ML20235F631
| ML20235F631 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 09/09/1987 |
| From: | Bacon T ABB IMPELL CORP. (FORMERLY IMPELL CORP.) |
| To: | |
| Shared Package | |
| ML20235F613 | List:
|
| References | |
| IM-P-011, IM-P-011-R00, IM-P-11, IM-P-11-R, NUDOCS 8709290248 | |
| Download: ML20235F631 (4) | |
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CLARIFICATION OF IMPELL CTH DESIGN VERIFICATION CRITERIA / METHODS FOR RESOLUTION OF CYGNA AUDIT CONCERNS Unbraced Lengths Used to Evaluate Lateral Torsional Buckling of Trapeze Support Post Members i
Prepared for:
l Texas Utilities Electric Company
]
Prepared by:
Impe11 Corporation 0210-040/041 IM-P-011 Revision 0 Prepared by:
2/ 7 h 9l9/87 Approved by:
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IM-P-011 Concern:
The length "A" for use in AISC equation 1.5-7 (which detennines the strong axis bending allowable stress for channels) may be calculated for trapeze type cable tray hangers' assuming bracing for the post members is provided by the tier members. CYGNA has a concern that this length should be based on the overall post length rather than a tier-to-tier or tier-to-anchorage' distance.
Background:
In an effort to reduce conservatism included in the initial stages of the design verification procedure for the CPSES cable tray hangers, Impe11 PI-11, Rev. 2 allows the unbraced length of trapeze su members to be based on tier-to-tier spacings (1, A. 2. 2 )pport post 1
2 3
4.as shown-below when determining the allowable bending stress for channels bent about their major axis.
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a a
2.1 t
12 L
23 1
14 l
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1 o
l l
I CYGNA has expressed a concern that the post unbraced length should be based on the overall post length "L".
By using tier-to-tier spacings, the major axis bending allowable stress is increased for the post member. This allowable stress is calculated using AISC equation 1.5-7.
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Discussion:
The AISC code [1] states that A used in equation 1.5-7 is the distance between cross sections braced against twist or against lateral displacement of the compression flange. The tier members of trapeze supports restrain the post members from twisting, satisfying code requirements for using tier-to-tier or tier-to-anchorage spacing for the unbraced length,f, in equation 1.5-7.
In addition, both the tier members and the cable trays provide a degree of restraint against lateral displacement of the post member.
Additionally, the cable tray hangers are evaluated using the Impe11 computer program SllPERPOST. This program does not include a term to account for moment gradient (C ) in the lateral buckling formula.
b Instead, a value of Cb = 1.0 is conservatively assumed.
In general, the post members for multi-tiered supports have positive end moment ratios, i
resulting in reverse curvature for the posts. Reverse curvature occurs when the posts are withstanding lateral loading at the tier location [2, p.202]. An exaggerated deflected frame showing this curvature is shown in Figure 1. It is acceptable practice to treat an inflection point as a braced point when there is no provision for moment gradient [3, p.481].
l As a result, the tier-to-tier spacings are acceptable for use as an j
unbraced length due to the existence of inflection points caused by reverse curvature bending shown in Figure 1.
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DEFLECTED SHAPE Figure 1 2/3 i
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Conclusion:==
i AISC requirements-for using tier-to-tier or tier-to-anchorage spacings
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as-the unbraced length "A" in equation 1.5-7 are met. Trapeze support i
tier members restrain twisting in the post members. The cable trays and.
l tier members also help to resist lateral displacement of the posts.
l Also, the inflection points caused by the reverse curvature can be appropriately considered as braced points since SUPERPOST does not account for moment gradient in the lateral buckling formula. Therefore, the current practice of using tier-to-tier or tier-to-anchorage spacings in AISC equation 1.5-7 is valid.
References:
1)
AISC Manual of Steel Construction, 7th Edition.
2)
Norris, Wilbur, and Utku, Elementary Structural Analysis, Third Edition,1976; McGraw-Hill, Inc.
3)
Salmon and Johnson, Steel Structures, Second Edition,1980; Harper
& Row Publishers.
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