A friend put one in his basement. Basically:
Wall frame built on 12" centers.
Ceiling and floor joists placed with "concrete" (coated) nails and Liquid nails.
Cross bars between the studs off-set, top-to bottom, at 18"
Solid wood outward openung door (recessed hinges)with lock and catch set 18" apart.
Face panelling of 3/4" particle board.
He had some rebar run through the studs somehow, too. I think he had 6' pieces that he dead-headed into the exterior wall stud, and drilled the subsequent studs, basically threading them onto the rebar, then deadheaded into the door frame.
Made a good family safe-room.
