Mr. Rawles,
I hope most readers that are considering building underground shelters that are 16′ by 20′ with a 6″ cap or roof, hire the expertise of a registered structural engineer. The design of an underground structure that have a 6″ cap or ceiling as proposed by Jim O., with 1/2″ rebar is not to be considered heavily reinforced by any means, and would probably be not to any CRSI design standards, unless it is braced underneath with several columns. It does not really matter if a house sets on top or several feet of earth, when properly designed.
I am not an engineer but I did hired a reputable structural engineer to design my underground room which is not connected to the house, as any fire which could occur would have the possibility of evacuating any air in the structure.This potential exists even with ventilation. My room is approximately 12′ by 18′ and has an 8′ ceiling. The roof or ceiling is composed of 12″ concrete with two layers of 3/4″ rebar at 9″ on centers. Concrete is of five sack [per cubic yard] design mix. The walls are 10″ thick with 1/2″ rebar 9″ on center, the floor is 12″ with 5/8″ rebar 12″ on centers. The ceiling is freestanding without any interior support.
Any design with 3/8″ steel or even 1/2″ steel for an underground shelter to support any differential movement or possibly seismic activity would in my opinion be totally insufficient in design and to proceed would be negligent without professional design. Otherwise, the result could be none other than a large concrete coffin vault.
I am a retired commercial contractor with a degree in architectural engineering. I have closed my company this year after being in business for 62 years. People please, consult with a structural engineer. I stress “Structural” as not all engineers are the same, as doctors and lawyers. All have specialties.
Otherwise, article is excellent and informative. – O.T.