In this PDF, the fallout shelter is a ceiling shelter in which protection is provided in a basement corner by bricks or concrete blocks between the overhead joists. A beam and jack column support the extra weight.
FEMA considered this one of the better plans in low risk areas. Remember this is a fallout shelter, not a bomb shelter.
It was designed to protect against radiological contamination by providing a relatively low cost mass.
According to the document, this fallout shelter should take approximately two “man days” of work, meaning 16 hours of labor. 2 skilled people could do this in a single day, 4 could do it in half a work day.H-12-A Home Fallout Shelter Modified Ceiling Shelter Basement Plan A