ML20206S502

Forwards Summary of Review of SEP Topic III-7.B Re Effect of Snow Load Conditions on Critical Plant Components,Per NRC 860310 Request.Heating Boiler Roof & Lower Roof of Primary Auxiliary Bldg May Not Meet Current Snow Load Criteria

ML20206S502
Person / Time
Site:	Yankee Rowe
Issue date:	09/08/1986
From:	Papanic G YANKEE ATOMIC ELECTRIC CO.
To:	Mckenna E Office of Nuclear Reactor Regulation
References
TASK-03-07.B, TASK-3-7.B, TASK-RR DCC-86-135, FYR-86-085, FYR-86-85, NUDOCS 8609220245
Download: ML20206S502 (8)

Text

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T1lephone (611) 8724100 TWX 710380 7619 YANKEE ATOMIC ELECTRIC COMPANY

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N 1671 Worcester Road, Framingham, Massachusetts 01701 DCC 86-135

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YANKEE 2.C2.1 September 8, 1986 FYR 86-085 United States Nuclear Regulatory Commission Washington, DC 20555 Attention: Ms. Eileen M. McKenna Project Manager, Project Directorate No.1 Division of PWR Licensing - A

References:

(a) License No. DPR-3 (Docket No. 50-29)

(b) Letter, USNRC to YAEC, dated March 10, 1986

Subject:

SEP Topic III-7.B. Snow Loads

Dear Ms. McKenna:

In your letter of March 10, 1986, Yankee was requested to evaluate the effect of two snow load conditions on critical plant components. We have completed that evaluation, and we provide the following summarization of our work. The two load conditions are:

1. 40 psf normal snow load

2. 125 psf extreme snow load Structures subjected to Load Case No. I were evaluated to the current (1978) AISC Code or to the current ACI Code using code allowable limits.

l Structures subjected to Load Case No. 2 were evaluated to faulted allowables identical to the SEP allowables which the Staff recommended for review of SEP Topic III-6.

A complete summary of our findings is presented in Enclosure A. The critical plant components, which you requested us to evaluate, are those necessary for safe shutdown. We have evaluated roof decking, structural framing, enclosures, and other components which either are a part of safe shutdown or house and protect safe shutdown components. The structures which were evaluated were selected based on information obtained from plant visits in addition to input from our Systems and Mechanical Engineering Croups.

Based on our review, we conclude that all plant structures have the capacity to support the snow loads for which they were originally designed.

This recent SEP evaluation, however, has demonstrated that the Heating Boiler Room roof and the lower roof of the Primary Auxiliary Building may not meet the current SEP snow load criteria. The specific roofs are described in more detail in the Enclosure. Both roofs will be redecked, and other appropriate structural modifications will be made upon satisf actory resolution of this O Topic, h0 3 I I i 8609220245 860908 PDR ADOCK 05000029 P PDR l

United States Nuclear Regulatory Commission September 8, 1986 Attention: Mr. Eileen M. McKenna Page 2 FYR 86-085 We believe the enclosed summary is sufficient to resolve the issue of snow loads for YNPS. We will submit a response to your remaining SEP Topic III-7.B questions separately.

Our commitment to make plant modifications is contingent upon your acceptance of the enclosed material. It is our understanding that the Staff will issue an SER for snow loads and a summary resolution of this topic in a future supplement to NUREG-0825. If this is not the case, please advise us.

Very truly yours, G. Pap c, Jr.

Senior Project Engineer- censing CP/dps l

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ENCLOSURE A SEp Topic III-7.B. Snow Loads criteria Evaluation for the extreme snow load was performed using the following faulted allowables for steel structures:

Ft 1 0.9 Fy Fb 1 1.5 Fb (Normal) but not > 0.9 Fy Fa 1 0.9 Fce but not > 0.9 Fy Where FL, Fb, Fa, and Fy are as defined in the 1978 AISC Specification and Fcr is the critical, i.e., buckling, stress in an axially loaded compression member.

Because this snow load is an extreme environmental load, it was combined with dead load only.

I. Turbine Room Structure o Location /

Description:

The Turbine Room is a five-story steel frame structure located at the north end of the Turbine Building. Roof framing consists of trusses, purlins, and decking. the only components necessary for safe shutdown in the Turbine Room structure are a two-inch pipe and valve which are part of the Steam Drain

System and a one and one half inch conduit and solenoid which are part of the containment Isolation System Train B. These components are located in the southeast corner at or below the mezzanine level 1 and are protected by the five-inch reinforced concrete operating floor slab which was designed for 500 psf in this area.

o Load Case 1, 40 psf Normal Snow Load: This load is identical to the roof live load used in the original design. An evaluation has

$ confirmed that the original design of the Turbine Room structure is acceptable for this load case.

o Load Case 2, 125 psf Extreme Snow Load:

The Turbine Room roof decking does not meet faulted allowable limits

_ for this load case. The maximum allowable snow load on the roof is 65 psf.

5- The decking was analyzed for the condition of two-span continuous with a maximum span of seven feet - nine inches. The purlins were analyzed as simply supported beams, laterally braced, spanning 22 7 feet 0 inches between trusses and spaced seven feet - nine inches apart.

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e ENCLOSURE A (Continued)

The truss members were analyzed as pin-connected members with K = 1.0. A faulted allowable stress of 0.90 Fcr is considered acceptable due to the conservative K value of 1.0 (connections at each end of member would provide partial fixity and reduce the actual value of K to less than 1.0).

The effect on the building columns of the extreme snow load is less than that of the turbine hall crane load. Therefore, the extreme

' snow load is not the controlling load on the columns and footings.

o Proposed Action Even though the Turbine Room roof decking does not meet the faulted allowable limits for Load Case 2, failure of the structurs will not occur and the failure of the-decking will not impair the func.6 ion of the components necessary for safe shutdown because of their location below the operating floor level. Therefore, no modifications are required.

II. Cable Tray House o Location /

Description:

The Cable Tray House is a block wall structure with a 4 1/2-inch deep metal roof deck spanning approximately 15 feet between walls. It is located on top of the Control Room roof.

o Load Case 1, 40 psf. Normal Snow Load: This load is identical to the roof live load used in the original design. An evaluation has confirmed that the original design of the Cable Tray House is l acceptable for this load case.

o Load Case 2, 125 psf Extreme Snow Load: The stress in the decking is less than the faulted allowable 0.9Fy. The bearing stress on the block wall is less than the ACI code 531-79 allowable value.

o Plant Modifications: The roof decking and the block walls are within allowable stress limits for both load cases. Therefore, no

modifications are required.

III. Control Room Structure i

l o Description / Location: The Control Room structure is very massive, l'

consisting of three and four feet thick concrete walls and three j feet - three inches deep precast inverted tee roof beams. It is

! attached to and shares common framing with the south side of the l Turbine Room structure. It is located above the Switchgear Room which, in turn, is located above the boiler feed pump area.

o Load Case 1, 40 psf Normal Snow Load: Meets code allowable limits.

l See Load Case 2.

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ENCLOSURE A (Continued) o Load Case 2, 125 psf Extreme Snow Load: The precast inverted tee roof beams were conservatively analyzed as rectangular beams (tee flanges were neglected) for dead loads plus 125 psf extreme snow loads. The maximum stress was found to be less than the ACI code allowable with no increase in allowable stress for extreme environmental loads.

Similarly, the concrete walls were found to be stressed to less than ACI Code allowable limits with no increase in allowable' limits.

The additional load on the footings from the increased snow load is less than the original design live load of 50 psf on each of the two floor levels. Therefore, the Control Room structure is acceptable for both load Cases 1 and 2.

o Plant Modifications: The stresses in the concrete are less than ACI Code allowable limits for Load Cases 1 and 2, even without the allowable stress increase for Load Case 2. Therefore, no modifications are required.

IV. Heatina Boller Room Structure o Location /

Description:

The Heating Boiler Room structure is a two-story steel frame structure attached to the southeast corner of the Turbine Room structure.

Roof framing consists of steel beams and 1 1/2-inch deep steel decking. The Heating Boiler Room structure houses piping which is part of the Steam Drain System and is necessary for safe shutdown.

o Load Case 1, 40 psf Normal Snow Load: Because the Heating Boiler Room structure roof level is adjacent to and below that of the Control Room roof on its west side and the Turbine Room roof on its north side, the potential exists for drifting against the higher walls. The current (1980) Massachusetts State Building' Code accounts for this drifting of snow against the higher walls up to a maximum of 120 psf.

The existing roof decking does not meet the current criteria with drifting because the original design code, ASA A58.1-1955, did not account for drifting.

The roof beams, columns, and footings do meet current criteria.

o Load Case 2, 125 psf Extreme Snow Load: The stress in the roof decking exceeds 0.9Fy. The decking was assumed to be continuous over three spans. The stresses in the roof beams, columns, and d

footings are less than the faulted allowables.

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gNCLOSURE A (continued) l l

I 0 Plant Modifications: Because the existing roof decking does not meet the criteria for Load Cases 1 and 2 modifications are proposed to upgrade the roof decking to meet all criteria for both Load cases.

V. Primary Auxiliary Building (PAB) o Location /

Description:

The PAB is a two-story structure which consists of reinforced concrete walls up to a reinforced concrete roof slab in the central portion. Each end of the building consists of steel framing with steel roof decking at the upper level. It is located south of the vapor container.

o Load Case 1, 40 psf Normal Snow Load: Portions of the PAB roof with steel framing and decking are adjacent to the higher level roofs of the Stack Monitoring Equipment Room and Upper Pipe Tunnel. The higher roofs can cause drifting snow to accumulate on the lower-adjacent roof similar to the situation for the Heating Boiler Room structure discussed previously.

The existing lower level roof decking does not meet the current (1980) Massachusetts State Building Code criteria because of the additional drifting load which was not part of the original design code.

The existing roof beams and columns do meet current criteria.

o Load Case 2, 125 psf Extreme Snow Load: The stress in the lower level roof decking exceeds 0.9 Fy. The decking was analyzed as continuous over two spans. The stresses in the roof beams and

. columns are less than the faulted allowables.

The central reinforced concrete portion of the PAB consists of up to three-foot thick walls and roof slab for shielding purposes.

Stresses due to the extreme snow load are low.

o Plant Modifications: Because the existing lower roof level decking does not meet current criteria for Load Case 1, and also exceeds the faulted allowables for Load Case-2, modifications are proposed to i upgrade the lower level roof decking to meet criteria for Load cases L 1 and 2.

l VI. Safe Shutdown System (SSS) Building i

i o Location /

Description:

The SSS Building is a one-story reinforced l

concrete structure with twelve-inch thick walls and roof. It is located north of the fire water tank (TK-SS).

o Load Case 1, 40 psf Normal Snow Load: Meets code allowable limits.

See Load Case 2.

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ENCLOSURE A

~(Continued) o Load Case 2, 125 psf Extreme Snow Load: The SSS Building was built in 1984 utilizing ultimate strength design per the current (1983)

ACI Building Code. The total factored vertical design load used in the original design was 500 psf. Using the 200 psf deed load from the original design and the 125 psf extreme snow load with the normal load factors yields less than 500 psf. Therefore, both the normal and the extreme snow load cases are enveloped by the original design.

o Plant Hodifications: No modifications are required.

VII. Fire Water Tank (TK-55) o Description / Location: TK-55 is a 350,000 gallon cylindrical steel water tank 44'-0" in diameter and 32'-0" high. The supported cone roof consists of steel plates supported by radial rafters and a center post. The slope of the roof is 3/4-inch per foot. TK-55 is located southwest of the VC.

o Load Case 1, 40 psf Normal Snow Load: The original design snow / ice load is 70 psf. Therefore, this load case is enveloped by the original design.

o Load Case 2, 125 psf Extreme Snow Load: The stresses in the 3/16-inch roof plate and the C8 x 11.5 rafters exceed the faulted allowable of 0.9 Fy. However, the accumulation of a 125 psf snow load around the edge of the tank is virtually impossible. This is because the roof has a 3/4-inch per foot pitch and is rounded off at the edge with no parapet. Two reasonable extreme snow load profiles are a linear variation of 125 psf at the center of the tank to 70 psf at the edge of the tank or a constant 125 psf from the center to 6 feet from the edge decreasing linearly to zero at the edge. Both profiles result in roof plate and rafter stresses which are less than 0.9 Fy.

The stresses in the center post which consists of two channels, the peripheral angle, and the tank shell, are less than the faulted allowables for the uniform 125 psf snow load.

The roof plate was analyzed as a membrane with in-plane and bending stresses. The rafters were analyzed as beams laterally braced continuously. The peripheral angle was analyzed as a circular ring subjected to uniformly distributed torsion. The tank shell was analyzed as a cylindrical shell subjected to axial and circumferential stresses.

For this snow load, the increase in load on the footing is approximately 33 percent which is equivalent to a one-third increase in allowable stress.

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ENCLOSURE A (Continued) o Plant Modifications: No modifications are required for the two extreme snow load profiles described above.

VIII. Vapor Container o Location /

Description:

The vapor container is a 125-foot diameter steel sphere supported on 16 steel columns. It is located south of the Control Room.

o Load Case 1, 40 psf Normal Snow Load: This load case is equivalent to the original design load which is identical to that specified in the current code.

o Load Case 2, 125 psf Extreme Snow Load: This load case is considered not applicable to the vapor container for the following reasons:

(a) The spherical shape will not allow snow buildup.

(b) The painted steel surface is very smooth, allowing snow to slide off easily.

(c) Due to the fact that the inside is heated and the steel shell is not insulated, the exterior surface temperature is always above 320F during operation.

o Plant Modification: No modifications are required for the Vapor Container.

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