ML19350B894
| ML19350B894 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 03/12/1981 |
| From: | Sylvia B VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | James O'Reilly NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| References | |
| 152, IEB-79-02, IEB-79-2, NUDOCS 8103231083 | |
| Download: ML19350B894 (2) | |
Text
_
Q VIRGINIA EI.ecrRIC AND POWE N CO MPANY Hzcatwoxo,VIMOINIA 2Ou61
'farch 12,1981 s ( '. I ' (Ty,
c, xW Mr. James P. O'Reilly, Director
/
p Serial No. 152 Office of Inspection and Enforcem g p
'l N0/RWC:ms
/Mgp"c,IO81 Docket No.
0-338-U. S. Nuclear Regulatory Commissi an-O u.s. g M yr % ro,, N A..}
License No..
Region II
\\
co b.
101 Marietta Street, Suite 3100 Atlanta, Georgia 30303 g
<t;.
W
Dear Mr. O'Reilly:
S 6
REQUEST FOR ADDITIONAL INFORMATION NRC I & E BULLETIN 79-02 NORTH ANNA UNIT NO. I 1
This letter is in response to the verbal request by your Mr. H. J. Wong for additional information concerning our letter Serial No.
- 984, dated December 17, 1980, on I.E.Bulletin 79-02.
As discussed in our telephone conversation with your staff members on February 2, 1981, the " base plate anchor bolt load ratio vs stiffness ratio" curves given in Figure 2 attached to our letter Serial No. 984 were developed from finite element studies of base plates subjected to moments and forces.
The studies. did not include cases with pure tension.
Thus, for symmetric 1
flexible base plates with six or more anchor bolts subject to pure tension loads, the forces in bolts closest to the structural attachment may be underestimated by our review procedure.
For the six bolt plate given ~as j
Example 2 attached to letter Serial No. 984, in the pure tension load case, the load factor calculated from the base plate review is approximately 1.1, whereas the load factor calculated from a ~ linear elastic finite element analysis is approximately 1.4.
We believe that this difference in load factor is not significant for the following reasons:
1.
The number of Seismic Category I symmetric flexible base plates with six or more bolts subject to pure tension is very low, and in general, structural members are not usual.
.nconnected to the base plate.
l 2.
The presence of even small moments reduces the loads in the bolts that are close to the structural attachment.
In general, most connections to' base plates induce moments and shears on the plates.
l i
81032310 %
Q.
t
F Mr. James P. O'Reilly 2
Visorwta Euctaic no Powra Con 4wv ro 3.
Base plate stresses are usually low.
Thus, redistribution of bolt 2oads can occur without overstressing the plate.
Prior to bolt failure significant load redistribution would occur since bolt displacements become large when ultimate bolt load capacity is approached.
4.
Construction tolerances and unequal slips in bolts will redistri-bute bolt loads more uniformly than indicated by the calculations.
5.
In the review of base plates for flexibility, the allowable tension loads in Hilti bolts had a factor of safety of 4 as required by the I&E Bulletin 79-02.
However, the original design of the. bolts had, a minimum factor of safety of 5.5 for tension loads.
Thus many bolts have a factor of safety larger than 4.
6.
The minimam factor of safety used for shear allowables in Hilti bolts is 7, which is much larger than the 4 required by the I&E Bulletin 79-02 Thus bolt designs with shear and tension are conservative.
7.
Allowable bolt loads used in the design are for anchors in concrete with minimum compressive strength of 3,000 psi.
Our experience at Surry Unit 1 and 2 and at North Anna (as shown in table 3.8.1-1 through 3.8.1-6 of the FSAR Amendment 63) indicates that the in situ concrete strength is likely to exceed 4,000 psi.
The allowable bolt loads in 4,000 psi concrete are at least 20%
higher than those in 3,000 psi concrete.
8.
The seismic loads on base plates. are conservatively calculated, and therefore factors of safety are larger than those indicated above.
As can be seen from the above discussion, our base plate anchorage design is conservative.
Please contact us if you have any further questions.
Very truly yours, B. R. Sylvia Manager - Nuclear Operations and Maintenance cc:
Mr. Howard J. Wong Office of Inspection and Enforcement t
5,*
e
'Y k
)
y
,