D1.5, 3.3.7.6

By etomlins Date 05-29-2002 17:27

The referenced section requires that backing bars on full pen welds either have tacks removed or be welded continuous. Our sub/ fabricator has a joint configuration that makes the backing bar inaccessible after the joint is welded out, and thus cannot grind off the tacks used to hold the backing bar in place at fit-up. Also, being responsible for the design, we do not see the necessity to have the fabricator weld the backing bar solid in lieu of grinding the tacks. Any advice is appreciated.

By TimGary (****)

Date 05-29-2002 18:07

If I'm not mistaken, the main concern about these tacks is that they have a tendency to crack, resulting in the possibility of the crack propagating into the base metal, especially in Cyclic stress conditions. I ran acroos this problem while working on cylindrical legs for Offshore Jack-Up Oil Rigs. We seal welded the backing straps as a solution.
As you are quoting from D1.5, I assume you are working on a bridge. If this weldment sees a lot of stress, I recommend that you seal weld as well, if possible. If it's not possible, can you re-design to allow accessibility? Or perhaps revise the WPS to eliminate the backing strap all together? If backing is a must, consider the stick on ceramic tape for backing. This will still require a WPS change, but it's an option.

By CHGuilford (****)

Date 05-31-2002 13:18

There is an option. D1.5 sec 3.3.7.5 says the tacks should be placed within the weld joint so they will be remelted and become part of the final weld. 3.3.7.6 applies only if 3.3.7.5 is not done.
That may not be the easiest way to do it, but usually not too hard if you watch your joint prep.

CHGuilford

By DGXL

Date 06-02-2002 16:15

DOT/Caltrans enforces this provision of D1.5 with vigor here in CA. I have been on a number of projetcs where tack welds are typically placed in the weld joint and ground (cascaded) before the the root pass is deposited as noted in the previous post. This has now become standard practice. Depending on the design, fillet welding continuously may be more detrimental at these locations.

Many SE's also prohibit contiuous welding of backing bars on girder/column connections (SMRF's) for the same reason.