Welding

IF they both are comprised of structural steel, then why not? It's either field weld the purlins to the truss, or bolt them.....

purlins are usually made of high tensile strength steel. welding will weaken them. mild steel angle cilps should be welded to the truss then the purlins should bolt to the clips.

Is there any in steel structural code that says it will weaken the purlins or alter its purpose?

i'm sure the manufacter would void any warrenty involved. you won't find specs because welding pulins is'nt allowed. they can only be bolted. normally one bolt near the bottom of the purlin an another near the top.

This is kind of an open ended question that needs a lot more info before an answer could be given. It largely depends on the type of perlin in discussion. If it is one of the old Z perlins of lighter guage materials, Factory Mutual had issues with these a few years back and required other fastening procedures and bracing. So given the fact you gave no info on what your working with, I would suggest you start with your insurance carrier. They will most likely have info that you can go on. Then you will also need to look at local building codes for your area.