DonT think that hideing in a ditch will protect you from a tornado. 2X4 s are driven 3 ft into the ground in ditch banks all the time. A ditch is better than nothing but "underground" is best.
Brick veneer houses will only have a couple of pipes sticking up after a tornado, a slick slab with 2 or 3 pipes sticking up.
You have to keep in mind that a tornado is pushing things like vans, mini and full size, mobile homes, tractors, buildings, trees, water and anything that a 200 mile per hour wind can push. This type of debree can destroy most any man made structure. ear