Interior Hurricane - Tornado Shelter

I am presently building a concrete storm cellar. I decided to do this after tornadoes came within a 1/2 mile of our property in January 2008.

Our shelter when completed will be about 6' high on the interior. It is made of 8" concrete block with 1/2' stell rebar in the holes approximately every 16". The holes are filled with concrete and small stones. I enhance our concrete mix with extra portland cement and use rubber gloves to push air bubbles out by hand, as well as with a trowel.

The shelter will be approximately 4' underground and 3' above ground when complete. It will be half inside and half outside the house. There will be a small entry door from inside the house and a larger door (though still small (2' X 4') as an exterior entry/exit.

the dimensions are rather large for a shelter but we also want to use it as a room for other functions. It is approximately 9' 6" X 7' on the interior. There is a 4' X 18" "bench" additional to this on one side. There will be an additional "berth" that will be approximately 7' X 5' and 4' high that wraps around the house on the outside. The exterior door will be at the end of this 7' long room. Hence this shelter could be used as a survival shelter in the event of an ongoing catastrophe, complete with a separate sleeping quarters. Overall area will be approximately 105 square feet (includes the bench and sleeping quarters)

I have read that several shelter building experts recomend the reinforced block filled with concrete construction. I plan on making my roof 2 X 4 beam with OSB covering and on top of this support frame will be 4 to 5" of steel reinforced concrete tied into the walls by rebar and 1/8" fence wire. I also use 1/8" fence wire randomly throughout the structure and the floor...