FEMA 453 Design Guidance for Shelters and Safe Rooms

For a standalone shelter, many sites will be constrained or site limited
for underground, and an aboveground structure may be the
only feasible alternative. For these sites, the siting considerations
include:
m Outside the floodplain
m Separation distance between buildings and structures to
prevent progressive collapse or impact from collapsing
elements
m Separation from major transportation features (road, rail)
m Access to redundant power and communications capabilities
1.6 occuPancY duration, toxicfree
area (tfa) fLoor sPace, and
VentiLation requirements
Occupancy duration (also known as button-up
time) is the length of time that people will be cBr collective Protection shelter Basics
in the shelter with the doors closed and in the m occupancy duration
protected environment. This period of time
m toxic-free area (tFa) Floor space
is determined by the building owner or local
m Ventilation requirements
authorities and can range from several hours
to several days. For off-site industrial accidents,
the occupancy duration is usually less than 24 hours; occupancy
durations longer than 24 hours are generally restricted to wartime.
Occupancy duration stops when the doors to the shelter
are opened. It influences the floor area requirements and the
amount of consumable and waste storage. Generally, occupancy
duration will not significantly affect the performance of the collective
protection system.
a. Less Than 24 Hours. An occupancy duration of less than 24 hours
does not require sleeping areas. The occupant load will generally
be a net 1.86 m 2 /person (20 square feet/person), depending upon
the classification of occupancy. The classification of occupancy,
as stated in NFPA 101, may require a higher or lower occupant
design considerations
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