Agricultral extension services and USDA may be able to provide some guidance, such as plan 5626 at http://www.ag.ndsu.nodak.edu/abeng/plans/Construction1.htm

I have seen such construction in WV but I have great reservations about using that pattern. The extension service pattern solutions were developed in the depression to serve necessity and may not pass modern code.

If it was my task to provide a TF solution, I would start my investigation with a principal purlin and queen post truss structure with common rafters, a very much more expensive solution than you seem to require.

If the lumber truss route suits you, contact a truss fabricator or a lumber company, that serves pro builders, will set you up with a fabricator. The fabricator will have design and testing resources that will solve your concerns.

I would also suggest you get an engineer for design overview and to forecast the life span of a concrete embedded post foundation before making improvements.