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Up Topic American Welding Society Services / Technical Standards & Publications / Backing Bar Thickness Allowable
- - By MichCabo Date 04-14-2014 11:43
I'm trying to find the maximum allowable backing bar thickness in D1.1, I suspect that the use of thicker backing bars than the base metal may be a reason to create a lack of fusion on the root.
For example a 3/4" plates with a backing plate 1 13/16" thick.
Parent - - By Milton Gravitt (***) Date 04-14-2014 12:01
MichCabo, if you have a D1.1 2010 book look on page 197 5.10.3 Backing Thickness.

                                  M.G.
Parent - By SCOTTN (***) Date 04-14-2014 12:11
As far as I know, there is no maximum backing thickness in D1.1  Only the recommended minimum thicknesses.  I personally don't see a need to increase the backing thickness as long as the root pass/passes are being retained.  If there is lack of fusion at the root, I would think that it's caused by something other than the backing being too thick.
Parent - - By 46.00 (****) Date 04-14-2014 12:22
Why would you have a backing bar thicker than parent metal?
Parent - - By MichCabo Date 04-14-2014 14:46 Edited 04-14-2014 14:49
Please take a look on the image, its supposed to be a V groove (Formed by a bevel and a fillet weld) over a tube 1.875" thick (Act as a backing).  The current root opening is 6mm or less (depends on the actual fillet weld size). The vertical plate restricts a better angle of the torch. I'm thinking in a small fillet weld between doubler and the tube as a first pass in order to focus the heat in both tube and doubler.

Parent - - By 46.00 (****) Date 04-14-2014 16:26
I thought you wanted to know about backing bar thickness?
Parent - - By MichCabo Date 04-14-2014 18:08
If the thickness of the backing (the tube in this case) is above the limitations, this joint may be considered as a lap joint instead of butt joint with backing bar.

In a lap joint, I can request a wider area to locate the GMAW torch in a proper angle.
Parent - By 803056 (*****) Date 04-14-2014 18:54
Your line of thinking is warped and twisted to make it fit the situation. The joint you depict in your sketch is a groove weld.

There is nothing in D1.1 that addresses backing that is too thick. A PJP on thick material is considered to be a weld with backing. That portion of the joint that is not penetrated is considered to be backing.

Al
Parent - - By jwright650 (*****) Date 04-14-2014 20:07
D1.1 only mentions minimum backing thicknesses to be used (to prevent melt through), says nothing about using thicker materials as backing for groove welds. See AWS D1.1:2010 Clause 5.10.3
Parent - By SCOTTN (***) Date 04-14-2014 21:30
MichCabo

If you are using the 1.8125 "tube" as backing, your detail shows a root opening of "currently" 12 mm (15/32") and indicates "will be 30 mm" (1 3/16").  Among other things, this is not a prequalified root opening.
Up Topic American Welding Society Services / Technical Standards & Publications / Backing Bar Thickness Allowable

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