and there is a

'Iargel reserve:for settlement.stresset.LSecond, settlement stresses

'are c self -'reli eving. : 1f 'l arge settl ement stresses occur and local cyieldingcof the reirif orcement occurs, the resulting deformations J.ofy.the structure,willoreduce the set 11ementEinduced loadings.

?The documentation of-the crack analyses used to determine Ap'

[s~ tresses fis (not suf ficient'. There is-no calculation on record

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Elich calculates stresces in'all of' the walls using this method.

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2TNere is.also no' written' justification, showing that.the method may o

cc 1be used for structures like the DGB.-

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! Concerns 4. : CRACK MONITORING e-

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This concern ' deals 1with-the lack of a

good crack monitoring

' system andxspecification ofzaction to be taken if the cracks

. e x c e e d' certain limits. As stated in Section 3.0, it is our opinion that the planned crack monitoring system is not adequate.

More reliable _ gages (e.g., Whitemore Strain Gages) should be placed in areas where cracking-is now evident. These gagcc can be used even

-after crack repairs are made..

Two limits :are now defined in the current crack monitoring program.' If the crack width' reaches.05" (Action Limit) a meeting will be held to~ evaluate what steps to take when the cracks reach the next-limit. The next upset limit is set at

.06" (Alert Limit).

-It is,our opinion that the form of this plan is adequate but that

[the specific threshhold numbers must be based on a resolution of the current. settlement stresses. A safety margin must be left for the.other potential loading events,'such as-tornado or seismic iloads, with the remaianing ellowable stress allocated to future potential settlements.

Once this limit was reached the only solution would be to make a structural repair. The exact form of this repair would depend on the _ location and extent of the crack which exceeded the limit. The planned response cou1~d not specify the nature of the repair but "1 couldEindicate 'that an exceedance of the Alert Limit would result 1:

in=al structural repair rather than performing additional analyses.

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3.0 CONCLUSION

S Based-on tne review of the studies performed to demonstrate

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the edequacy of the DGB, the-following conclusions are drawn:

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1.fThe.settlementEdhta indicates'that primary consolidation of the

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fillLis completed. However it -is recommended that-the anomolies kin ' the 'docu'mentatt'on of the. settlement hi stcry be resolved.

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1 jt i's unl"i ke19 that yP a.

satisfactory stress anal ysi s can be

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performed -based on t,he. measured settlement data.

It-is

..k recommended?that settlement stress'es be estimated from the ferack. width i ata.' 1The existing work that has been done in this d

area must be completely' documented.

T f

,W A jIt.[ appears that' cracks in,the DGB are continuing to grow. It is Iessential that a better. crack monitoring program be-established asLoutlined'in-Section 3.0.

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4.?The: upset' crack: width' levels' _specified in.the crack monitoring I

programJshouldibe.Lchosen so that a. sufficient stress r.iargi n is

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' : available~ to ' resist ' the critical load combinations.

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iff.

z S. :If : the Alert' Limit (in crack ' width) were

exceeded, specific

structural-: repairs should.be mandated.

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REFERENCES l

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21. Memorandum.'For R.F.Warnick Thru J.J. Harrison From R.B. Landsman q,

,e 1, - Subj ect/ Diesel Generator Building Concerns at Midland, dated

.4'+-

L Jul y..: 19s 1983.

2. Straight'Line. Analysis y..

'. Ni 4'

m

Eh 4

4

-f' 3.Bechtel ' Calculation RNo.

DD-52. 7 (O)

Finite Element

~ Calculation of Settlement Stresses Using. Actual Displacements

4. Structural Rea'nal ysis of. Di esel Generator Building. Utilizing Actual Measured. Deflections as Load: Input, by John

Matra, Naval. Surface-Weapons Center

5. Evaluation of the Effect-on stru-tural Strength of Cracks in the Walls of the Diesel Generater Building Midland Plant Units 1 and 2, by Mete Sosen, February 11,1982.

6. Effects of' Cracks.on Ser.yiceabi.lity of Structures ct Midland

~

Plant, by W.G.Corley

.A.E.Fiorato.

and' D.C. Stark, April 19,1982.

7. Executive' Sunimary, Diesel Generator Building, Midland Plants Units l'and-2, Augurt, 1983.

8. Letter from CPCo to NRR dated Oct 21 1981; Enclosure 1, Tech.

Report, Structural Stresses Induced by Differential Settlement of-the DGB..

~?.

4 9.'

i' L i

.o j.

3.Bechtel Calculation No.

DO-52. 7 (O)

Finite Element Calculation of Settlement Stresses Using Actual Displacements

4. Structural Reanalysis of Diesel Generator Building Utili=ing

-Actual-Measured Deflections as Load

Input, by John

Matra, Naval Surface Weapons Center

5. Evaluation of the Effect on Structural Strength of Cracks in the Walls of the Diesel Generator -Building Midland Plant Units 1 and 2, by Mete Sosen, February 11,1982.

6. Effects of Cracks.on Ser.yiceabi.lity of Structures at Midland Plant, by W.G.Corley,

A.E.Fiorato.

and' D.C. Stark, April 19,1982.

7. Executive Summary, Diesel Generator Building, Midland Plants Units'1 and 2, August,.1983.

8. Letter.from-CPCo to NRR dated Oct 21 1981; Enclosure 1,

Tech.

Report, Structural Stresses Induced by Differential Settlement of the DGB.

.Q..