You can join together as many lengths of backing as needed to span the entire length and contour (geometry), but the joints connecting the individual lengths must be joined by CPJ groove welds and the backing must be continuous. The only exception I am aware of is for pipe that permits the use of a split ring.
Al
First off, get yourself a newer edition of D1.1. BUT, even in the 2006 it should be in Clause 5 at about 5.9 BACKING.
In the 2015 edition you also have Clause 9 for Tubulars and in Clause 9.23 of Part E Fabrication there are guidelines as well.
I am not sure I totally understand your part description. Can you attach a photo or sketch?
For the most part Al and John are correct but there are exceptions. All depends on joint geometry, welding symbol, and approved drawings with details.
He Is In Control, Have a Great Day, Brent
If you don't CJP the splices in the backing bar the UT guy will mostlikely find them. As Al stated, you can make them as long as needed. I've seen really large sections of rectangular tubing made with continuous flat bar inside for the full length.
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