ML20215B384
| ML20215B384 | |
| 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-005, IM-P-005-R00, IM-P-5, IM-P-5-R, NUDOCS 8706170309 | |
| Download: ML20215B384 (4) | |
Text
_.)
i l
1 i
CLARIFICATION OF IMPELL CTH OESIGN VERIFICATION CRITERIA / METHODS FOR RESOLUTION OF CYGNA AUDIT CONCERNS
-)
l
)
Evaluation of Composite T-Channel Stitch Welds for Torsionsal Stresses l
Prepared for:
1 Texas Utilities Electric Company Prepared by:
I f
Impell Corporation 0210-040/041 IM-P-005 Revision 0 Prepared by: bgM g/ffg Vg Approved by: ((M s-/f-Es 7
/
8706170309 870600 PDR ADOCK 05000445 A
l l
CONCERN:
Impe11 evaluates stitch welds (i.e., intermittent fillet) in composite T-Channel sections for stresses resulting form flexural loads about the composite section major and minor Stresses in the weld resulting from torsional loading of axes.
the section are not considered.
j BACKGROUND:
Stitch weld allowables for composite T-Channel sections were developed in Impell Calculation M-59.
Allowables were determined for a conservative weld configuration consisting of 1/8" fillet welds 1 1/2" long on a center to center spacing of 6".
Shear flow in the section due to flexural loading was considered as y. V_Q It 1
Using this relationsh'ip, along with the conservativo weld configuration assumed, allowable shear forces in the major and 1
minor axis directions of the composite section were derived.
For design verification of. the stitch weld the following interaction equation is used:
M+E
-< 1.0, Vy Vz i
where vy and vz are the observed shear forces in the major and minor axis directions of the section and Vy and Vz are the allowable shear forces.
DISCUSSION:
To assess the impact of torsional stresses on the qualification of composite T-Channel stitch welds the behavior of the 4
composite section under torsional loading was studied.
Torsional loading results in three types of shear stress; pure j
torsional shear stresses (St. Venant), warping normal stresses, and warping shear stresses.
Rigorous evaluation of the stress distribution in the composite T-Channel section indicates that the torsional loading is carried mainly as pure torsional (St. Venant) shear in the section.
Harping rigidity of the section is low and, therefore, warping stresses are small.
Harping normal stresses were evaluated in detail using the program HARP (Reference Impell Calculation M-46).
It was found that warping normal stresses in the extreme fiber used for member qualification can be large.
However, the distribution of warping normal stresses is such that the resultant stress on the stitch weld is negligible.
2/4
IM-P-005 l
Harping shear stresses were found to be much smaller than warping normal stresses.
The distribution of warping shear stress also results in negligible stress in the weld.
Thus, the warping components of torsional loading are insignificant and need not be considered in the stitch weld evaluation.
- However, although warping in the composite section has negligible impact, the stitch weld is' subjected to additional loading due to the torsional moment in the form of St. Venant shear.
The stitch weld qualification should include an evaluation of St. Venant shear.
Review of the stitch weld qualification procedure employed by Impell and a sample of actual weld qualifications in hanger calculations indicates the following:
a)
Stitch weld qualification generally involves the comparison of SSE loads to OBE allowables, creating significant safety margins and conservative interactions.
b) The weld configuration chosen in developing the allowables is conservative. A 1/8" fillet weld 1 1/2" long on 6" centers was assumed.
Most often the weld is larger and the stitches are longer.
Held allowables may be factored up for-the change-in weld configuration, however, generally the conservative lower bound allowables are used.
c) Observed weld interactions based on a sample of stitch weld qualifications are very low (generally less than 0.20).
The maximum interaction encountered was 0.30.
Significant margin exists in the weld qualification.
d) Based on a sample of supports with T-Channels, torsional loading is very small, resulting in moments less than 0.5 in-kips.
An assessment of pure torsion on a composite T-Channel section indicates a contribution to weld interaction of 0.40 under unit torsional moment. Observed torsion in T-Channel sections, j
however is typically much smaller than the unit torsion I
assumed.
Torsional moments are generally less than 0.5 l
in-kips. Thus, stitch weld interactions could typically be expected to increase by about 0.20.
Hith this 0.20 increase maximum interactions would still be well under allowable limits.
Therefore, the consideration of torsional shear stresses presents no design adequacy concern, due to very low stress levels in the welds and small torsional loading.
I 3/4 i
IM-P-005
-,l CONCLUSIONS:
Torsion in composite T-Channel sections was not considered in evaluating the stitch welds connecting the two channels..
Torsion.in the composite section creates pure torsional-(St.
3 Venant) shear stresses, warping normal stresses, and warping shear stresses. However, the' warping rigidity of the composite section is low and.thus,' torsion is taken mainly through St. Venant shear in the section.. Resulting warping stresses are negligible.
Stitch welds evaluated using the current Impe11' procedures are not very highly stressed.
Maximum interaction ratics, are at 0.30 and conservatively compare SSE. loads to OBE allowables.
Including the,effect of St.'Venant shear stresses due to-torsional loading would. increase the interaction.to approximately 0.50.
Thus, the consideration of torsion in present design verification efforts 'will have:
no impact on the design adequacy of the sections and need not be rigorously addressed.
St. Venant shear stresses resulting from~ torsional loading in composite T-Channel sections will be included in the~
criteria for stitch weld evaluations.for future design considerations.
i
~
4/4