Im currently doing the same but in a red iron building. Im using several options. The metal beams, 2x6's & 2x12's.
A word on the structual screws that cj7jeep81 shows. I believe that they are great for interior use. Be thoughtfull on outdoor use.
I am working on 8 windmills for a farm. There are two derricks that were in use & moved to his property. They layed for three years, now I'm inspecting the derricks & found they are built with these structual screws. Found several screws rusted about 1 inch from the head & a few broke at that spot. This was a wood derricl. Also, the top of the derrick had a angle iron top to hold the fan. The angle iron was screwed to the wood derrick with the same structual screws. When checking them for tightness a couple broke off. Again rusted about 1 inch from the head. More checking, found 95% broke off & all rusted the same. Replaced all screws with lags, & building 6 new derricks useing bolting.