ML18031A105

Final Deficiency Rept on spot-welded Struts

ML18031A105
Person / Time
Site:	Susquehanna
Issue date:	12/29/1978
From:	PENNSYLVANIA POWER & LIGHT CO.
To:
Shared Package
ML18031A103	List: ML18031A100 ML18031A105
References
P23-9, NUDOCS 7903010285
Download: ML18031A105 (22)

Text

Revision 1 JN 4'~'- '0'8012 FINAL REPORT ON SPOT-WELDED STRUTS FOR SUSQUEHANNA STEAM ELECTRIC STATION UNITS 1 AND 2 Prepared by:

Checked by:

Approved by:

BECHTEL POWER CORPORATION San Francisco, California September 29, 1978 December 29, 197U (Revision 1)

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JN 4 >"-" >O JSG'3 2 TABLE OF CONTENTS Section Title ~Pa e 1.0 Purpose 2.0 Spot-welded Struts 3 ~0 Background 4 ' Deficiency and its Safety Implications 5 ' Immediate Action 6.0 Test Program 7 ' Technical Evaluation of Deficiency 8.0 Corrective Action 10 9.0 Repair Methods 10 ' Conclusion 14 ATTACHMENTS Test Report by 'Pittsburgh Testing Laboratory'est Results Specifications

• The Attachments were previously transmitted via PLA-295 dated October 9, 1978 and are not forwarded with this report.

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Revision 1 1 ' PURPOSE The purpose of this report is to provide final data and information as required by 10CFR50.55(e)(3) subsequent to the notification of a reportable deficiency. The subject deficiency is associated with spot/resistance welding in strut material.

2.0 SPOT-WELDED STRUTS 2.1 Struts Basic strut sections are light gage (thickness varying from 0.105 to 0.109 inch) channels manufactured by cold forming mild steel strip. These channel sec- V tions are connected to each other in various configura-tions such as back-to-back, back-to-side, or side-to-side by using a welding process commercially known as spot-welding or resistance welding. The channel sections and built-up sections used on Susquehanna project are either mill-galvanized or hot-dip galvanized after spot-welding and are supplied by Unistrut Corporation, Wayne, Michigan; Power Strut, Division of Van Huffel Tube Corporation, Warren, Ohio; and B-Line Systems Incorporated, Highland, Illinois. These were procured as standard off-the-shelf, items with no formal Quality Assurance. Various configurations used on Susquehanna are given in Figure I.

Strut members are used in field fabricated supports for electrical raceways, HVAC ducts and instrumentation

>e lines. The governing documents are drawing 4 r-'69BQj2..

8856-E-53 for electrical raceways, Specification 8856-M-323-C for HVAC ducts and Drawing 8856-JG-16 for instrumenta-tion.

The supports are designed in accordance with 'Specifi-cation for the Design of Cold-formed Steel Structural Members'1968 Edition), published by American Iron and Steel Institute.

The spot/resistance welding process consists of passing high current through the thicknesses of adjoining plates resulting in metal-to-metal fusion. The quality of a spot weld is dependent on many variables such as pres-sure on the electrode tips, finish of the material, presence of impurities on the material, build-up of zinc and other contaminents on the welding electrodes, contact between joining surfaces, voltage and amperage.

Therefore, unless these parameters are closely moni-tored and controlled, the result may be inadequate fusion.

3.0 BACKGROUND

Since the use of strut material began on the Susquehanna Pro-ject, there were three F

isolated instances when the adjoining channels separated during handling or assembling. These member lengths were rejected.

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.)( z-r=. E 8612 However, in recent months there have been significant in-stances on other nuclear projects where inadequate fusion was observed at the weld. spots. As a result of th's', on March 25 and 26, 1978, Bechtel field engineering personnel performed an inspection on Unistrut member P-1004A, Power-strut member PS-3022 and B-Line member B22-X. The tested members were part of installed electrical raceway and HVAC duct supports. .The method of inspection consisted of

'sounding'he members with a ball peen hammer to detect separated spot welds and verification of the separation by insertion of a card between the members at the spot welds.

The inspection indicated a high incidence of spot-welds with inadequate or no fusion. Therefore, Project Quality Assurance (QA) issued Management Corrective Action Report (MCAR) 1-23 on March 28, 1978.

4.0 DEFICIENCY AND ITS SAFETY IMPLICATIONS Based upon the resul'ts of the inspection as described in Section 3.0, it was determined that:

a. The deficiency is related to spot-welding technique and/or procedure.

b. The quality of the spot-welding is indeterminate, without further investigation.

Various combinations and configurations of channel struts have been used in the support systems. Individual members

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vision 1 0.8012 have been designed as composite sections for which the con-nection (spot-welding used in this instance) between adjoin-ing channels is relied upon to carry the postulated loads within established design margins. Thus, inadequate fusion at the spot-welding may result in inadequate strength, and may adversely affect the safe operation of the plant under design loading conditions.

5.0 IMMEDIATE ACTION A 'Hold'as imposed on further installation of member P-1004A or its equivalent unless the member was stitch-welded to develop equivalent design strength of the spot-welding.

6. 0 TEST PROGRAM 6.1 General.

Although it was determined that a deficiency existed in some spot-welded members, the extent of the de-ficiency was unknown. So the first step was to es-tablish the scope of the problem. Since there is no practical nondestructive I method for examining the sound-ness of spot welds in the erected material, it was de-cided to initiate a destructive test program.

6.2 Basis of Test Pro ram The underlying approach is described below.

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a. Obtain representative samples, selected at random, from the'nstalled and stock material for all shapes and manufacturers.

b. Perform destructive shear test on the samples to obtain failure loads.

c. Analyze the test results statistically for each shape and manufacturer to compute corresponding expected strength per spot-weld at a certain confidence level.

6. 3 ~sam les Samples were obtained by Bechtel field engineering personnel and sent to a recognized testing laboratory.

Samples were typically 6" long containing two spot-welds.

For the stock material, two samples were obtained from each 20'-0" length and only one sample from any given installed member selected at random. For thc installed material, generally one sample was obtained for every 100 feet of the installed quantity.

6. 4 ~Testis Testing was done by Pittsburgh Testing Laboratory (PTL) in Pittsburgh, Pennsylvania. Test method and procedures are fully described in PTL's report. (See Attachment A).

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>e 4-is gasp:g.p 6.5 Statistical Anal sis Based upon the test results, histograms were plotted for each shape and manufacturer. Mean and standard deviation were computed for each case. The test results generally follow the normal distribution.

The expected failure load per spot weld was determined by using the mean of the failure load based upon the test results and subtracting one and a half times the standard deviation. This approach provides more than 90% confidence level for the expected failure load.

Based upon above criteria, the expected failure loads were computed and are given in Table I.

7.0 TECHNICAL EVALUATION

OF DEFICIENCY 7.1 General The technical evaluation in this section is limited to all strut members installed and/or at the jobsite which are not fabricated.

7.2 Desi n Criteria and Theoretical Considerations AISI specification specifies allowable shear strength per spot-weld to be equal to 1.65 kips with a factor of safety of 2.5 for 0.109 inch thickness. This al-lowable shear is based upon "Recommended Practices (9 23-9)

Revision 1 Jg g-(c g,gg.Q'j'f for Resistance Welding," AWS C1.1, by American Welding Society. Based on this criteria, the failure load for a spot should be over 4,000 pounds. However, in light of the problem associated with spot-welding, it is necessary to assess bases of code requirements and evaluate structur al adequacy of the installed material without compromising the basic design philosophy.

The connection pr ovided by spot welds between adjoin-ing channel sections is relied upon to maintain the integrity of the built-up sections. The calculated shear in a spot weld in a member depends upon many variables such as loading, sectional properties, end conditions and if the member is used as a beam, brace, column or tie.

The allowable shear of 1.65 kips/spot specified by the code is the upper limit for designing purpose.

However, from the evaluation point of view, it is more realistic to consider actual maximum design shear cal-culated individually as required for each shape. Sec-ondly, for the strut material (Fy = 33 k.s.i.) the allowable bending stress (Fg ) per code is 20 k.s.i. for 0.105 inch thickness. Therefore, the factor of safety for the bending stress is considered to be 1.65 while for the spot-welds, it is 2.5, which is rather high.

Reason for this could be attributed to many variable, w7 (P23-9)

Revision 1 Ju 4 ra 098012 affecting the weld strength. For the material in ques-tion on Susquehanna, an extensive test program has been carried out and expected failure loads for each shape are well defined. Therefore, it is reasonable to as-sume that a factor of safety of 2.0 for spot-welds, which is still higher than the factory of safety for bending of the strut material would still be adequate and consistent with the basic design philosophy.

7 3 The design of framing members in the suppor t system is based upon allowable shear of 1650 lbs. per spot weld.

Therefore, to maintain a minimum safety factor equal to 2.0, the expected failure load must be 3300 lbs. or greater.

7.3.1 The strut sections with the expected failure load equal to or greater than 3300 lbs/spot weld, are considered to be structurally adequate.

7.3.2 However, for the other sections, further evaluation is necessary.

Design loads on the support systems are governed by many considerations such as type of support, structure, elevation, etc. For individual mem-bers in a support, design shear forces addition-ally depend upon if the members are loaded flex-urally or axially. Et is noted that for flexural members, shear forces are generally high.

Revision 1 JhN g <c g>GOj p For the earlier investigation, a 5'-0" long member 'sup-porting uniformly distributed load was considered to be a representative case for various loading conditions, and member shapes, and comparison was made between com-puted design shears and expected failure loads.

However, in the final analysis, actual design shears were computed in individual members for all support

~ types in various buildings and at different eleva-tions.

These shear values vary considerably; therefore, the existing members, which may be adequate at a certain location, may need strengthening elsewhere depending on the aforementioned variables.'n any case, a fac-tor of safety of 2.0 has been maintained.

7. 4 ~Summa r Based upon a factor of safety, of 2.0, all shapes are grouped in three categories.

Cate or I Unistrut: P-5501, P-1001C, P-3301, P-1001A P-1001C3 Powerstrut: PS-151, PS-202 These shapes are structurally adequate and need no repair.

Cate or II Unistrut  : P-1001, P-5001 Powerstrut: PS-201, PS-101, PS-204 PS-3080 B-Line  : . B-12A These sections are considered deficient at certain locations and need to be repaired as required.

Cate or III Unistrut  : P-1004A Powerstrut: PS-3022 B-Line B-22X, B-11A All members of above shapes are to be repaired/

strengthened to provide shear connection capacity equal to twice the design strength required.

8.0 CORRECTIVE ACTION 8.1 General Corrective actions are grouped in several categories. Each category and corresponding action is described below.

8.2 Installed and Stock Material 8.2.1 Powerstrut PS-3022, I

Unistrut P-1004A B-Line B-22X.

Based upon the inspection by Bechtel field engineering personnel, all material of this (P 23-9)

evision l configuration was deemed to be deficient, and will be completely repaired without taking any credit for the existing spot welds.

8.2.2 B-Line B-11A.

Installed and stock quantity for this shape was small; therefore,. no samples were obtained.

In the absence of any test data on the existing spot welds, this section will be repaired to develop required design capacity to meet the design criteria.

8.2.3 Powerstrut

PS-151, PS-202 Unistrut: P-5501, P1001G, P-3301, P-1001A, I

P-1001C3 Based upon the technical evaluation in Section 7.0, existing spot-welding is adequate and will perform satisfactorily under design loads.

Ther efore, no further action is deemed neces-sary.

8.2.4 Powerstrut; PS-201, PS- 101, PS-204, PS-3080 Unistrut: I P-1001, P-5001 B-Line: B12A (P23-1/1)

R evision 1 ig u7 .09 12 These members do not meet the design criteria, therefor e, a detailed re-analysis and redesign has been performed to compute required additional strength and design drawings have been prepared to show the necessary repair details. The re-quired repair will be performed on the installed and stock material. On completion of the repair, the strut material will satisfy the design cri-teria.

8.3 Strut Material to be Received in the Future Until now, the strut material was procured as a stan-dard 'off the shelf'tem. However, in light of the problem experienced with the spot-welding, a new speci-.

fication 8856-C-92 (see Attachment C) was developed for procurement of,all spot-welded struts in the future.

The results of the testing by PTL positively indicate that the spot-welds on a plain material have much higher strength than spot-welds on mill-galvanized material.

Therefore, the specification stipulates that strut material to be coated after spotwelding. In addition to this, to pr ovide a reasonable assur ance of attaining the design strength, the supplier is re-quired to introduce a destructive testing program on the pr oduction samples and no material will be shipped to Susquehanna jobsite unless it meets the acceptance

.criteria. This action should prevent the recur rence of this problem in the future.

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Revision 1 9.0 REPAIR METHODS N 4' Kid+Q3,+r,'.

1 General Construction has been provided the option to select any one of the following methods on a case-by-case basis.

9.

This method is to replace the deficient material with new material to be procured per Specification 8856-C-92 (see Attachment C).

9.3 Mechanical Fasteners Second method is to provide 1/4" dia. self-drilling and self-tapping metal screws to obtain r equired shear strength. These, screws will be procured per Specification 8856-C-91 (see Attachment C). In order to assure strength requirements for the screws, it is required to perform a destructive shear test on samples and each lot must meet the acceptance cr iteria.

In addition to self-drilling and self-tapping screws, other mechanical fasteners are being considered such as pop-rivets or clamps. If these are determined to be technically acceptable, they will be used as alternate to the screws.

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9. II ~Veldkn Third approach is to provide intermittent .stitch welding at the joint between channels and/or chan-nel and side plate.

10.0 CONCLUSION

On completion of the required repair/replacement of the existing spot-welded struts determined to be deficient, the existing support systems will be structurally ade-quate to satisfy the design requirements. Secondly, pro-curement of new spot-welded strut material per new spec-ification (see Attachment C), will provide adequate as-surance of preventing the recurrence of this problem in the future..

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FE8 2'80 C 99480 TABLE 1 EXPECTED FAILURE LOADS Standard Expected Failure Load Mean Deviation =A-1.5B Meuufactur er Member (1bs) (1bs) (lbs) Remarks (A) (B)

Unistrut P-1001 4140- 1213 2321 Cat. II P-5501 5499 1229 3656 Cat. I P-5001 4357 -, 1072 Cat. II P-1 001 C 5386 1033 3837 Cat. I P-3301 '465 1025 3928 Cat. I P-1001A 4192 554 3361 Cat. I P-1001C3,5186 629 4243 Cat. I (Back to back)

P-1001C3 5126 458 4439 Cat. I (Back to side)

Powerstrut PS-201 4418, 1194 2627 Cat. II PS-151 5533 892 4195 Cat. I PS-1 01 3855 782 2682 Cat. II PS-204 4150 2233 801 Cat. II I PS-3080 4126 1252 Cat. II (Back to back).

PS-3080 4666 1511 2400 Cat. II (Back to side)

PS-202 5783 3950 Cat, I B12A 3565 1317 1590 Cate II Note'. 1. Above numbers in the table ar e based upon one spot veld.

2. For explanation of Remarks" column, refer to section 7.4.

3. For shapes in Figure 1 and not in table above (i.e. those in Category III), r efer to section 7.4.

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Revision 1 FIGURE 1 STRUT SHAPES AND IDENTIFICATION

~Sha e K Unistrut P-1001

~Sha e R Powerstrut PS-201 Unistrut P-5501

~Sha e 0 Powerstrut PS-151 Unistrut P-5001 B-Line B-12A Powerstrut PS-101 B-Line B-11A 3~ha e N ~Sha e L Unistrut P-1001C3 Unistrut P-1001C Powerstrut PS-3080 Powerstrut PG-204

.~Sha e N Unistrut P-1004A Powerstrut PS-3022 B-Line , B-22X

~Sha e 0 Unistrut P-1001A Shaue P Powerstrut PS-202 Uuiattut P-3301