That's probably more of the contractors know how to do it one way, the way they've always done it, and don't want to follow what the engineer says as that's usually more complex. I would show what the engineeer says how to do it to the county and get them to pre-approve and sign off on it, and then find a contractor willing to do it the way the engineer says. It takes a 5 hour class to become a contractor, but many years of study to be an engineer. I'd rather an engineer sign off on how to do it than a contractor.I have had an engineeer spec the joists, girders, rafters, etc. and now have contractors saying "you can't do that by code"..