Wow!
The thread scared me. How do the UT Techs write their reports if they don't examine the corners! Is that one of the "realities" UT techs understand but designers don't? If they blink, that's very scary. Not my job, however.
This is my job, however.
Yes, the backing must be continuous all around the joint. If the weld symbol is CJP all around, that means all around even where it's tough in the corners. The easiest way to achieve this is to cut a slug out of Pl. 1" to fit the inside of the tube. Watch your tolerances so you don't get more than a 1/16" gap at any location along the joint. One can use a hand torch to cut the middle out of the slug for really large tubes to reduce the handling weight. Just leave enough metal to help the UT tech. This solution is very common in Utah, and I think the rest of earthquake country because a lot of Utah fabricators ship to Las Vegas and CA.
You can't bend or weld (with full pen weld) a flat bar with sufficient accuracy to maintain 1/16" max gap at the "faying" surface between the tube corners and the back-up bar. We've wasted hours trying to do this over the years.
New testing work has been done concerning the radius'd corners of tubes and it isn't pretty. The steel work hardens there and does not have the ductility of the rest of the tube. Some designers now specify no weld around the corners or CJP along the flats only with a "Typ" symbol and no weld all around symbol. This is good practice nowadays even if it is poor use of a weld symbol. If the designer doesn't want the corners welded, don't weld them! Do what the designer tells you. It might be worth a Request for Information for clarification.
I agree with the concern about weld dams and I would bevel all around, too, even if the corners aren't welded.
We should discuss how to finish the weld ends of these new "flats only" welds. I haven't run into it yet. Any thoughts?
Bkoz
By ctacker 
Date 09-02-2007 09:23
Edited 09-02-2007 09:35
In the past i would make 1 bend(for 2 sides) on 2 pieces bar, take the ID of the tube minus bar thickness for the lengths of each leg, weld them corner to corner, grind the inside to the weld and weld a fillet inside,then grind the radius needed,on the bent corner i would butter up the backup bar with low hydrogen on the outside of the corner then grind it to suit, never had a complaint!
As for welding only the flats, IMO thats the best way if water is not an issue or cosmetics allow it. that way if a weld cracks,it doesnt crack all the way around! I've had jobs where welding in the K region of a beam and wrapping the flanges was prohibited on columns for that reason!
