The very thick walls of straw-bale houses (also rammed earth, double-thickness adobe, etc.) need an equally wide foundation (18-24 inches or more) to support them. In mild climate with sandy soil this is not a big problem, as the concrete foundation can be modest in depth (1-2 feet). But where deep frost-heaving or expansile soils are present (much of the country) you may require foundation depth of 4 feet or more. Making a 2 ft wide and 4 ft deep foundation takes a hell of a lot of concrete and rebar.
I have 2 ideas to reduce the materials required in such conditions. One is to engineer your foundation in the shape of an I-beam instead of a solid rectangular block. The I-beam principle is widely used in steel, engineered wood, and reinforced concrete to maximize strength while minimizing weight and material (hence cost as well).
--see cross section drawing below -- At the bottom of the foundation you'd have a 1 ft high by 2 ft wide base, then a web 8-10" thick by 3 ft high, then another 1 ft by 2 ft section sitting on top of the web -- total height of your reinforced concrete I-beam ~5 ft.. You would undoubtedly need an engineer to certify this design, but it would be easy to build and save a lot of cement and iron.
The second scheme I've thought of could be substantially cheaper and even "greener" -- an old idea but seldom used, why I don't know -- the grade-beam foundation. I associate this idea with FL Wright, but he may not have invented it. You excavate a trench to required frost-stable depth, say 5 ft deep by 2 ft wide, fill the trench with rocks to approximately grade level. Then build your foundation form, say 14" deep by 24" wide, directly on top of the rock base. On the rock base, Wright even used heavy wood timbers fastened together, instead of concrete, for some of his foundations; and if you had a bunch of used railroad ties or creosoted bridge timbers you could easily concoct some mixed-media foundation, sandwiching the timbers and reinforced concrete, using the timbers on the outside as "form," reinforced concrete in the middle, all tied together with rebar and reinforcing mesh.
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Straw Bale innovations
These are great ideas alex_m. You and spuds should start a business to market your SB concepts.
- maxmsf
bamboo
Way to save on cement, pier and beam is much easier for plumbing and wiring.
Bamboo can be used for the columns and beams repacing the wood frame.
Bamboo and straw bale a great way to build.
Alternative Foundations..
I concur Max! Alex_M & myself march outside of the box! Box?? -What box?!
The RTF (rubble trench foundation), most commonly associated with FLW's use of it, is a viable and stable system that has been used sucessfully for hundreds of years. It does have it's drawbacks (like a sore back) from intensive labor... But does work. Soil moisture mitigation measures, and upward weeping would be a consideration I'd pay very close attention to.
The closest I can come to bein' called an Engineer... Is my Sheep-Skin from the Jethro Bodine School O' Higher Cipherin'... -Or playing with my toy trains! Having made that distinction, to my qualifications? I think Alex's concrete I-beam, is a stellar idea! -Close attention to the rebar connections at the points of highest stress (inside corners of the top and bottom beams) would be critical. to prevent distortion. I'm certain, that there is a ratio to the overall height, to width of the base and top beams in relationship to the thickness of the backbone. After all, they've been using this type of construction for highway over pass beams for decades.
An idea that crossed my mind, in Cipherin' this through? Why not combine the I-beam configuration with the RTF? In other words... Infilling to either side of the backbone of the beam with mixed grade aggrigate. You'd kill two birds with one stone.. actually many! Sorry for the bad pun. But? With proper planning, the RTF could also be designed to function as a french drain to lead ground water (via a downhill grade channels) away from the foundation! A moisture barrier membrane (or coating) applied to the outside faces of the concrete I-beam, would also be a wise consideration to eliminate wicking action.
Further investigation, worthy of consideration? Would be to seak out one of those highway beam manufacturers'. Not only would they know the engineering standards for the load requirements. But, they might be interested in making them in pre-cast form to be delivered to your building site and set in place! -Then, all you'd have to pour on site, would be the intersecting corners!
Well? -That's certainly Enuf4now, babbling outta me. I'll say it again, I think it's a stellar idea...
Regards,
Spuds
aka: Mark
alt/foundations
Both David and Mark make good points. I didn't mention another variation, which might be best design under some conditions: pour round concrete piers going down a few feet, spaced horizontally every 8-12 feet, then pour your grade beam to rest on them (instead of the rock/rubble trench). This would certainly save concrete, need to excavate massive trench, fill with massive volume of rock, etc. Piers are often the best solution in post and beam as well as typical frame construction where the ledger beams will sit directly on them.
Pre-casting the I-beam foundation pieces offsite might be the best way to go. Beams made offsite under controlled conditions, by guys that typically build to rigid high-rise and bridge specs, are almost sure to give you higher quality product -- more uniformly mixed and cured concrete and better reinforcing systems. Might even save money over on-site pouring.