ML20107H673

Review of Waterford III Basemat Analysis

ML20107H673
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Site:	Waterford
Issue date:	04/16/1984
From:	BROOKHAVEN NATIONAL LABORATORY
To:	NRC
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FOIA-84-455 NUDOCS 8502270238
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Text

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REVIEW OF WATERFORD III BASEMAT ANALYSIS

[ Structural Analysis Division Departnant of Nuclear Energy Brookhaven National Laboratory Upton, NY 11973 April 16,1984 t

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TABLE OF CONTENTS Page No.

INTRODUCTION ................................................. 1

GENERAL COMMENT

S ............................................. 2 STRUCTURAL ANALYSIS TOPIC REVIEWED ........................... 3

1. Dead Loads .......................................... 3

2. Buoyancy Forces ..................................... 5

, 3. Variable Springs Used For the Foundation Modulus .... 6

4. Vertical Earthquake Effects ......................... 6

5. Side Soil Pressure .................................. 7

6. Boundary Constraints ................................ 7

7. Finite Element Mesh and Its Effect .................. 8 CONCLUSIONS AND RECOMMENDATIONS .............................. 8 APPENDIX A LIST OF CONTRIBUTORS ............................. A-1 s

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INTRODUCTION At the request of SGEB/NRR, the Structural Analysis Division of the Department of Nuclear Energy at BNL undertook a review and evaluation of the HEA Waterford III mat analysis documented in Harstead Engineering Associates (HEA) Reports, Nos.'8304-1 and 8304-2. Both reports are entitled, " Analysis of Cracks and Water Seepage in Foundation Mat". Report 8304-1 is dated September 19, 1983, while Report 8304-2 is dated October 12, 1983. Major topics addressed in the first report are:

(l') Engineering criteria used in the design, site preparation and con-struction of the Nuclear Power Island Structure basemat.

(2) Discussion of cracking and leakage in the basemat.

(3) Laboratory tests on basemat water and leakage samples.

(4) Stability calculations for the co.ntainment structure.

The second report concentrates on the finite element analysis and its results.

-Specifically, it describes:

(1) The geometric criteria and finite element idealization.

(2) The magnitude and distribution of the loads.

(3) The final computer results in terms of moments and shear versus the resistance capacity of the mat structure.

Supplemental information to these reports were obtained at meetings held in Bethesda, MD, on March 21 and 26,1984, at the Waterford Plant site in Louisiana on March 27, 1984, and at Ebasco headquarters in New York City on April 4,1984. At the close of the EBASCO meeting, a complete listing of the HEA computer run was made available to BNL.

Because of the very short time interval assigned for the review and preparation of this report.(i.e., April 4-13,1984), it was decided to concen-trate the BNL efforts on the review of the results presented in report no.

8302-2 and on the supplemental infonnation contained in the computer run given to us by HEA. This run contains 9 load cases and their various combinations.

The input / output printout alone consists of roughly two thousand pages of in-formation and thus only selected portions could be reviewed with some detail .

The other sections were however reviewed from an engineering judgement view point. Comments regarding the reviewed work are given in the sections that follow.

GENERAL COMMENT

S Basically, the HEA report concludes that large primary moments will pro-duce tension on the bottom surface of the mat. For this condition, it is shown that the design is conservative. Furtherinore, the shear capacity vs.

the shear produced by load combinations are concluded to be adequate although ,

a few elements were found to be' close to the design capacity. Accordingly, the cracking of the top surface is attributed to " benign" causes such as _ Y shrinkage, differential soil settlement. and temperature changes. Yist S Basp on the discussions held with EBASCO and HEA, and on the review of data given to BNL, it is our judgement that the bottom reinforcement as well as the mat shear capacity is adeouate. The statement that the cracking of the g% top surface is attributable to " benign" causes however has not been analyti-M* cally demonstrated by HEA. In the BNL review of the reports and data, an at-tempt was made to ascertain the reasons for the existing crack patterns that appear around the outside of the reactor shield building as depicted in Figure 0-1 Appendix D of the HEA Report 8304-2. Other effects influencing the structural behavior and safety were also investigated. Specifically, the structural analysis topics reviewed in more detail include:

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(1) Dead loads and their effects.

(2) Buoyancy forces and their effects.

4 (3) Variable springs used for the foundation modulus.

(4) . Vertical earthquake effects.

(5) .The side soil pressures.

(6) .The boundary constraint conditions used for the mat.

(7) Finite element mesh size and its effects.

STRUCTURAL ANALYSIS TOPICS REVIEWED

1. Dead Loads .

As mentioned EBASCO in their discussion and HEA in their reports have not shown analytically, the cause of the top surface cracks. In reviewing the HEA canputer outputs, it was found that element moments and shears for individual loadings are explicitly given. Thus, for the case involving dead loads only, a number of elements in the cracked regions exhibit moments that can produce tension and thus create cracking on the top surface. This situation is shown in Table I which gives moment data for elements in some of the cracked re-gions. From the HEA report (page C-2-1-9) it seems that the top reinforce-ment, which is #11 0 6" in each direction

• is the minimum requirement for temperature steel according to the American Concrete Institute Building Code .

• ln a subsequent phone conversation, P.C. Liu of EBASCO stated that some additional reinforcement was added.on the top surface in one direction. Even if this is the case the statement that follows is true for the unstrengthened direction and perhaps even for the strengthened direction.

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' TABLE 1 ~

Normal Pressure ~

Hx Hy Mxy Side Pressure ,

- + '

-(  ; -

ELEML NT D B D B D B Mx Mx Mxy 437 -242 173 -574 19 7 116 - 31 -294 -196 93 212 +644 1595 +207 + 91, 106 - 25 -663 -392 79 211 -605 205 -412 217 -296 48 -2 19 -416 - 76 5 207 + 64 99 -136 136 - 81 15 -319 -193 50 "7 441 -105 168 +172 -170 39 - 12 -347 -489 66

• Fi 436 -719 269 -1193 357 +531 -130 -274 -258 117 JE 'T 4 38 269 14 2 -159 158 - 60 26 -730 -347 27

• f 447 665 59 210 88 248 - 55 -653 -339 -127 l2 204 193 87 569 72 -143 28 -361 -420 24 208 350 32 898 - 24 -241 75 -354 -771 - 49 .

203 -676 260 -995 236 39 - 21 -574 -247 30 426 -542 157 -705 31,0 332 - 65 -171 -486 61 259 62 148 -133 ' 81 +154 - 36 ,

2S3 5 11 531 + 75 0 18  ?.

255 30 58 670 5 41 10 S 252 86 24 611 - 55 87 8 NOTE: 0 - Dead Load

. 4. 254 50 26 412 - 41 69 9 E */ 251 37 5 162 - 23 44 12 B - Bouyancy

"' *E"C 257 320 - 38 57 15 - 81 - 15

• 248 255 - 26 29 16 - 29 - 6 .

. 2b7 -236 80 87 118 - 64 28 269 -173 59 434 10 - 82 32 4 19 -314 137 -635 313 - 30 12

$i 410 -371 71 -642 238 270 - 29

.E 400 -315 108 -774 275 - 44 41 3 Et 401 -180 42 -201 102 +108 - 23 ~

JE JL 414 -304 118 -130 178 + 44 - 19

• t 4 17 -200 93 440 41 - 17 ~ - 15

" - 18 404 - 64 17 4 28 - 32 98 e . O

.. 1 ,. s., .

Specification (i.e., As = .0018 x 12 x 144 = 3.11 in2/ft). The resisting moment capacity based on working stress design is about M = Asf sjd = 3.12 x 24 x 131/12 = 817 ft-kips /ft. The steel reinforcement strain for this

.. moment is equal to e

s (" *c )

• s

• 29 000 = 0.00083 in/in while, the corresponding concrete stress is, U = 3 ksi f g= cec s/n = 0.00083 8

.I In checking the data in Table 1, it can be seen that element 208 has exceeded the working load capacity under the dead load condition and, thus the local area could have exhibited a crack when this load acted alone. Similarly,

- concrete cracking could occur under this load condition in elements 447, 212, WM.

O 204, 253, 255, 269, 257, 417, and 404. Thus, the cracks on the upper surface o  : P outside of the shield, wall could have been initiated after construction of the superstructure, before placement of the backfill. It should be noted that 4 Wt.fnd since no analysis is available for dead load without the superstructure, the reason for the basemat cracks inside of the shielded wall cannot be explained by this reasoning.

2. Buoyancy Forces The moment results from this analysis show that these forces when acting alone would mostly cause tensile stress on the upper surfaces. The moments causing these stresses are tabulated in Table 1 for groups of elements in the cracked regions. As can be seen, these moments are not as severe as those due ,

to dead weight. By superpositon they could in some cases contribute to higher

- tensile stresses and thus result in further cracking in some of the upper surface areas.

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3. Variable Springs Used for the Foundation Modulus Moments and shears developed in the basemat were computed using the con-cept of the Winkler Foundation; namely the soil is represented as a series of relatively unifonn independent springs. The stiffness of the springs is ob-

. . tained from relatively crude analyses which are based on some generalized

{ analytic solutions available for rigid mats on the surface of elastic soils.,

The actual design of the mat was based on a series of interactive cc:q:. tar

-; runs in which the soil stiffness was varied until the computed contact pres-e sures under the mat were fairly uniform and equal to the overburden stress at

'the elevation of the foundation mat. This approach appears to be reasonable in that the long term consolication effects can be anticipated to cause effective redistribution of loads and cause the mat to behave in a flexible manne r.

, 4 Vertical Earthouake Effects .Q ggg7 g,4 "3 Vertical earthquake ef fect was not discussed in the HEA reports. Howe ve r, fran the finite element an,alysis' print out and the conversation with HEA engi

• neers, it was told that this effect was included in the load combination cases by specifying an additional factor of 0.067, which was then applied to the dead and equipment load case., From the discussions and the review BNL is not

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clear whetner an amplification factor due to vertical mat frequency was used or not. A quick check by the reviewers indicates that this factor could have some influence on the results.

Horizontal ear:15 Fake effects were input into the HEA finite element analysis as an equivalent bending morrent and in plane (fx2) shear acting on the pertinent nodes of the foundation mat. The reviewers however, are not  !

~

certain whether the dynamic interaction effiths between the superstructure and the mat were accounted for fn the analysis, nor are they certain about its ,

importance in ef fecting the results.

S. Side Soil Pressure _

Accordin,g to the STARDYNE cunputer results obtained fran HEA, the nonnal side soil pressures produce large monents that are opposite to those caused by the dead loads. As shown in Table 1 where moments of elements located in one of the cracked regions outside of the shield building are compared. The total '

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moments in some cases (i.e. element 447 or 208) become quite small. In other regions there is infact a reversal in the total bending moment which causes i tension on the bottom surface and compression on the top. This compression

'would tend to close the cracks on the upper surface. Thus,-it appears that j,, this ' pressure is a very important load case for the mat.

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For the static or nonnal operating condition the lateral pressures are based on the at-rest stress condition and are uniform around the periphery of -

the structure. For the seismic problems the pressures are computed to ap-

. proximately account for relative movements between the structure and the soil.

On one side the structure will move away from soil (active side) and reduce the pressures while the opposite will occur on the other side (passive side). ,

The actual computations made use of triaxial test data from site soils to arrive at the soil pressures rather than use the standard Rankine analyses.

^

However, no dynamic effects on either the lateral soil or pore pressures was included. The sensitivity of the calculated responses to these effects are

. currently unknown. Since the lateral pressures have a major impact on the computation stresses in the mat the dynamic effects can significantly in

- . fluence the stresses computed in load combination studies.

6. Boundary Constraints For equilibrium calculations no special consideration need be made for ,

vertical case since the soil springs prevent unbounded structural motion.  !

However, the same cannot be said for the horizontal case since soil springs l are not used to represent the soil reactions. Rather the lateral soil forces  !

are directly input to the modal. To prevent unbounded rigid body motion arti- ,

'=

ficial' lateral constraints must be imposed on the model From the output pre- '

sented in the ESASCO and HEA reports, it is not possible to evaluate the im-l

. pact of these assumptions. The stresses caused by the artificial boundaries ,

must be calculated and compared with those presented.

l .

7. Finite Element Mesh and its Effects In general finite element models for plate structures require at least four elements ,between supports to obtain reasonable results on stress comp-utations. The models used by both EBASCO and HEA violate this condition in the vicinity of the shield wall. The significance of this effect is demon-strated in Figure D-3 which presents a plot of moment taken through the center of the slab. The computed moments in adjacent elements 193, 194 and 455 are

-3800, -2500 and +400K. The elements used in the EBASCO analysis are constant curvature elements so that the computed moments will be constant within each element. The steep moment gradient in the elements listed indicates that a finer mesh would be required to obtain a better representation of element stresses. A similar effect was also noted when investigating the elements forming the junction between the lateral earth retaining walls and the base mat. In general, it is felt that the finite element grid used for the structural modeling is too coarse.

CONCLUSIONS AND RECOMMENDATIONS (a) The Waterford plant is primarily a box-like concrete structure sup-ported on a 12 foot thick continuous concrete mat which houses all Class 1 structures. Tha plant island is supported by relatively soft over consolidated soils. To minimize long term settlement effects, the foundatior, mat was designed on the floating foundation principle.

The average contact pressure developed by the weight of the structure is made approximately equal to the existing intergranular stresses developed by the weight of the soil overburden at the level of the bottom of the foundation mat. Thus, net changes in soil stresses due to construction and corresponding sellements can be anticipated to be relatively small.

W--- - - _ _ - - - - _ . - _ _ _ - _ - - _ . - - - - - _

(b) In reviewing the information reports and computer outputs sup-plied to BNL by EBASCO, HEA, and LPL, it is concluded that nor-mal engineering practice and procedures used for nuclear power plant structures were employed.

(c) Accepting the infomation pertaining to loadings, geometries of the structures, material properties and finite element ideal-ization as correct, it is the judgement of the reviewers:

(1) that the bottom reinforcement as well as the shear capacity

, of the base mat are adequate for the loads considered.

(ii) that computed dead weight output data can be used to explain some of the mat cracks that appear on the top surface. The cracks that appear, would have occuredi after the construction of the super;tructure but before the placement of the backfill.

Their growth would be constrained by subsequent backfill soil .

pressure.

(d) Due to the existance of the cracks, it is recommended that a sur-vetlance program be instituted to monitor cracks on a regular basis.

Furthermore, an alert limit (in tems of amount of cracks, and or crack width, etc) should be specified. If this limit is exceeded, specific structural repairs should be mandated.

(e) , it is also recommended that a program be set up to monitor the water leakage and its chemical content.

(f) The validity of the BNL conclusions depend mainly on the infor-mation supplied by EBASCO, HEA and LPL, either vercally, in re-ports or in computer outputs. While some checks for accuracy and engineering approa'ch were made pertaining to the supplied infomation some open questions still remain, especially those mentioned in the text under topics 4 thru 7 under the heading,

" Structural Analysis Topics Reviewed". It is recommended that the particular issues raised under these items be resolved.

- -<- -' 8 Since the Waterford plant is located in a low seismicity zone, there is a low likelihood of occurrence of an SSE and its as-sociated effects. Thus, although the inherent safety margins in the design of the basemat are as yet unquantified (due to cracking effects and the other items mentioned above), they seem to be sufficiently adequate to permit the performance of a confirmatory evaluation for their resolution in the near future.

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