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Ever think about just buying a tornado shelter? Might not be something thats common up there but down here all the companies that cast septic tanks also cast shelters. Its all pre cast and you just bury it.
Thanks for the comments glenn. The arch would more likely be slight. The purpose is not just structural, but functional. Root cellars require humidity and condensation can accumulate on the roof. Most designs recommend a slight arch for the condensation to not drip on the food.The examples run through by the Engineering Professor at the University were for 3/4 of an inch coverage of #4 rebar. That is also the requirement in a footnote in the I.C.C. "Concrete cover for reinforcement measured from the inside face of the wall shall not be less than 3/4-inch." I also remember a low requirement from the concrete design books, but don't have the exact figure in front of me. From the equations, the closer to the bottom of the slab, as long as it has the minimum coverage, the more weight the slab can bear. In reality it would be hard to keep the rebar at exactly 3/4 of an inch.It is probably a little beefier than some of the floors you had seen, if I did it right. A few of the examples I read for building designs didn't use close to the dead and live loads I used for this design. I used over 500 lbs per square foot, and most examples I read were less than 200 lbs per square foot. I am also looking into additional fiber reinforcing. It is more expensive, but from what I was reading the impact, crack, and failure properties were pretty amazing.