So we're planning on building an octagon ICF house.  I posted on here a couple months ago asking about the suitability of an ICF basement for a geodesic dome home, but now we're pretty much convinced on all ICF.  One reason we had been looking at a dome was the efficient shell to living space ratio, but an octagon approximates the efficiency of a circle pretty closely (E.g. it'll use about 9% less wall materials to enclose the same area compared to square), plus we get straight (not curved) walls (ICF-wise, we use regular and 45 degree forms, not radial). 
Anyway...

In working on our floor plans, we're having to figure in supports for the floors and roof.  First floor is about 1000 sq. feet, each wall section 14.5' long, and the interior span from one wall across to the other is ~33'.  Trying to figure out the best way to design floor joists and supports. 

Was originally thinking we'd have to run a beam or wall across the house, meaning the joists would be around 16.5', which (looking at span tables) might mean deeper (2x12) and/or more tightly spaced (12") joists (=more materials).

Then, thinking of octagonal gazebos, and come to think of it, circular homes I've seen as well (Deltec comes to mind), would a "web" approach be better?  That is, beams coming in towards the center from the corners, with the joists then spanning each triangular section rather the width of the house?  Would this then just require a large central support?  It would also, I think save having to attach ledgers to the ICF, instead either incorporating the beam ends, of just placing supports flush against the wall (for a bit of a timber frame look perhaps).

Any thoughts on the best approach for this?

I guess I'm partly wondering about costs and material efficiency for floor framing - fewer girders or beams with longer joist spans (and less walls/columns underneath), or more girders/beams with fewer and shorter joists (with more vertical support below)?

One possibility for beam:

And another:

have you considered using a flush, poured in place concrete beam, or beams.
lets you build and pour the beam (s) and floor and the together, and gives you more head room than a steel beam placed below the floor. you could even pour the floor and walls together on the same day, from the top.

peter @insuldeck.com

I was planning on having ducts to a heat exchanger, so good point about passing through the beam. While I was originally thinking flush (or near flush) beam, we think we're okay with exposed, and that'll save the expense of joist hangers. That's with a wood beam though - haven't looked into poured in place concrete.

Am planning on a regular wood framed floor, not a concrete system. While we'll probably put radiant tubing in the finished basement areas, we want a big ol' masonry heater for the main part of the house (the support of which isn't factored in or placed yet).

I'm thinking 2 beams/girders may be preferable to 1, can get by with less joist strength, but I haven't priced anything so can't say for sure. Any thoughts on 16" OC vs 24" OC joists? Span tables suggest either are easily doable with regular dimensional lumber. Seems 24" would save time installing and maybe costs (less joists of less depth), but am wondering about potential sag between the joists in the long term? Maybe means going up to plywood subfloor rather than OSB (and so trading one expense for another)?

Two exposed smaller beams instead of one large deep beam should look better from below with your floor plan.  In fact, by adding some fake beams you could make the ceiling look coffered.

Be sure to determine the amount of deflection in the beams before deciding upon type of beam.  Compare prices for beams with a deflection of L/360 versus L/720.

Are you going to have tile or stone on any part of the floor? If so, pay extreme attention to deflection. I would suggest you look at using the two smaller beams with your joists extending all the way across from one wall to the other, resting on top of the beams. With your spans that way you quite possibly can get by with 9 1/2" deep I-joists on 12" or 16" centers. Don't go 24" centers. You could end up with a spongy floor. Use I-joists, not dimension lumber. More consistent in dimension, won't change dimension as they dry, are a lot lighter, and I'm sure a lot cheaper. They also are available up to 40' length so you would have no problem getting them to go from wall to wall. Do not use short joists cut to fit from the wall to the beam. A continuous joist resting on top of the beams will give you a much stiffer floor with shallower joists. For subflooring take a close look at Advantech subflooring. It's priced between OSB and plywood and stronger than either one. My floor is tile and 1/2" engineered hardwood on 3/4" Advantech on 9 1/2" I-joists spaced 16", except under the Jacuzzi and showers where it's 12". The chime rods in my grandfather clock do not rattle when walking heavy footed next to it. Joist spans are 22' and 26' with a pony wall halfway. Longest span from wall to pony wall is 14' and the floor is plenty stiff there.

For some really good discusssions about making good stiff floors visit the John Bridge tile forum. I think it's johnbridge.com. Do not skimp on the floor structure. You want it to pass the 'china closet' test, i.e., the dishes don't rattle in the china closet when you jump on the floor.

Dmaceld,

I agree with your comments.  Thank you for taking the time to explain it.  Although the shape of this house is unusual I think your advice can apply to most designs.  I-Joists lend themselves to being cut to length in the field.  Maybe this thread will also help other people.

If not for the octagonal shape I would have suggested using 4x2 floor trusses.  I did not suggest using floor trusses because of so many jig setups could make them too costly and one should avoid shortening trusses in the field.

Thanks for the all the advice. Probably only tile in a mudroom, and maybe bathrooms, otherwise we're thinking cork, bamboo, or maybe wood laminate. Pretty much sold on 16" OC.

One more question: house will be built on a bit of a slope, so one face of the basement will be exposed/walkout, then sloping up to a fully submerged back half or so. The exposed side is also towards the prevailing wind (west). From a structural standpoint, given the walk-out and/or wind, does it make more sense to orient the floor beams towards the open face, or perpendicular? Or is it just pretty much irrelevant, given the strength of the concrete in the first place?

I think the concrete walls and the shape of the house will make it strong enough to withstand the wind unless you have some exceptional strong winds.  What is the fastest wind on record there?  Have you had a chance to give some thought to which direction exposed beams would look the best from inside the home?

Yeah, I don't think the winds are anything too exceptional that would require anything special, and the beams will probably be more visually appealing oriented otherwise.