Would a concrete room integrated with a concrete foundation do the trick?
The rebar in the room walls needs to be tied carefully to the foundation metal, as well as the rebar in the ceiling. Wind forces and debris impacts are going to pull (tension) the shelter off the foundation otherwise.
By the way, I heard basements weren't common in this town because the land is mostly rock, or whatever, and so it's difficult (expensive) to dig a hole that big without making the house too expensive.
I believe it's clay. But the bigger issue is that the water table is very high and so it's hard to get water out of the shelter.
It appears that about a quarter of the Moore victims *drowned* in a basement that flooded.
With those odds, I'd definitely spend some money on a reinforced room! Plus, home insurance would probably go down.
That's easy to check - does anyone have a policy that offers a discount if there's a shelter?
Not everyone can afford to protect against every eventuality, and the alternative (apartment, etc) may be no safer.
I just listened this morning to a piece on NPR, a discussion with a research meteorologist, who opined that any single piece of real estate was likely to be struck by a tornado once in 4,000 years.
That's for any tornado. For one like this probably much longer than once in 10,00 years, maybe once in 50,000. A time scale that long exposes other risks!
I would regard construction of a safe room as a reasonable requirement in that particular neck of the woods.
Homes there are not bubble-priced. You're looking at likely a 5% price increase, and more if ADA-compliance is needed or you want it big enough to stand up in. Much more if it's supposed to protect against an EF5 with debris impacts.
$5,000 is a lot of money for some people when it is very unlikely to ever make a difference.