D1.5 has different welder performance qualification testing requirments (stricter) so unless the Engineer in authority is willing to sign off, I don't see welders "certs." easily crossing from D1.1 to D1.5 for the purposes of production.

For Welding Procedure Specifications to cross pollinate, it would have to be researched by an expert to be sure that all essential varibles, seismic requirements, as well as specific contractual requirments had been met or approved by the Engineer in authority to make that kind of decision.   The rigor I would think might be greater trying to get D1.1 procedures accepted into a D1.5 production scenario.

I bet Al and Brent have something even more complete to say about this.

Oh......... Welcome to the Forum!

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With a handle like that, I wish you had put more info in your profile so I knew your name,  But...

WELCOME TO THE AWS WELDING FORUM!!

Now, It is not really a matter of one being over or better than the other one.  Both codes have their own applications.  It is also not that a company can use one for all the paperwork and then do any work that comes along in either of the two categories.  BUT...

You can have 'dual' certifications and/or WPS's.  You would get the majority of information from the D1.5 Bridge Code, because it is more restrictive in many areas, and then make sure you utilize parameters that cover both codes.  The main conflict is with welder qualifications which have some restrictions in D1.5 that are not required with D1.1.  Namely, use of only a slag hammer and wire brush during the test for D1.5.  But, if you test all your welders to that standard and any other essential variables for welder qualification then their cert can read that they are qualified to both.

The same applies to the WPS you are working to in production.  It is totally possible, if memory serves me right, to have WPS's that are acceptable to either code. 

Now, that doesn't mean that absolutely anything goes.  There will be applications where each will need a WPS specific for the job and code.  But, there are many areas where you can do it.  They need to be carefully researched and filled out and make sure they are applicable to the task at hand.

A couple of examples: 1) Pre-Heat, Inter-pass temps, and Maximum Temps-  D1.1 has no maximum and as a minimum standard leaves some things to be desired with it's low pre-heats.  D1.5 and the Seismic codes, D1.8, have a maximum inter-pass temp of 550°F.  So where the form has a spot for max inter-pass you would need to state it as such.  That restricts some applications of D1.1 but how often does one go over that temp and why would you want to even if it were allowed?  Production and profits is the obvious answer.  Not for quality and safety sake.
2) Materials- the two codes would need to be carefully compared to make sure the materials in question were covered in both.  But a dual WPS may be used where the materials are the same. 

Bottom line:  Most D1.1 welders will have to be qualified to D1.5 as well.  But, often, the D1.5 welders are good for D1.1 but need their cert to read that they are dual qualified or get the engineer on the job to accept their qualifications.

Hope that was helpful.  Just my two tin pennies worth.  And no books in front of me to get specific.

Have a Great Day,  Brent