I haven't built my shed yet but have bee gathering and sawing the material , rafters 1 1
/2"x9"x12' ad two sections mfg home frame H beam 8"tall with 2 1/2" flangex30' long.
I plan to stack the two mfg home beams and weld. my opening will be 20'.
I mention this because I have found old frames for free or very cheap and recycled for different projects, such as welding table frames, 20 ton hydraulic jack press, several 2-3 axle fb trailers.
I think they could be a cheap source of beam for an ag/saw building.