[Magnum Wheel Man]

If you click the links I included in my 1st post... the Missouri Storm Shelters one includes a video of them supposedly shooting 2X4's at the shelter at 250 mph...

that to me is a weaker design than the one pictured in the other link, that is constructed entirely of heavy 3/16" C channel

I would put my "room" in the corner of my basement, with 2 sides covered by concrete ( but would still put steel on those sides, though in reality a lighter weight steel could probably be used on the sides facing the concrete, so long as the corner pieces would support the weight of house debris ) it would be concrete anchored to the floor, & would have a steel ceiling built of the same either bent steel sheeting or C channel ( BTW... my buddy has a big steel break, that could easily bend steel 5/16" thick ) but depending on how far apart your ribs are, the 3/16" design made from 8" or even 10" C channel would be plenty strong... the panels are designed small enough so that 2 people can carry them into place & assemple the unit...