There are above-ground,
below-ground, dedicated, dual-purpose, and potential vault designs.
Dedicated blast shelters are built specifically for the purpose of blast
protection.
Dual-purpose blast
shelters are existing structures with blast protective properties that
have been modified to accommodate people in their seeking protection
from a blast or fallout.
Generally, fallout shelters are much deeper than typical storm shelters.
The basic plan
is to provide a structure that is very strong in compression and fire
resistant. The actual strength specification must be done individually,
based on the nature and probability of the threat.
It is safe to say
if a shelter has a covering of 14" or more of soil and is built
of concrete with reinforced steel with a ceiling thicknessof 12"
or so of concrete would be considered 'safe' for most security considerations.
Adequate venting is a priority for a below ground structure.