Retro fit framing

[Dave_]

I want to shore up an old outbuilding with timber framed trusses/purlins.  The building is 20' x 20' with an 8' plate height and an 8/12 roof.  I want to keep the existing roof system in place (traditional 2x4 on 24"oc).  However, I want to eliminate the bottom cord of the existing "trusses" to open up the space.  My plan is to assemble three trusses/posts inside the building and pivot them up into position with my forklift.  I plan to hang the purlins with steel hangers.  I'm sure I'm not the first one to attempt this, so what issues do I need to account for?

Thanks.

[witterbound]

Put up a ridge beam 16 feet up with two 'bents' to hold the middle up, in an old 24 x 60 one story sagging barn.  Putting the ridge beam up was an effort, with pump jacks and ladders built out of 2x4s.  Neither the floor nor the ridgeline were level, so we had to do some adjusting of post lengths on the fly.  The walls had spread, so I thought by raising the ridge it would bring the walls back to plumb. That didn't happen, as it started pulling out the nails before then.  I learned that some of the out of plumb and square wasn't attributable to spread of the sagging ridge, but to the way it was originally built in the 60s.   

[Jim_Rogers]

Tough to call without seeing pictures or drawings of what you intend to do, and what is there already.