it all comes down to mass in motion — basic physics.
Having worked in the structural engineering field for a few years, I can see the corrolation between the different levels of intensity of a tornadic event and the structures it “could” effect, depending on that force acting on the structure...
But...
To me its all destructive, and dependent upon a few other factors in how a structure “might” fail...And it all happens so fast it is just impossible to design effectively enough to counter all the variables you could possibly compile into the design...
You can do your best, and there are guidelines and industry standards...But you can only do so much...
I would say that its important to go as far as you can with what you have available, but always look for ways to encourage people to prepare for the worst, and create plans and shelter you can get to very quickly that anyone can take advantage of to protect lives...
Anyone who has looked at the devastation in that town, if you notice how many (below ground) shelters beside those destroyed homes in those pictures, showed the doors open, know that those shelters saved lives, and that is the best thing yet out of this whole ordeal...