ML19208B359
| ML19208B359 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 07/06/1979 |
| From: | Mattimoe J SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | Engelken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| References | |
| NUDOCS 7909200076 | |
| Download: ML19208B359 (5) | |
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[ly!$ MUD ACRAMENTo MUNICIPAL UTILITY DISTRICT O 6201 s street, Box 15830, sacramento, California 95813; (916) 452-3211 Y
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Attention: Mr. R. H. Engelken, Director Xlti{ggd
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Enforcement 1990 North California Boulevard Walnut Creek Plaza, Suite 202 Walnut Creek, California 94596 Docket No. 50-312 Rancho Seco fiuclear Generating Station, Unit No.1
Dear Mr. Engelken:
I.E. Bulletin No. 79.02 was received by the Sacramento flunicipal Utility District on March 12, 1979 This Bulletin requests a design review of concrete expansion anchor bolts used on Seismic Category 1 piping supports at our Rancho Seco Nuclear Unit.
The attached report responds to the questions raised in this Bulletin. A field verification testing program is in progress.
Testing and installation and repair procedures are available at the site for review. We will submit a final report when all the testing is complete and when the modi-fications, if required, are done.
Sincerely yours, hk, !bAl R
NohnJ.Mattimoe Assistant General Manager mod Chief Engineer cc:
John G. Davis, Acting Director Division of Reactor Operations Inspections b
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SACRAMFNTO MllNICIPAL UTILITY DISTRI_C_T REPORT ON PTPE 91PPORT BASE PLATE DESIGNS USINC CONCRE EXPANSION ANCHOR BOLTS (In Response ta:
HRC IE Bulletin No. 79-02, dt. March 8,1979)
Introduction This report is in response to NRC IE Bulletin 79-02, dated March 8,1979, requiring all licensees and permit holders for nuclear power plants to review the design and installation procedures for concrete expansion anchor bolts used in pipe support base plates in systems defined as Seismic Category I by the NRC Regulatory Guide 1.29, " Seismic Design Classification", Revision 1, dated August,1973 or by the applicable SAR.
In accordance with the intent of the Bulletin 79-02, the following types of supports have been considered in the present review.
a.
Pipe Anchors (Seismic Category I) b.
Pipe Supports (Seismic Category I)
Numbered Responses Correspond to Those Given in Bulletin 1.
Pipe anchor and support base plates using expansion anchor / bolts are being analyzed to account for plate flexibility, bolt stiffness, shear-tension interaction, minimum edge distance and proper bolt spacity.
Depending on the complexity of the individual base plate configuration one of the follow-ing methods of analysis was used to determine the bolt forces:
A quasi analytical method, develcoed by Bechtel was used for base plates with eight bolts or less.
A review of the typical base plates used in supporting the subject piping systems indicate that the majority of them were anchered either by 4, 6 or 8 bolts. The plate thickness usually varied from 1/2" to 2" and are not gene. ally stiffened.
For thesc types of base plates an analytical formulation has been develaped which treats tae plates as a beam on multiple spriag supports subjected to moments and forces in three orthogonal direci. ions. Based on analytical con-siderations as well on the results of a number of repre-sentative 'inite element analyses of base plates (using the "ANSYS Code), certain empirical factors were intro-duced in the simplified beam model to account for (a) the effect of concrete foundation (b) the two way action of load transfer in a plate.
These factors essentially provided a way for introducing the interaction effect of such parametric variables as plate dimensions, attach-ment sizes, bolt spacings and stiffnesses on the distribution of external loads to the bolts.
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_ The re.'ults of a number of case studies indicated exceller.t correlation between the results of the present formulation and those by the finite eltment method (using the "AllSYS" Code). The quasi analytical method generally overpredicts the bolt loads than the FEM.
Although the effect of plate flexibility has been explicitly considered in the quasi analytical for-mulation described above, the impact of prying action on the anchor bolts was determined not to be critical for tr.e fcilowing reasons:
Where the anchorage system capacity is governed by the concrete shear cone, the prying action would result in an application of an external compressive load in the cone and would not there-fore affect the anchorage capacity.
Where the bolt pull out determines the anchorage capacity, the additional load carried by the bolt due to the prying action will be self limiting since the bolt stiffness decreases with increas-ing load. At higher loads the bolt extension will be such that the corners of the base plate will lift off and the prying action will be relieved. This phenomena has been found to occur when the bolt stiffness in the Finite Element Analysis was varied from a high to a low value, to correspond typically to the initial stiffness and that beyond the allowable design load.
A computer program for the analytical technique described above has been implemented for determining the bolt loads for routine applications.
The program requires plate dimensions, number of bolts, bolt size, bolt spacing, bolt stiffness, the applied forces and the allowable bolt shear and tension loads as inputs.
The allowable loads for a given bolt are determined based on the concrete edge distance, bolt spacing, embedment length, shear cone overlapping, manufacturer's ultimate capacity, ar.d a design safety factor.
The program computes the bolt forces and calculates a shear-tension interaction value based on the allowable loads.
The shear-tension interaction in the anchor bolts has been accounted for in the following manner:
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. Where the applied shear force is less than the frictional force developed in the shear plane between tne steel and the concrete surface for balancing the imposed loads, no additional provisions are required for shear.
Othemise, the total applied shear is required to be carried by the bolts in. accordance with the following interaction formula.
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5, 1. 0 Where T and S are the calculated tensile and shear forces and T nd S allowable valres. A A are the respective For special cases where the design of the support didn't lend itself to the foregoing method, the finite element method using the "ANS(S" code and/or other standard engi-neering analytical techniques with conservative assumption were employed in the analysis.
2.
In the current design review, based on the manufacturers ultimate static load test data, a factor of safety of four was used for wedge type anchors and a factor of safety five was used for shell type anchors.
At the Rancho Seco Nuclear Generating Station, some pipe supports have concrete anchor bolts that have a factor of safety of two to five.
For these specific pipe supports, all of the concrete anchor bolts on the support will be tested to the calculated design load of the highest loaded anchor bolt, thereby validating the integrity of anchor bolt installation.
3.
The original design of the piping systems did consider deadweight, thermal stresses, cyclic vibration, seismic loads, and dynamic loads (including water hammer in the feedwater and main steam systems) in the generation of the static equivalent pipe support design loads.
To the extent that these loads include cyclic considerations, these effects were included in the design of the hangers, base plates and anchorages.
The safety factors used for concrete expansion anchors, installed on supports for safety related piping systems, were not increased for loads which are cyclic in nature.
The use of the same safety factor for cyclic and static loads is based on the FFTF Tests *.
The test results indicate:
- Drilled - In Expansion Bolts Under Static and Alternating Loads, Report No. BR-5853-C-4 by Bechtel Power Corp., January,1975.
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The expansion anchors successfully withstood two million cycles of long term fatique loading at a maximum in isity of 0.20 of the static ultimate capacity. When the mu.imuin load intensity was steadily increased beyond the afore-mentioned value and cycled for 2,000 times at each load step, the observed failure load was about the same as the static ultimate capacity.
The dynamic load capacity of the expansion anchors, under simulated sei,mic loading, was about the same as their correspondi.ig static ultimate capacities.
4.
It is not necessary that the bolt preload be equal to or greater than the bolt design load, ripe supports and anchors are subjected to static and dynamic loads.
The dynamic loads are seis,nic loads which are short duration cyclic loads. This type of cyclic load is not a fatigue load, so the amount of preload on the bolts will not greatly affe:t the per-formance of the anchorage.
Therefore, if the initial installation torque on the bolt accomplishes the purpose of setting the wedge, then the ultimate capacity of the bolt is not affected by the amount of preload present in the bolt at the time of cyclic loading.
For vibratory loads during plant operation, the expansion anchors have successfully withstood long term fatigue environment as discussed in the previous section.
A testing program has been started to verify all expansion anchors proper location, minimum numbers of anchor bolts, spacing and edge distance as shown on design drawings, type of anchor used, embedment length and pro-jection of anchors, washers, anchor bolt diameter and anchor bolt length.
Also, expansion anchors are being tested for preload using the sampling technique specified in the Bulletin Appendix A.
The documentation, indi-cating the location of expansion anchor and group represented, method of test, test results, date of te+ along with name and signature of the irspector, are maintained at the jobsite.
5.
The testing program is currently being iniplemented for accessible pipe support plate concrete anchor bolts.
If any suppoi
~ ate anchor bolts are found deficient and their failure would render u system inoperable, the bolts will be replaced or repaired within 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.
If repairs cannot be made to this schedule, then the plant will be brought to a hot shutdown.
6.
Not applicable to Rancho Seco.
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