ML19260D922
| ML19260D922 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 01/02/1980 |
| From: | Mills L TENNESSEE VALLEY AUTHORITY |
| To: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| References | |
| IEB-79-02, IEB-79-2, NUDOCS 8002120621 | |
| Download: ML19260D922 (17) | |
Text
{{#Wiki_filter:* M TENNESSEE VALLEY AUTHORITY M -597 CH ATTANOOGA. TENNESSEE 37401 M 400 Chestnut Street Tower II January 2, 1980
'Mr. James P. O'Reilly, Director Office of Inspaction and Enforcement U.S. Nuclear Regulatory Commission Region II - Suite 3100 101 Marietta Street Atlanta, Georgia 30303
Dear Mr. O'Reilly:
SEQUOYAH NUCLEAR PLANT UNITS 1 AND 2 - NRC-0IE REGION 11 LETTER RII:LM INSPECTION REPORT - ADDITIONAL INFORMATION ON IE BULLETIN 79-02 The subject letter dated October 24, 1979, identified two areas wh'Ere NRC-01E Inspector L. Modenos needed additional information before he could close IE Bulletin 79-02 for Sequavah. Enclosed is the additional information as requested on anchor bolu .ifety factors and thread engagement. . If you have any questions concerning this matter, please get in touch with D. L. Lambert at FTS 854-2581. Very truly yours, TENNESSEE VALLEY AUT110RITY A.
- 1. M. Mills, Manager f
Nuclear Regulation and Safety Enclosure cc: Mr. Victor Stello, Jr., Director (Enclosure) Office of Inspection and Enforcement
, U.S. Nuclear Regulatory Commission Washington, JC 20555 1950 314 8002120 O(gnent N+V An Equal Opportun y ni toyer
EucLOSURE
' ' ' ADDITIONAL INFORMATION ON IE BULLETIN 79-02 FOR SEQUOYA11 NUCLEAR PLANT UNITS 1 AND 2 AREA 1 Based on data from the tests conducted for BFN at Singleton Laboratory for correlations of thread engagement in self-drilling anchors to tensile capacity, TVA will submit a revised acceptance criteria of 60 percent diameter as the minimum thread engagements. "
- Response A sampling inspection of concrete expansion anchors for safety-related piping systems conducted at Sequoyah from September 27 to October 3, 1979, revealed that thread engagements of some of the anchors did not conform to the requirements of TVA General Construction Specification G-32.
In order to respond to similar concerns on Browns Ferry Nuclear Plant, tests were conducted at Singleton Materials ingineering Laboratory to determine the requirements of thread engagen 'nt to develop anchor load capacities. Based on the results of these t:sts, a minimum thread engagement crit'eria of approximately 0.60 diameters was developed. NRC-0IE inspectors have determined this criteria is acceptable for Sequoyah. This criteria is shown in Figure A-1 and has been informally supplied to the NRC-01E inspectors. All of the inspected anchors at Sequoyah fall within the limits of the criteria. Therefore, all anchors installed to date are considered to be adequate for thread engagement. However, all rachors to be installed after October 21, 1979, are to have thread engagements as required by G-32. O s Apwe 9 9
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AREA 2 Item 2 of Bulletin 79-02 requires the factor of safety (FS) f r self-
. drilling anchors to be at least 5. Taking into account the decrease in FS due to base plate flexibility, TVA's FS is below 5. TVA will submit a justification of their present factor of safety. -
_ Response flRC-01E Bulletin 79-02 requires that factors of safety between design and ultimate capacity for expansion anchorages for the maximum loading condition be at least 4 for wedge-type and 5 for self-drilling type expansion anchors. The following is a justification of the factors of safety used by TVA for the design and installation of expansion anchors. Self-Drilling Expansion Anchors The minimum factor of safety utilized in the design of anchorages before the issuance of TVA's Design Standard for Concrete Anchorages. DS-C6.1, in September 1975, was 4 for the maximum loading condition. Designs utilizing 05-C6.1 have factors of s?fety of 4.5 for the service load and 2.8 for the maximum loading conditions. TVA's factors of safety are actually much greater when the following items are considered:
- a. TVA Allowables Versus Manufacturer's Allowables Design allowables for expansion anchors are specified in TVA's Design Standard for Concrete Anchorages DS-C6.1. Installation and testing procedures for these anchors are specified in TVA's General Construction Specificition G-32 for Bolt Anchors Set in Hardened Concrete. Qualifi-cation tests described in G-32 require that all expansion anchors (each brand, type, and size) be tested to failure in job concrete. It further requires thgt the concrete for qualification testing be between 3000 and 4000 lb/in at the time of installation and testing. Each size and type of expansion anchor is required to meet minimum specified tensile capacities. These capacities are based o' minimum factors of safety applied to the service load design allowables specified in DS-C6.1. If anchors of a given size and type fail to meet the required capacities, then the design allowables for those anchors at that project are reduced to maintain the minimum safety factors. For service load conditions minimum factors of safety of 4 and 4.5 are applied to wedge-type and shell-type expansion anchors, respectively, tio increase in design allowables is made for capacities in qualification testing which exceed minimum requirements and no increase in design allowables is made for higher strength concrete. Prior to the issuance of DS-C6.1 the same procedure was followed utilizing G-32, but the factor of safety was a minimum of 4 for the maximum loading condition. TVA's qualification requirements are approximately 10 percent less for a given size and embedment depth than the quoted capacities for most manuf acturers.
Actual safety factors are thus generally higher than the minimum specified by at least 10 percent. 1950 317
b. TVA Design Concrete Strength Versus Actual Concrete Strength Allexpansionanchordesignvalugsarebasedonminimumstructural concrete strengths of 3000 lb/in and project qualification testing is required to be performed2 n anchors it concrete having strength
- between 3000 and 4000 lb/in . Inplace strength and long range strength of TVA concrete containing fly ash exceed design sgrengths in these relative massive structures by at least 2000 lb/in . Tests were performed at Sequoyah on 3-foot-thick wall sections to demonstrate the strength gain of all basic classes of concrete used at Sequoyah.
The results of those tests are shown on attached Figure 1. structural class concrete which reached a strength of 50MMix 3 in lb/in 730 days was primarily used in walls in structures other ,,an the reactor building. Mix 3 structural class concrete which reached a strength of 7000 lb/in 2in 730 days was primarily used in slabs in structures. In the reactor building mixes only mixes 3, 4, and 5 were used. Anchorage capacity is effected by the tensile strength of concrete which varies approximately by the square root of the compressive strength. This strength increase furnished an added measure of safety which would result in 29 percent increase in the factor of safety for pix 2. For mixes 3, 4, and 5, a minimum strength increase of 3000 lb/in occurs which constitutes a 44 percent increase in the factor of safety.
- c. Effect of Plate Flexibility on TVA Designs See attached memorandum from R. W. Cannon to the Civil Engineering Branch Files dated November 5, 1979. TVA has reviewed the Sequoyah anchorage designs using the table for Anchor Design for Self-Drilling Anchors to Develop Attachment Capacities which shows a comparison of rigid plate moments for various size attachments utilizing various anchor sizes and configurations with various plate sizes. The results of the review indicate that no capacity reduction is required for TVA designs due to plate flexibility.
- d. TVA Production Testing Program To date, inprocess production tests have been conducted on approximately 11,000 anchors at Sequoyah Nuclear Plant in accordance with TVA General Construction Specification G-32. The overall failure rate was less than 2 percent. In addition TVA conducted a. random inspection of approximately 139 anchors of various sizes at NRC's request and found only 3 questionable anchors. The failure rate for the inspection was approximately 2 percent which confirms TVA's G-32 testing program results.
1950 318
The effects of the Concrete Design Criteria and Concrete Degradation under SEE events as well as statistical discussion are covered in R. W. Cannon's attached memorandum. The minimum factor of safety for TVA's designs utilizing self-drilling expansion anchors is as follows:
- 1. By DS-C6.1 FS = 2.8
- 2. Consideration of TVA's allowables versus manufacturer's allowables FS = 2.8 + 0.l(2.8) = 3.1
- 3. Consider use of actual concrete strength Mix 2 FS = 3.1 + 0.29(3.1) = 4 Mix 3, 4, 5 (min) FS = 3.1 + 0.44(3.1) = 4.46 In sumary, since TVA's G-32 production testing program has indicated only 2 percent failure of installed anchors and NRC has had additional inspection by TVA done that verified the 2 percent failure rate, combined with tha minimum factors of safety for the maximum loading cond. tion which range from 4.0 to 4.46, it is TVA's position that for Sequoyah Nuclear Plant designs the requirements of NRC-0!E Bulletin 79-02 have been met for self-drilling anchors.
9 Wedge-Type Expansion Anchors The minimum factor of safety for anchorage designs utilizing wedge-type expansion anchors was 4 for the service load condition and 2.5 for the maximum loading condition. TVA's factor of safety is actually much greater if the following considerations are taken into account:
- a. TVA Allowables Versus funufacturers Allowables Design allowables for expansion anchors are specified in TVA's Design Standard for Concrete Anchorages 05-C6.1. Installation and testing procedures for these anchors are specified in TVA's General Construction Specification G-32 for Bolt Anchors Set in Hardened Concrete. TVA limits wedge anchors on a jobsite to the two longest anchor bolts for each size anchor. For example, only the 8-1/2 and 10-inch long 3/4-inch anchors are allowed on a particular jobsite. TVA's qualification requirements are approximately 10 percent less for a given size and embedment depth than the quoted capacities of most manufacturers.
Actual safety factors are higher than the minimum specified by at least 10 percent.
- b. TVA Design Concrete Strength Versus Actual Concrete St.ength See b under self-drilling expansion anchors.
The minimum factor of safety for TVA's designs utilizing wedge-type _ expansion anchors is as follows:
- 1. By DS-C6.1 FS = 2.5
- 2. Consideration of TVA's allowables versus manufacturer's allowables FS = 2.5 + 0.l(2.5) = ?.75
. 3. Consider use of actual concrete strength Mix 2 FS = 2.75 + 0.29(2.75) = 3.55 } 950 ,$ } ')
Mix 3FS=2.75+0.44(2.75)=3.96 957027/11
In summary, since TVA's General Construction Specification G-32 for Bolt Anchors Set in Hardened Concrete requires wedge-type expansion anchors to be torque tested to ensure proper installation of the anchors combined with minimum factors of safety for the maximum loading condition which range from 3.55 to 3.96, it is TVA's position
- that for Sequoyah Nuclear Plant designs, the requirements of NCR-0IE Bulletin 79-02 have been met for wedge-type expansion anchors. ,
1950 320 2161?1/11
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. TENNESSE PVh(JEY'X.D',bidblTY
( IN I I A to : Civil Engineering Branch Files
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gItoM : R. W. Cannon, Principal Civil Engineer (Research & De ,eyopint-lStifff)d,_, Civil Engineering Dranch, W8D219 C-K i i a _3". ' ' . DATE : l l r' $2i w... i l '. i i i i MInJECT: lEQUOYAH UUCLFAR FLAITP - EXPAllSION AI.'Ci ?RS - SAFETY FM E --! I 5 t O .COMPARISOH OF T/A STAIR 0ARDS UITH OIE BULLETIN 79-02 dr i i i e .-^= i i i
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i . __i i g 6 , t,*4 e i g i- l 1;C i t i t i D8 ' ' TVA design standards for concrete anchorages specificOTeliT h_nllijral'- a g for expansion anchors t:hich appear to be in conflict i i-tr,t-Olli-linlinhn "- w 79-02 requirements. It is the intention of this memor gidjiiilto_iB9.thTti . the combined effect of T/A's design standards and constrtiet-Ion-spc' cifi'a'-- ! - tion results in a more conservative anchorage installatToh t*tshithe .... ' ', acceptable limits of the 79-02 Bulletin. j-l l
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l t i0. : i . Relation Ectueen Safety Factor and Plate Flexibility .. For a given plate thickness there is a maximum eccentricity of anchor location with respect to the attachment for which the plate has the ability to develop the full capacity of the tensile anchors. Frcs tests * "" to failure for A-36 stccl plates this maximum eccentricity "e" may be approximated by: g e
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e max
= 15t (u + ht) + t Dy 9 n( p where: v = width of attachment in inches 8 bd. l,_
n = nw::bcr of anchors Tu = ultimate tensile strength of anchor . t = plate thickness in inches . Prying action has no effect on the ultimate capacity displacements and vill therefore not be considered further. (See TVA's generic response to 79-02 for additional discussion of prying action.) Q Excluding the prying action of under designed plates the flexibility of plates only effects anchor stress in moment connections by effecting the location of the resultant compressive force in the anchorage. For the typical single line of ten::ile e. ichors the location can be connervatively estimated. Uhen stress in bot!. plate end anchors is lou the entire width of plate is effective in resisting plate bending on the tensile side, h On the compression side the effective plate vidth is limited to approximately the width of attachment plus twice the distance cf the resultant compressive force from the ecmpressive flange of the attachment. The combi,cd effect of small anchor displacement (uith respect to load) and elastic plate deflec-tions is to restrict the resultant to a close proximity to the compressive flange of the attachment. With increasing moment and stress in both plate h and anchors the resultant shifts toward the compression edge of the plate. 3 Plate yielding will occur first on the tensile anchor side due to the larger 44 i 1950 321 -
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- Civil Engineering Dranch Files .
i SEQUOYAll liUCLEAR FLAUT - EXPAU3 ION AUCHORS - SAFETY FACTORS - COMPARISON OF TVA STANDARDS iTITH OIE BULLETIU 79-02 , O cecentricity of tensile anchors. As yielding continues the effective width of plate resisting bending vill be reduced to approximately the ' vidth of attachment plus four timos the plate thickness. This results in a si Cnificant increase in plate rotation and shift of the resultant compressive force, It si.ould be duly noted at this point that tha '- moment resistance of the anchorage increases with both anchor stre.:s an:' increased moment arm. Anchor stress does not therefore increase in a direct proportion to increased load or applied moment in a moment W connection. A:: long as the effective moment arm of a flexible plate at ultimate capacity equals or exceeds the moment arm assuced in a rigid analysis then the factor of safety is unaffected by plate flexibility
- irrespective of the fact that anchor stress at design loading ma-yhave been underestimated by rigid assumptions.
Minimum plate size to develop a given size attachment is covem21 largely by the minimum spacing requirements of the anchors since the combined -- cffect of anchor size and minimum spacing controls the eccentricity of' anchors within the previously described limits. In the attached com-parison we found no instances of reduced ultimate capacitics due to plate flexibility which did not have an excess capacity over thr.t of the attachment. Under these conditions vc do not feel that any increase g@ - int design loading for flexibility effects is justified in evaluating factors of safety. TVA Qualification Recuirements for Anchors Versus Varintions in Manufacturer's Data - TVA General Construction Specification G-32, " Bolt Anchors Set in Hardened Concretc," requires that each size, type, and brand of anchor be qualified in project concrete to specified minimum tensile capacities. Design load allowables are based on these minimum qualification require-cents. They are not increased for any manufacturer's anchors 12 respective of capacities in excess of minirrxn requirements. In those rare cases ,; hen a particular size or type of anchor has difficulty qualifyinC at a given g. project the design values for that particular anchor are reduced accordingly. Thus the safety factors utilized in establishing TVA design allcuables are based on a minimum acceptability level for cach size and type of anchor and
' not on manufacturer's data which may be derived from a limited test experience.
The net result of continuing qualification testing at the various projects i:Q an increased level of confidence in these requirements as realistic minir:um values. . 30 322
. g
... r . 3 . Civil Engineering Branch Files Q SEQUOYAH l'UCLEAR ILCT - EXPA!:3IO.'l ANCHORS - SAFCTY FACTORS - COMPARISON CF TVA STANDARDS UlTH ole BULLETIN 79-02 g _G-32 Test Recuirements Versus 79-02 " The proof locd requirements of G-32 for inprocess testing average 50 percent of the ninimum qualification test requirements for 1/2-inch and larger size self-drilling type anchors and a corresponding 60 percent
.for the shell type anchors uhich are not self-drilling. This establishes g proof loads approxi ately 50 percent above maximum design loads and establishes a much higher level of assurance for a given failure rate than testing only to the maximum design loading.
In the application of statistics to the establishment of design and material strength requirement the key factors are the permissible rate of failure and the confidence level for testing acceptability. Once these are established the minimum acceptable test level or design value is then based on the variance of test values from the average value. This is standard practice in materials testi g. Once the design value . - . . is established then an acceptable level of variance is established for a given strength, or uhen the variance level is knoun then the strength levels are adjusted accordingly. I'or the same test failure rate of 5 percent G-32 testing uould provide sn assured maxirers standard deviation of 0 32 in comparison trith the 0.h8X maximum under the 79-02 Bulletin testing requirements. At the same time the G-32 test failure rate vould have to execed 11 percent for TVA's minimm design safety factor of 3 to have a failure rate of 5 percent. At Sequoyah, in the unit 2 shield building, uc replaced all : elf-drilling anchors then ne discovered the G-32 test failure rate exceeded 10 percent in some areas. We do acknowledge that a major portion of the test failures are a result of improper installations and therefore the total test failure rate does not represent a true picture of the standard deviation of the total g installation. Under these circumstances any time the G-32 failure rate exceeds 5 percent it uill be necessary to record failurc loads and do a statistical variance on the failure loads in order to substantiate that the design load failure rate is less than 5 percent. At Sequoyah the overall G-32 failure rate is less than 2 percent and rarely exceeds this in any structure or support systems. For a minimum design safety factor of 3 (for factored loading) this statistic:lly indicates a design failure rate less than 0.3 percent. No matter how you view this failure rate it must be concluded that the Sequoyah installa-tion provides a greater assurance of meeting design requirements than required by 79-02 irrespecti the differences in minimum applied design h safety factors.
~
[)) - hd{}50.523
Civil Engineering Branch Files , . SEQ,UOYA!! !!UCLEAR PI.Attr - EXPAICIC;I A!ICl! ORS - SAFETY FACTORS - COMPARISoli 0F TVA GTAliDARD3 UITil OIE BULLETIII 79-02 O t m ' Effect of Concrete Design Criteria and IIRC Assumotions of Concrete
. Degradation Under SSE Events on Deci;n Safety Factor We have been told that a principal consideration in the minimum
- cafety factor requirement of 5 by IrRC is their concern for the effect of concrete degradation (under SSE events for stnictures designed to "
ACI zone of349the strain concretelimits) and the on the strength location of anchors in the tensile of anchors. First of all we do not agree that such degradation vill occur under h the limited number of cycles (assumed 200) associated with an SSE event and tests are in progress (specimenscast) to verify the capacity of anchors under these conditions. Secondly, Sequoyah strength allouables of was not designed ACI 318 utilized. by the 3h9 Code vor was the ultirate For the maximc's factored load conditions vas limited tocor. crete stresses were limited to 0.75 f'c and "*steel str 0 9 fy. At this stress level the number of cycles required cycles.- for any degradation would be in the hundreds of thousands of
's If ITRC is serious cbout the assumed degradation of anchorege strengths under SSE events then they must have alloued for an increased rate of / failure under the SSE event since the minimum specified rate is not based on degraded strengths.
Assuming the variance is unaffected by strength degradation, attached figure 1 shous the increase in failure ; rate for the 79-02 requirements and for the TVA desicn and testing standards. Any assumption of increased variance will merely increase i the rate of failure but will not effect the relative comparison or the . strength degradations at which the tuo criteria produce equal failure rates. For the 2-percent failure rate at Sequoyah figure 1 indicated tbc strength degradation would have to be more than 50 percent before ! TVA's criteria would be less conservative than 79-02 requirements . Inplace Strength Versus Design Strength All expansion anchor design values are based on minitrxn structural concrete strengths of 3000 psi and project qualification testing is 3000 and 4000 psi. required to be performed on anchors in concrete having stre ; Inplace strength and long range strength of TVA concrete containing fly ash exceed design strengths in these relative massive structures by at least 2000 psi. Tests were performed at 0 . Sequoyah on 3-foot-thick wall sections to demonstrate the strength gain > D OD D
'9'lS
,U X .fA L
, JL 6 1
. 1950 324
,I I
?:
l . 5 -- Civil Engineering Branch Files h SEQUOYA!! NUCLEAR PL/u'*r - EXPAUSI0li AUCHORS - SAFETY FACTORS - COMPARISON OF TVA STMib.MDS WITH OIE BULLETIN 79-02 e of all basic classes of concrete used at Scquoyah. The results of those tests are shown on attached figure 2. The louest strength structural class into which anchors might be placed is mix 2. ,g
, Anchoroge capacity 'is affceted by the tensile strength of concrete which varies approximately by the square root of the compressive g strength. This strength increase furnishes an added measure of safety which may either be evaluated as a 29-percent increase in safety factor or as an offset to any possible degradation during SSE events.
Conclusions
- 1. When the ba.se plate of moment connections is properly sized to develop the full capacity of expansion anchors, while maintaining minimum dimensions, the resisting moment arm of the bacc at ultimate capacity vill equal or exceed that derived from rigid plate escump-tions based on nominal stress strain limibations for concrete.
Therefore, plate flexibility need not be considered in establishing g factors of safety for troperly designed base plates. "
- 2. When the plate is incapable of developing the full capacity of the anchors then the safety factor of the anchors does not govern design.
- 3. Plate ficxibility only effects ultimate capacity with inefficient utilization of anchor sizes and unsteful proportioning of plates.
Such is not considered reasonable design practice.
- 4. TVA's method of qualifying anchors on each project by brand size and type provides a higher level of confidence and more realistic minimum capacities than the normal industry practices assumed in Bulletin 79-02.
O 5 TVA's inprocess testing under G-32 provides a much higher level of confidence in installed capacities than the minimum testing requirements of 79-02.
- 6. The test failure rate of less than 2 percent at Sequoyah (which has h, been verified by inspections) provides a much higher 1 cycl of conser-vatism with TVA's standard minimum design safety factor ~of 3 than
- the minimum factor of safety of 5 stipulated in 79-02 with an acceptable 5-percent failure rate. / '
3' D D D V $", oNS a ,b- a
. 1950 325
- s. .. .
, Civil Engineering Dranch Files .
i - I I SEQUOYN! IIUCLCAR RAlff - EXPAllSIO:I AIICi! ORS - CAFITY FACTORS - . i COMPARISOIT OF TVA STAIIDARDS WIDI OIE BULLETIIT 79-02
- i i
'7 Since TVA is more con:crvative in every accepted method of
.,cvaluation than the 79-02 Bulletin requirements, we see no
.. .'c.just..i.f.ica..- tion
. . for altering TVA designs.
g .
. . - - - O r i c M' M '"'"
- g,yj,cannoa RUC:GT
- R. W. Cannon h
Attachment , cc: R. G. Domer, W9D224 C-K - . n/2/79 - RGD:W' cc: Roy H. Dunhon, W11A9 C-K (Attachment) J. L. Parris, W11C126 C-K (Attachment) D. R. Patterson, W10C126 C-K (Attachment) R. M. Pierce, 204 GB-K (2) (Attachment)
.- .4
. n/5/79 - RitD:s cc (Attachment):
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