ML13330A761
| ML13330A761 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/06/1979 |
| From: | Head J SOUTHERN CALIFORNIA EDISON CO. |
| To: | Engelken R NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V) |
| References | |
| NUDOCS 7909200070 | |
| Download: ML13330A761 (5) | |
Text
Southern California Edison Company P. 0. BOX 800 2244 WALNUT GROVE AVENUE J.T.
HEAD, JR.
ROSEMEAD, CALIFORNIA 91770 TELEPHONE VicE PRES-DENT vJ235217 July 6, 1979 U. S. Nuclear Regulatory Commission Office of Inspection and Enforcement Suite 202, Walnut Creek Plaza 1990 North California Boulevard Walnut Creek, California 94596 Attention:
Mr. R. H. Engelken, Director Docket No.
50-206 San Onfore Unit 1
Dear Sir:
References:
(1) NRC (R. H. Engelken) letter to SCE (J. B. Moore) dated March 8, 1979, Docket No. 50-206 (2) NRC (R. H. Engelken) letter to SCE (J. B. Moore) dated June 21, 1979, Docket No. 50-206 The following information is in response to References 1 and 2, which forwarded IE Bulletin 79-02 and Revision 1 thereto concerning pipe support base plate designs using concrete expansion anchor bolts at San Onofre Unit 1.
In the responses that follow, the design and installation of safety related piping systems as defined in the San Onofre Unit 1 FSAR as amended was considered. These responses.correspond to the item numbers given in 79-02, Revision 1.
Item 1
.The design of piping support base plates and anchor bolts for safety related piping systems has been reviewed to verify whether base plate flexibility was accounted for in the calculation of anchor bolt loads.
The results for small (2-1/2 inches or less in diameter) and large (greater than 2-1/2 inches in diameter) piping systems are presented below.
- a. Large Lines (greater than 2-1/2 inches)
The utilization of concrete expansion anchors in support base plate design at San Onofre Unit 1 falls into two chronological categories.
7909200 700
USNRC 1
Page 2 Namely, those supports for the original plant and early modifica tions which were designed in accordance with the original design procedures, and those supports which were designed for recent back fit modifications (subsequent to 1975).
Flexibility considerations for each of these categories are discussed below.
(1) Original Design For base plates using concrete expansion anchors in accordance with the original design procedures, there is no documentation to verify that support base plate flexibility was accounted for in the calculation of anchor bolt loads.
We have reanalyzed these base plates to determine bolt design loads, taking into consideration base plate flexibility.
For these base plates, an analytical formulation was developed which treats the plates as a beam on multiple spring supports subjected to moments and forces in three orthogonal directions.
Based on analytical considerations as well -as -on--the results of a number of representative finite element analyses of base plates, certain empirical factors were introduced in the sim plified beam model to account for the effect of concrete foun dation and 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 dimen sion, attachment size, bolt spacing and stiffness on the dis tribution of external load to the bolt. The results of a number of case studies indicated excellent correlation between the results of the present formulation and those by the finite element method. Based on comparisons with calculations utilizing the finite element method, this formulation conservatively pre dicts the bolt loads.
Although the effect of plate flexibility was explicitly con sidered in the formulation described above, the impact of prying action on the anchor bolts was determined not to be critical for the following reasons:
Where the anchorage system capacity is governed by the concrete shear cone, the prying action would result in the application of an external compressive load in the cone and would not, therefore, affect the anchorage capacity.
Where the bolt pull out determines the anchorage capa city, the additional load carried by the bolt due to the prying action will be self-limiting since the bolt stiffness decreases with increasing load. At higher loads the extension will be such that the corners of
US NRC Page 3 the base plate will lift off, and the prying action will be re lieved. This phenomenon 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.
The shear-tension interaction in the anchor bolts was accounted for.in the following manner:
Where the applied shear force is less than the frictional force developed in the shear plane between the steel and the concrete surface for balancing the imposed loads, no additional pro.vi sions are required for shear.
Otherwise, the total applied shear is required to be carried by the bolts in accordance with the interaction formula (see equation below).
(2)
Backfit Modifications For plant modifications made subsequent to 1975 the flexible base plate approach was utilized. In this approach the resulting con crete expansion anchor tensile loads, due to moment, are calculated by positioning the centroid of the compressive area at the compres sion flange of the pipe support member attached to the base plate.
Other tensile loads are divided by the number of anchors.
The total tensile load on an anchor is the sum of these two tensile loads.
Since the allowable value for shear and tension may differ, shear tension interaction is accounted for by the vector sum of the ratio of each to its allowable. Shear-tension interaction is within al lowable limits when the sum of the squares of these ratios is less than 1 (see equation 1).
f )2 (f
2
+
1.0 (1) where f
= bolt design tension FT = ultimate pullout divided by 5.0 for shell and 4.0 for wedge anchors fv = bolt design shear Fv = ultimate shear divided by 5.0 for shell and 4.0 for wedge anchors
USNRC Page 4
- b. Small Piping (2-1/2 inches or less)
The majority of small piping supports have been designed using the chart analysis method. For such piping, the support locations were dictated by practical layout considerations and in most cases the the supports carry loads well below the weight carrying capacity of the pipes themselves. It is considered that the design of such small piping is highly conservative with respect to support loads and there fore detailed analyses justifying the assumption of rigidity for sup port base plates have not been performed.
There are several base plates utilizing concrete expansion anchors in small piping systems which were installed during recent backfit modifications and which were designed using computer analysis.
For these base plates, the response in 1.a(2) above applies.
Item 2 For those expansion anchors with supporting design documentation of a base plate flexibility analysis, a minimum safety factor of four was main tained for wedge type and five for shell type. A reanalysis of base plates without this supporting documentation was performed using the method outlined in (1) above. Of these, nine supports were found to have safety factors less than the value of five recommended for shell type expansion anchors.
In all cases the safety factors exceed a value of two.
The nine supports, seven of which are located within the containment building, will be modified at the next refueling such that the anchor bolts will meet the appropriate minimum safety factor.
Based on the facts that (1) the calculated safety factors do not reflect consideration of such factors as the higher as-built strength of concrete versus the design strength and (2) significant transients over 11 years of operation of the unit have not resulted in any concrete expansion anchor failures which would be revealed by. routine visual inspection, inservice in spection activities, snubber inspections and recent comprehensive visual in spections, it is concluded continued plant operation pending completion of these modifications does not represent an undue risk to public health and safety.
Item 3 In recent backfit modifications to safety related piping systems at San Onofre Unit 1, consideration was given to deadweight, thermal stresses, seismic loads, and dynamic loads in the generation of the static equivalent pipe support design loads.
To the extent that these loads include cyclic considerations, these effects would be included in the design of the hangers, base plates and anchorages.
USNRC Page 5 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 conducted by Bechtel Power Corporation in January 1975.*
The test results indicate:
- a. The expansion anchors successfully withstood two million cycles of long term fatigue loading at a maximum intensity of 0.20 of the static ultimate capacity. When the maximum load intensity was steadily increased beyond the aforementioned value and cycled for 2,000 times at each load step, the observed failure load was about the same as the static ultimate capacity.
- b. The dynamic load capacity of the expansion anchors, under simulated seismic loading, was about the same as their corresponding static ultimate capacities.
With respect to the original design, seismic loads were considered in developing supporLing loadings for certain safety related systems as-des cribed in the FSAR. As demonstrated by the FFTF tests as discussed in (a) and (b) above, the static ultimate capacity for the support anchor bolts in cluding the safety factors provides adequate resistance to seismic induced cyclic loads.
Item 4 An inspection and testing program is being developed for implementation during the next refueling outage for those expansion anchors without adequate QC documentation.
The inspection and testing program will verify sub-items (a) and (b) of IE Bulletin 79-02, Item 4.
Should you have any questions concerning our response or require additional information, do not hesitate to contact me.
Sincerely,
- Drilled -
In Expansion Bolts Under Static and Alternate Loads, Report No.
BR-5853-C-4 by Bechtel Power Corp., January 1975; prepared for USAEC Hanford Engineering Development Laboratory cc:
Director, Office of Inspection and Enforcement, Division of Reactor Operations Inspection