46 CFR § 116.440 - Atriums.
(a) The atrium opening area must be a minimum of 93 square meters (1,000 square feet) or 20 percent of the gross deck area of the largest deck within the accommodation space containing the atrium, whichever is smaller.
(1) Each side of an atrium opening must be a minimum of 6.1 meters (20 feet) in length. If the opening is circular or ellipsoid, it must measure at least 6.1 meters (20 feet) across in any direction.
(2) Any deck opening within an atrium must fit wholly within the horizontal projection of any deck opening of an upper deck.
(3) The horizontal projection area of stairs, escalators, statues, etc. within the atrium must not be included for purposes of computation of atrium opening area.
(b) The entire main vertical zone containing an atrium must be protected throughout with a smoke detection system of an approved type which is installed in accordance with § 76.27 in subchapter H of this chapter. However, on vessels with no overnight passenger accommodations, smoke detectors may be omitted from the accommodation space containing the atrium.
(d) The atrium must be provided with a smoke extraction system that complies with either:
(1) The smoke extraction system must be capable of exhausting the entire volume of the space within 10 minutes. The smoke extraction system must be capable of being activated by both the smoke detection system and by manual control, and designed with sufficient plenum air openings to prevent excessive negative air pressure in the atrium; or,
(2) The smoke extraction system may be designed in accordance with the principles of NFPA 92B “Smoke Management Systems in Malls, Atria, and Large Areas.”