I have been at the center of 12 Hurricanes, 3 Direct hits from Tornadoes, and offshore in a sailboat through a Tropical Storm. I worked as a Disaster Recovery Coordinator in Florida, Georgia and Alabama after numerous Floods, Storms, Tornadoes, Chemical Spills and FireStorms. I was a Firefighter and EMT, and did work as a contractor for FEMA and various State and Local agencies. I am now retired and live in Tennessee.
FEMA has done extensive work developing and testing various ideas for storm shelters. The have published plans for Masonry, Steel and Wood Shelters that can withstand the strongest winds (EF5) in a storm. They also explain how to evaluate your risks and different levels of protection.
Not everyone NEEDS to have a shelter for a Tornado, or a Cat 5 Hurricane, but IF you feel that you want one, there are easy sets of plans that you can download from FEMA.
For myself and my family, I used a 20 ft shipping container as my base structure. I dug 6 post holes (3ft deep) then used Mobile Home screw type 4ft anchors down another 3 feet. Topped that with regular strength concrete for extra mass. Then I got 6 inches of "Road Base" rock over the entire site for good drainage. The Container is raised above that by 3 railroad ties which are drilled through for 4 ft rebar (#5 or 1/2") and bermed, or surrounded by more rock, compacted and settled.
To meet FEMA suggested criteria, I added 2 layers of 3/4" Exterior Plywood to the exterior of the box, and on the interior, I added 1" of EPS foamboard and 1/2" of Interior Plywood. Did the same to the roof. Also added 2X6X10' boards onto the roof 16 inches on center, to provide a good airspace, and simply stapled ShadeCloth over the top to reduce the effects of the sun, which can turn that box into an oven.
With the $2000 for the box, only $200 for all the anchors and concrete, about $750 for the rock, and about $1000 for all the lumber and connectors. I estimate I ahve about $4000 for the entire project, and by MY engineering, should be sufficient to protect my family and a few friends through just about ANYTHING!
By the way, a group of Miami-Dade Florida Students made a few videos trying the make a shipping container fail the test of shooting a 15 pound (8ft) 2X4 at 130 mph at various weak points at the walls, doors, seams, etc. They did a lot of damage, and even got one seam to start to open (a bottom seam in the center of the wall of a 40 footer), but failed to penetrate the box at any time.
FEMA had UL test their plan designs at a cost of almost a million dollars, and found that the 2x4 standard test could deform a wall of 2 layers of 3/4" plywood alone, but the addition of just 14 GA sheet metal stopped everything. Shipping container walls are NOT made to be bulletproof, but they are made from 10GA steel and bent (corrugated) to improve the impact resistance.
I use my shelter as storage for garden tools and supplies. I keep a small generator and battery chargers for some tools there, with a few back up supplies and snacks.