ML20215B224
| ML20215B224 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 05/15/1987 |
| From: | Ashley G ABB IMPELL CORP. (FORMERLY IMPELL CORP.) |
| To: | |
| Shared Package | |
| ML20215B058 | List:
|
| References | |
| IM-P-004, IM-P-004-R00, IM-P-4, IM-P-4-R, NUDOCS 8706170262 | |
| Download: ML20215B224 (4) | |
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l CLARIFICATION OF IMPELL CTH DESIGN VERIFICATION CRITERIA / METHODS FOR RESOLUTION OF CYGNA AUDIT CONCERNS i
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Safety Margin Against Buckling Using AISC Factored Allowables j
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I Prepared for:
Texas Utilities Electric Company Prepared by:
Impe11 Corporation 0210-040/041 IM-P-004 Revision 0 Prepared by: d./?J.MA 4.w s s'//s-/g 7 Approved by: (( M J-/r-g7
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8706170262 870699 PDR ADDCK O g5
'IM-P-004-CONCERN:
Increasing _AISC allowables for SSE loading conditions may.not.
provide a safety margin against buckling.
In particular,.a concern was raised for brace ' angles in longitudinal cable tray.
hanger supports.
8ACKGROUND:
For cable tray hanger. design verification, the project l applies a 1.6 increase factor to SSE allowables.
Per the CFSES FSAR, normal stresses are never allowed to. exceed 0.90 Fy. [1]
The ASME Code, Subsection NFl[2] Ifmits normal compressive'-
stress to two-thirds of the' critical buckling stress. _ The CPSES FSAR list of structures to be qualified per the requirements of ASME NF excludes cable tray support. systems.
Other nuclear codes examined.for guidance (e.g., ANSI /AISC N690 [3] allow'a 1.6 increase factor for SSE loading conditions without providing an additional limit as a. function; of the criti_ cal buckling stress.
DISCUSSIONS:
AISC Equations 1.5-1 and 1.5-2 are' used to compute axial
=comprssive allowables.
The ATSC equations providp/r.= Cc when-k' a factor of.
k /r = 0 to 1.92 at safety varying from 1,67 at compared to SSRC test data for steel columns [4].
Examination of SSRC test results shows that rol. led steel sections provide the indicated factors of: safety with a.high-degree at confidence since they have.less residual. stress than welded (built-up) sections.
Shorter columns have a lower inherent factor of safety in the AISC equations.
However, the strength of these columns is limited by yielding rather than buckling.
Regardless of column length, the use of a 1.6 allowable
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increase factor under no circumstances allows SSE stresses to exceed the critical buckling stress.
This is illustated.in l
Figure 1 4
Angle brace.s act primarily as axial members.
E'nd connection is provided by relatively flexible gusset plates.
Flexural stresses in these members are typically not significant.
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For angle braces design verified to date, the axial stress interaction ratio has been very small..The average SSE interaction ratio was shown to be less than 12% of the c
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allowable, with an extreme value slightly exceeding one-half l
the allowable.
Tha actual factor of safety against buckling is therefore significantly greater than that provided by the allowable limits.
i CONCLUSION:
Project design verification procedures meet FSAR licensing committments.
4 Project design verification' procedures under no circumstances allow stresses to. exceed critical buckling stresses, l
Longitudinal angle braces design verified to date indicate axial stresses fall far below allowables, providing substantial margin against buckling,
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REFERENCES:
1.
Impe11 Project Instruction PI-03, " Design Verification of Cable Tray Supports", Rev. 4 2.
ASME 1985 Subsection NF 3132.3.
3.
ANSI /AISC N690 1984.
4 Gaylord & Gaylord " Design of Steel Structures", 2nd Edition.
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