Hello Alex;

The code isn't always as clear and precise as we would like it to be. This may be one of those situations.

I look at the weld thickness (size) in a transition between thick and thin members to be the critical consideration. If one of the plates is 4 inches thick and the other is only 1/4 inch thick, the weld between them can be on the 1/4 inch side (the weld size would be 1/4 inch) of the joint or it can be in the 4 inch plate and it would be somewhat thicker depending on the weld detail involved. In the case of a PJP groove weld the weld size can be as small as that permitted for a PJP weld or nearly as thick as the thinner member if the weld isn't back gouged.

I use the thickness of the weld deposit as a rational approach to a problem that isn't clearly addressed by the code. It may be a moot point if the WPS was qualified on 1 inch plate because the procedure would be qualified for unlimited thickness in that case.

Why couldn't you have asked how to select the preheat temperature? That would have been an easier question to resolve.

The attached sketch depicts a couple of different configurations that could be encountered. However, in any event, the CJP weld can not exceed the thickness of the thinner plate, which is also a valid argument and I would not take a position opposing it. However, that wouldn't address PJP welds that are apparently qualified by any CJP weld on any thickness material. Another fly in the ointment and somewhat of an inconsistent approach to the problem. The approach that I take, i.e., thickness of the weld deposited, takes into consideration both CJP and PJP joints in a consistent manner. It may be conservative, but in the absence of a clearly stated position by D1.1, it works for me. 

As is always the case, this is only my opinion. You should direct this question to the D1.1 Code Committee. You might be surprised and get a response in a couple of months. I asked a question that took nearly two years to resolve and it might even be included in upcoming edition of D1.1. I doubt it will be addressed in the 2008 edition, but you never know, maybe that's why it's being held up.

Best regards - Al