We added a storm shelter to our addition when we built it. They used a boatload of 2x4 to build essentially stud walls inside the poured walls. Then they put a couple layers of 1" plywood on top, then rebar, then poured the roof. They tied the roof in with the walls with rebar.
I don't have a great photo, but I have a photo. You can see the shelter in the foreground.
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