Good effort!

Here in the D/FW area, I've gotten to see some structure damage analyses by Tim Marshall of Haag Engineering. Tim does a lot of tornado damage assessments for the National Weather Service. Here's his damage survey from the '99 Moore, OK Tornado outbreak.

http://tinyurl.com/2eled3

Here are a few things that happen when a tornado strikes a house.

1. Peaked roofs act like aircraft wings. High speed airflow lifts the room off the structure, which is then more vulnerable to collapse. Once this happens, the house tends to come apart like a deck of cards.

2. High speed air striking the structure can blow holes in structurally weak areas (garage doors, picture windows, etc.), damaging the structure and rendering it more vulnerable to collapse.

3. High speed debris can poke holes in the structure, again rendering it more vulnerable to collapse.

A factor in determining what happens to a house is the quality of the home construction. The nail joints between structural members help determine how fast winds need to be before the roof is damaged. Also, the way the house is fastened to the foundation makes a tremendous difference. Tim's presentation had several pictures of homes that looked only lightly damaged - except they had been picked up off the foundation and set in the swimming pool. When you see those pictures of two houses, one destroyed and the one next to it untouched, it was probably because of different construction specs.

Since you have a basement to put a shelter in, you probably don't have to worry so much about flying debris. You may have to worry about having the house falling on top of you. If you roof your shelter, you might want to think about running some cast iron pipe in it to help resist collapse due to debris on top of it. If you have the opportunity, having your roof joists secured with hurricane clips may help keep it from crashing in on you to begin with.