So, to sum up that bit, I'm building a straw bale chicken coop on top of a poured-concrete-walled root cellar.
The big thing right now is the roof of the root cellar/floor of the chicken coop. I'm looking at a size of 12 by 15 feet (exterior dimensions). The root cellar will probably be divided into two equally sized rooms. My root cellaring book recommends a concrete ceiling, as root cellars are supposed to be damp and wood will rot in such a space. Also, it would make an easily washed floor in the chicken coop (I would put a sealant on it). I ran this past a friend of my husband's who has construction experience, and he suggested that the walls (or maybe the ceiling?) would have to be very thick to hold up the weight of the ceiling AND the weight of the straw bale walls. He thought that it was a bad idea and we should just use wood. This isn't for my house or anything, so I don't want to hire a structural engineer. I just want to figure out if it would be feasible to use concrete for this application, because I think it's preferable functionally.
In case it's pertinent, I'll try to give a good idea of the overall plan. From the bottom up, the root cellar will be dirt-floored, with a gravel-filled sump to drain into if required. The walls will be built on a wide footing, and they will be poured into insulated concrete forms. The proposed concrete ceiling can sit on top of that? Or maybe only on top of half of the wall? The walls will stick up out of the ground at least a foot but under two feet. After addition of moisture barriers to prevent bale wetting, bales will be stacked up. They are going to bear the roof load. The roof plate will be tied down through conduits in the concrete under the bales. The bales will be stuccoed, probably with a concrete stucco. As you can see, I feel fairly confident in my root-cellar knowledge, and fairly confident in my straw-bale knowledge, but the interface between the two is kind of scaring me. The more input the better!
I'm not a structural engineer either, but concrete in roughly the shape depicted on the left, with a few narrow sections "webbed" to have a section like I've depicted on the right, might work.
We are planning something similar, but we are planning a wood/beam floor from standing dead timber. Walls with earthbags on rubble trench foundation with a bond beam, dirt floor. Vapour barrier under the floor/ceiling, floor beams, straw insulation, then vapour barrier on top of the straw under the top floor. I think if I was putting a chicken coop on top instead of a screened gazebo I would use sheet metal/roofing tin for the floor and have the floor with a slope(s) to the door for cleaning. But I don't see a problem with making your form for the cement floor right on your flooring/roof beams as long as you don't make your pour too deep. Think I might want to add a supporting post or two underneath depending on the span though. I would call my local building inspector and ask him though, he's a great help if I don't pester him too much! When my mom had tile put on her floor they just poured the cement right on top of the old subfloor, then the thinset, then the tile.
If there is one thing I know about cob and strawbale construction, even in Cascadia.... do not use vapor barriers!! The walls need to breathe properly in order to not decompose and collapse.
I had the pleasure this year of visiting Cob Cottage over at Mountain Homestead in Coquille, OR. A fascinating place, great people. Anyway, Cob Cottage tour showed me the process of making cob, cob / strawbale, and strawbale construction of many buildings, and as I said, if there is one thing I did learn that day... do not use vapor barriers!
The humidity in the root cellar will be high and in order to keep the straw insulation dry a vapour barrier is needed from the bottom. As the root cellar needs to have its own ventilation system, to vent the whole roof would only cause the straw to rot. As we are putting a screened gazebo on top the second vapour barrier serves to help keep the insulation dry from the top since the screens will allow some moisture to come in from the top.
If we were making the structures separately the plans would change, and if we are on the wrong track of course feedback is always welcomed as the idea to combine the two is only a plan. Mostly the reason for this smaller project is to test the results of the earthbags underground in our lowlying 'misty meadow'. I would love to have a traditional root cellar but alas we have no hills. I am calling the gazebo "the bug house" a necessary evil for my 2 and 4 year old boys to have a place to play without being eaten alive (priority one). The whole structure will probably be turned into a play area with an underground fort once the house is built.
We could and will be making a hill (winter sledding) from the pond construction (aka the old lakebed) but it will be incorporated into an earth sheltered greenhouse next year, as our meadow is too wet right now for any digging.