FEMA has a good source for related info at http://www.fema.gov/hazard/tornado/to_saferoom.shtm

My understanding is that the primary focus is on protection from flying projectiles, and protection from falling structures is somewhat secondary.

If I understand the power of hurricanes and tornadoes properly, the winds will grab anything that is protruding, and can shove or lift things that aren't anchored down.

IMHO, if you didn't want to totally bury the concrete box, you could bury it halfway, then mound a fairly thick (2 feet?) layer of soil over it, and then plant the mound with something that has a good rooting system, like clover. But I think you would have to have a smooth plate or something for the access hole, something that provided no edge for the wind to grab.

A guy I was talking to a few years ago used to live in OK, and he mentioned that Grade 5 tornado that peeled up the road asphalt: "I wasn't there, but I would bet some money that the wind was able to grab an edge of that asphalt and peel it up like a roof shingle".

Re: safe room in a house -- what would that help if the wind grabbed the house itself? It's just my opinion, but Kevlar is good for some things and not for others. I'll bet that Kevlar grabbed by wind would act just like Tyvek. It MIGHT stay together, but you and your family might be 'mailed' to Oz by TEx (Tornado Express). <img src="/images/graemlins/smile.gif" alt="" />

Sue

Shelters that are designed to be buried, even partially, are designed (or should be designed) to withstand different stresses than an above ground shelter. A buried shelter needs to withstand stresses like ground freezing and shifts in the water table, while an above ground shelter would put an emphasis on wind and projectile protection. Installing a shelter in a way that the manufacturer did not intend could put your safety in jeopordy when you really need it. Could have a problem with an insurance claim, too. So, unless this "someone" is an engineer and knows the specs on this particular shelter, I would stick with what the manufacturer recommends. If you're going to go to the trouble of purchasing a tornado shelter, and you want it above ground, then get a good above ground tornado shelter. But all things being equal, I would much rather be below ground if a tornado hits.

Pardon me while I dig up a post from October. Ok October 2006.

Blast,

There is a shelter designed at the wind lab of Texas Tech, that is really what you are looking for. Generally they are built at the time of new construction because they are tied into the foundation beams and piers of the slab on grade foundation.

I am a custom homebuilder in DFW area, and have put one in almost every home I build. They are MUCH less expensive than DuPont's offering and secured by more than 1/2 inch drop-in anchors drilled and epoxy'd into the concrete slab 6 inches.

As a recent retrofit to a home, I built an "out building" that looked like a pool equipment room / storage for a customer. The damn thing should withstand even close 105mm HE rounds much less a cat5 tornado. (he really had this one over engineered).

Susan,

The May 9 1999 tornado is the one in Oklahoma you referred to hearing about. It is the reason the Fujita scale (F1 - F5) was changed. There was no Fujita F6 so it is now the Enhanced Fujita scale (EF1-EF5). All tornados are now measured against this one which is EF5. The only other EF5 since then was Greensburg, Kansas.

We were visiting my DW's family in that area when it hit. I was the last into the underground shelter as I wanted one more look. For the rest of my days on this earth, I shall never forget what was to be seen.

Two of the aforementioned (From Texas Tech) shelters were directly hit by that tornado. When it was over, the occupants opened the door to find their shelters were the only things standing from horizon to horizon.

Blast PM me for any questions, the really aren't that hard / expensive and perfect for your area.