reading some of the comments to this,
I'm an old man. As a kid in the '50's (yes, I'm old), my father actually had a fallout shelter built in our house. Probably around 1956 or so. Lowered the ceiling in one part of the basement by putting 8" industrial steel I-beams across an area that was probably 20' x 12. Above the lowered ceiling, they placed about 6" of concrete blocks on top of plywood. Probably between existing floor joists.
(Like a brick pizza oven!) If you were over 5'5", you had to duck to get under the beams. There were a couple of couch/beds, chairs, and he had s storage closet full of food and medicine. Dad was a doctor and it was full of penicillin, syringes, and God only knows what else. He also kept a loaded gun there in case the zombies wanted in. Ventilation was through the door, which had a cement block L built in so that you had to turn and turn again to get in the room There was also a vent pipe that went through a former casement window to a point about 2' off the ground.
More curiously, we made this our family room. The family TV was there, and I remember as a young teenager taking my friends there to watch TV and hang out. We called it "The Shelter." Rolled off our tongues like it was normal.
My family moved in 1964 and the crisis with the Soviets was kinda over, so our new house didn't have a shelter and my father didn;t see the need to build one.
Many many years later I drove by the old house with my brother and we decided to introduce ourselves to the current owners. We met with the briefly, saw most of the house, but when we asked about the "shelter", she said it was just packed with junk storage.