The Victoria's Cottage plans call for a 6x12 DF beam when built on concrete piers. While these beams are available in my area, the cost is outrageous (@$14 per linear foot). I would like to substitute a PT 6"x6" as a sill and then single or double 2x12 as a rim joist (with tie-ins to the sill). Will this work (i.e., be strong enough) ok? Do I need double 2x12s for the rim joist? Thank you.
Your suggested configuration should generally be in the same strength range. This would be a difficult analysis for an engineer and I am not doing that of course. I would suggest you tie the 6x6 sill together with a double 2x12 rim with metal straps or splice plates well nailed to get them to work as one unit. Then hang the joists from metal hangers from this new composite beam. This would make a beam approaching the value of a 4x16 which would carry as much as a 6x12