ML20215B206

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Rev 0 to Clarification of Impell Cth Design Verification Criteria/Methods for Resolution of Cygna Audit Concerns, Analysis of Welds Connecting Base Angles to Embedded Plates
ML20215B206
Person / Time
Site: Comanche Peak  Luminant icon.png
Issue date: 05/15/1987
From: Ashley G
ABB IMPELL CORP. (FORMERLY IMPELL CORP.)
To:
Shared Package
ML20215B058 List:
References
IM-P-003, IM-P-003-R00, IM-P-3, IM-P-3-R, NUDOCS 8706170256
Download: ML20215B206 (5)
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-CLARIFICATION OF IMPELL CTH DESIGN VERIFICATION  ;

CRITERIA / METHODS FOR RESOLUTION OF CYGNA AUDIT, CONCERNS 4

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Analysis of Welds Connecting Base Angles f to Embedded Plates 4 l

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1 Prepared for:

Texas Utilities Electric Company

  • Prepared by:

Impell Corporation 0210-040/041 IM-P-003 Revision 0

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Approved by: ((M T-/T-47 y

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CONCERN: Impell assumes an even distribution of loads in evaluating welds connecting base angles to embedded plates. A concern was raised regarding this distribution

,of loads since the loaded member is attached to the free leg of the angle and, thus, loads could be concentrated in the weld closest to the free leg. l BACKGROUND: The base angles for cable tray hangers are sometimes attached to embedded plates if the plates are . located in .

proximity and if the plates are not occupied by other items within the zone of influence. When cable tray hanger base angles are attached tosembedded plates they  !

are usually attached with stitch fintermittent) weld i patterns. Intermittent welds are used for two reasons:

1) Continuous welds are not required to attain the  ;

necessary design capacity, and, more importantly,

2) To avoid problems with warping and residual stresses i in the embedded plate due to excessive heat during j wel ding.

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The typical base angle connection to an embedded plate is shown below in Figure 1.- I I l

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S E~CTION A-A SEcrION 8-8 FI G UR.E 1 2/5 ,

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l impell's procedure for; evaluating'such intermittent welds provides a very conservative analysis' approach.while assuming an equal distribution of loads to' the welds. In Impell's current procedure, loads from the attachment are transformed to loads'at the weld C.G.' considering all the >

, necessary eccentricities land ; increasing applied moments accordingly. The weld 'is treated as a continuous weld pattern of length equal to.the sum of the stitch welds on

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either side of the base angle'(lesser of 1 +12 1 and.13+14 )

as shown in Figure 2. This provides very conservative weld properties in relation to the properties. achieved through rigorous consideration of the intermittent weld, i

A similar approach is employed for base angles'with .l multiple attachments and/or additional intermittent welds. Impe11.'s analysis procedure again provides a very ~1 conservative analysis approach. Loads from the attachments are either; a) transferrred to one location at the weld C.G. for all . attachments considering the appropriate eccentricities, or b) transferred to the C.G.-  !

of the immediate interrittent weld for each attachment.

The weld pattern is then treated.as a' continuous weld considering all of the intermittent welds for case 'a' above or a continuous weld considering only the immediate-intermittent welds for case 'b' above. For case 'b' resultant weld forces are added together directly for -

combined weld stress where-necessary.

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p .c. m DISCUSSION: Clearly the assumption of an equa1' distribution of loads presents no concern for the applied torsional moment and.

direct shear forces. The. base angles commonly used are very rigid in shear transfer and provide an adequate F

mechanism for an even distribution of the load.-

Similarly, all welds would be effective for. resisting moments applied to the base angle. Increased bending moments resulting from load eccentricities (with respect to weld C.G.) are used'to calculate average stresses in the weld. However, since the force applying direct tension to the welds is located'directly over_ the weld, and since the loaded member is' attached to the free leg of the base angle, the load would be transferred directly to the welds nearest'the free leg. The relative stiffness of the angle.and'the weld is such that a -

mechanism'for transfer of this load to the weld at the toe of the angle does not exist. An even distribution of load to all welds in the tensile direction does 'not appear' appropriate.

However, the procedure employed by Impe11'in evaluating intermittent welds is very conser_vative and offsets any -

stress concentration due to this uneven load distribution. -For the typical configuration presented in-Figure 1 and assuming unit forces and moments in each direction, Impe11's evaluation' procedure predicts. results .

almost identical to those achieved assuming an unequal distribution on tensile loading on the. intermittent wel d. Furthermore, this comparison assumes an equal distribution of loads in each direction. However, the true behavior mode of the cable tray systems results in a much different load distribution. In actual system analyses direct. tensile forces are typically very small-and the critical design concern is applied moments.

Hence, the Impell procedure provides very conservative results as properties for weld resistance to bending are underpredicted significantly.

The typical configuration shown in Figure 1 was evaluated assuming a more realistic distribution of forces and moments. Results of this evaluation indicate thet Impe11's procedure significantly (more than 60%)'

overestimates the resultant weld stress attained assuming an unequal distribution of the tensile force on the weld.

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r . sn-e-uus-CONCLUSIONS:

Impell's analysis procedure for intermittent welds significantly underpredicts weld properties and provides a j conservative approach to weld qualification. Although the- j direct tensile loading on the weld through the base angle may be distributed only to the weld directly below the free leglaf i the base angle, the conservatisms in Impell's analysis procedure greatly offset the effect of an uneven load distribution.

Therefore, the procedure used by Impell in evaluating the weld of base angles to embeddment plates provides a conservative method of determining average stresses on the weld material, i

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