My wife and I now live in Tornado Alley (central southern Missouri) and the house we just moved into has no viable way of taking shelter.
I used my foundation plugin and basic SketchUp tools to quickly mockup a tornado storm shelter:
6x8x7.5H interior dimensions
12" thick reinforced concrete walls and roof.
30"x72" door opening with steel door
I have’t determined yet where the grade line will be but I want to have it at least half buried in the hill side next to our house, so some excavation will be required. This will also affect the final geometry required for the stairs. L-shaped entrance also helps protect the door from large flying debris (ie. vehicles).
I like your thinking. I realize tornados don’t typically last for long periods of time, but I imagine you could be stuck (literally, debris blocking the door) So you may want to provide storage for water, and perhaps a toilet (if only a 5-gallon bucket with a seat) Perhaps a Cap over the stairs to help keep debris from blocking the door. I’m probably over thinking it…
I like the idea of a vapor barrier under the concrete and then coating the walls with the typical foundation/basement coating. Also a drain pipe that collects the rain water and sends it down the hill away from the structure.
Welcome to Missouri. If you ever make it to St. Louis, let me know so I can buy you lunch for all the help through the years. There are a number of companies in the region that manufacture shelters. Most are aboveground steel structures. It would be interesting to compare cost. They might be a good resource for ventilation fans and accessories.
I did a full basement under a 2 car garage once with precast concrete plank. I first learned about plank and building with it on a 5 story apartment building in the 80’s. When I asked the precast company if the ever did this in a residence, they said, “Oh, yeah, we get a couple a month or so.”