In my garden there were two Concrete roofed brick shelters. Each held 16 people.
I say “Were”, I removed one of them with a 16lb sledgehammer and a lot of fun. Walls were approx 12″ thick mades from interweaved Engineering brick.
Roof was brick formed (Like you did with lego) then 8 inches of concrete. NO REINFORCING. I had the roof down in 4 hours. A direct hit would have gone straight through…………..but, these shelters were not made to take a hit, they were made to keep you away from rubble and flying glass.