Done all the time. Look in the ceiling of a typical 2 car garage ranch. See 2009 national code below ( I don't have more recent version handy) I believe spans were even longer in the prior codes, they are typically downgraded in every revision.

http://forums.pelicanparts.com/uploa...1514652239.jpg

In OP's case, I would not assume roof trusts can be tied to used to support additional weight of ceiling or sheetrock.

Typically there would be 1 or 2 strongbacks for a long span like that.http://forums.pelicanparts.com/uploa...1514652716.jpg

The pic in post 43 is what I was referencing earlier when I mentioned a "L" bracket to fasten to the top of the existing ceiling rafters. At 3' OC, I'd put metal on the ceiling to support the weight of the insulation and be done with it. The PVC might tend to sag with a bit of insulation on top of it at 3' OC. I would think he could use 2 L brackets as Dad911 mentions with 2x4's basically splitting the ceiling into thirds and then use the PVC since he'd be at 18" OC. I don't think the pvc would sag at that spacing. You'd have to weigh the time and cost for lumber to split the rafters into smaller spacing versus just using metal and keeping the spacing as is.
In my pic below my truss spacing is 4' OC with a metal ceiling. Also shows how I installed my door track support. I said earlier my door is 12' tall but it's 12' wide x 10 feet tall. You'll also see the jackshaft opener I referenced earlier.
http://forums.pelicanparts.com/uploa...1514654663.jpg

strongback is just common framing practice, it's still supported at ends.

Having said that, 14' ceilings aren't that tall. I'd frame in walls, keep 14' ceiling height, and add a few ceiling fans.

Thanks for all the input. After mulling this over for a couple of weeks I’ve decided to frame up a little shop in one corner of the building and add decent lighting to the rest. I do want doors at each end so I don’t have to move everything I own to get a car out of the back.