Section 704 defines fire barriers as complete rated assemblies, including all openings and penetrations, that have a specified fire-resistance rating and separate different occupancies, areas of refuge, or other protected spaces. Fire barriers must extend from the foundation to the underside of the floor or roof deck above, through all concealed spaces. The fire-resistance rating of a fire barrier ranges from 1 hour for certain interior separations to 4 hours for separations between hazardous occupancies. Most fire barriers separating mixed occupancies require 2-hour fire-resistance. Openings in fire barriers must be protected with fire-rated doors and frames. The door rating must be not less than 75 percent of the barrier rating, with a minimum 20-minute rating. Door assembly must include the frame, hardware, and self-closing or automatic-closing device. Doors must close upon activation of the fire alarm system. Multiple doors in series are permitted but require staggered openings to prevent line-of-sight passage. Lobbies separate the exit stairwell from the occupancy area, typically with 15-30 feet of additional travel through the lobby. Ductwork penetrating fire barriers must include fire dampers that close automatically upon temperature rise or smoke detection. All other penetrations must be protected with through-penetration firestopping systems meeting ASTM E814 standards.