Help with straw bale cabin,unsure of beam spans.

Hello all, just have a few questions and am looking for some collaborative insight about how I should proceed with building my Octagonal wood pier foundation. The foundation uses a spoked wheel design to achieve the octagonal shape. The spokes are 12f spans of laminated 3ply 2x10s supported each by 2 piers: one at the center of the wheel and one 9 feet out.

My first question is what size of beams should I use to connect the spokes? these are 10 foot spans between the ends of each spoke. I think I need to add another ply to make these 4 ply beams but I'm not sure. Looking for any pointers from someone more knowledgable than I a photo of the layout is attached.

That photo scares me. You're planning to put the weight of strawbale walls and roof plus live loads on posts sticking up as much as 4-5 feet without real sway bracing? Even if the concrete piers are epoxy-pinned to the boulders or go 4' below ground, the connections to the base of the wood posts appears insubstantial, and likewise at the tops of the posts. I believe you need heavy diagonal braces from the base of each pier to around the center of the future joists topping them to give enough resistance to possible high winds (or earthquakes if that is a risk).

Beyond that, it really needs an engineer with the right load tables to give an accurate answer... an experienced builder could give a good ballpark. We would have to know the exact proposed structure, materials and dimensions, to calculate weights and loading.

My gut would be to sister 2x10s on each side of those beams at the outer half or 8', as the wall load will be centered about 2' out from the posts, and by far the greatest bending moments and shear will be at the outer posts. The inner half of the beams might even be in reverse bending, trying to lift the center, if the walls go up before there is a significant load at the center. What do you plan at the center of the structure? Roof support posts, loft, masonry fireplace,

The walls will most likely require doubled joists at least, at the inner and outer edges of the bales. Again, it needs weight information and someone with load tables.

Shawn, I'd have to know the species of 2 x 10 but you're at least at a safety factor of 2 over a 12' span on the spoke beams, perhaps 3, assuming they are well bonded and nailed. Plenty good although as Glen said finish off the steel simpson strong ties at all joints to resist over turning moments. 6 x 10's would have been a better choice, treated if exposed. I'd attach intercostal beams blocking between spokes @ 24" OC in from the outer end using the same beams to keep the loads even w/simpson ties and hangers. That should keep the spoke beams from buckling and resist bending along with the strong ties. Put some T&G 24" OC rated plywood down and you should be good to go. Assuming you did your piers per local code.