Welcome back to our 10th Edition Major Changes blog series. In this post, we will be reviewing the new code provisions for Atriums.
The upcoming 10th Edition of 780 CMR Massachusetts State Building Code, which will adopt and amend the 2021 International Building Code, introduces additional clarification and changes regarding atriums.
Atrium Definition
In the 9th Edition, an atrium is defined as “An opening connecting two or more stories other than enclosed stairways, elevators, hoistways, escalators, plumbing, electrical, air-conditioning or other equipment, which is closed at the top and not defined as a mall.” The code includes allowances for unenclosed floor openings, such as two-story convenience openings or stairs/escalators protected with draft curtains and closely spaced sprinklers. This definition raised questions about whether all unenclosed floor openings connecting two or more stories were required to be designed with atrium features. To provide clarity, the 10th Edition revises the atrium definition to “A vertical space that is closed at the top, connecting two or more stories in Group I-2 and I-3 occupancies or three or more stories in all other occupancies.”
Smoke Control System Exception
Under the 9th Edition, atriums connecting three or more stories were required to be provided with a smoke control system for most occupancies. The 10th Edition introduces a new exception that allows the omission of the smoke control system if only the lowest two stories are open to the atrium, and all other stories are separated from the atrium with shaft construction. This new exception recognizes the effectiveness of shaft enclosures and the improved egress conditions by having a natural smoke reservoir with a two-story floor opening. The image below illustrates an example of this type of atrium described by the code.
Atrium Separation Exception
In the 9th Edition, atriums typically needed to be separated from the remainder of the building with 1-hour rated construction. The 10th Edition introduces a new exception that allows the omission of a horizontal assembly between atriums and stairs/escalators protected with draft curtains and closely spaced sprinklers. The image below provides an example illustrating this code change.
Atrium Exit Access Stairways
Exit access stairways in atriums are permitted to be used for as a component in a means of egress system under the 9th Edition. However, the path of travel to an exit is not permitted to pass through more than one adjacent story (Sections 1019.3-5 & 1006.3). Interior exit stairways within an atrium are permitted to be unenclosed and there is no limit to the number of stories occupants can traverse for egress (Section 1023.2 Exception 2). A new exception has been added to the 10th Edition that allows egress along exit access stairways to pass through multiple stories within an atrium without being designed as an interior exit stairway.
Atrium Interior Exit Stairways
The 10th Edition adds a new section that outlines requirements for atrium interior exit stairways. These requirements include:
- The entry to the exit stairway is the edge of the closest riser of the exit stairway.
- The entry of the exit stairway shall have access from a minimum of two directions.
- The distance between the entry to an exit stairway in an atrium and entrance to a minimum of one exit stairway enclosed in accordance with Section 1023.2 shall comply with the separation required by Section 1007.1.1.
- Exit access travel distance shall be measured to the closest riser of the exit stairway.
- Not more than 50 percent of the exit stairways shall be located in the same atrium.
Fire Protective Curtain Assembly
Fire protective curtains are not prescriptively permitted by code to be used as opening protectives. These assemblies have commonly been used over the years as part of the performance-based design for atrium smoke control systems. Though this will not change under the 10th Edition, the code will contain new labeling and performance requirements when curtains are permitted to be used by the Authority Having Jurisdiction (AHJ) as part of an alternative method or performance-based design. These requirements include testing in accordance with UL 10D without the hose stream and installation in accordance with NFPA 80, Standard for Fire Doors and Other Opening Protectives.
Our current projection for the adoption of the 10th edition is Q2 of 2024 – see this which outlines the overall process and timeline.
Stay connected for more Insights posts as we continue our “10th Edition Major Changes” series as part of CRC’s 10th Anniversary!