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Stair Support in Exit Enclosure

V

VermontDrafter

Member
Joined
Jan 22, 2016
Messages
4
Location
Vermont
I have read the earlier discussion on the topic of structural penetrations of a rated exit enclosure, but I remain unclear as to what is permissible.

We are working on a 5 story, Type II-B building. Vermont has replaced IBC chapter 1000 with NFPA 101 for matters of m.o.e.

We have a 2-hr wall assembly (steel studs and GWB UL assembly) protecting the (five-story) exit stairs. The structure supporting those walls (beams and columns) will be spray-fireproofed to 2-hrs as well. My questions:

1. The metal studs of the stair enclosure sit on the concrete slab of the (unrated) floor system. So the slab is interrupting the building side of the 2-hr assembly (a membrane penetration) Is this allowed, and if not, what alternative detailing could make this work (other than rating the whole floor system)? Will an approved joint protection system at the flutes of the deck where it penetrates the enclosure suffice to maintain the continuity of the enclosure?

2. The stair landings are supported by beams that penetrate the rated enclosure and frame into the spray-protected structural frame. Do these need to be sprayed as well? NFPA 101(6) reads: Structural elements, or portions thereof, that support exit components and either penetrate into a f.r.rated assembly or are installed within a f.r.rated wall assembly shall be protected . . ."

3. In general, has anyone seen a satisfactory way to detail a rated exit enclosure with steel stud walls rather than CMU? Does the code require that the support for the stairs be totally independent of the building frame outside of the stair enclosure?

Your comments are appreciated.
 
Hi SteveRay, thanks for the (almost immediate!) response:

1. By continuous walls, do you mean a 5-story balloon-framed tower for that rated enclosure? With a gap to allow GWB to slip past the concrete slab opening? And lateral support via clips maybe? I've never seen this done.
2. Almost every building I've ever seen has the steel stairs hung off the main structure of the building structure in some way (unless it is a short stair - 1 or 2 stories . . .) and thus, necessarily, penetrating that enclosure from the inside?
3. Your question mark indicates that this probably isn't a common system. I've certainly never heard of it for steel framing. . .
 
1. I think has been covered pretty well by previous posts. Supporting construction (i.e. The floor in your case) must have a rating equal to or greater than the assembly being supported.

2. The beams do not need to be fireproofed as they are no part of the primary structural frame; however, the penetration of the shaft wall assembly must be protected as required for membrane penetrations.

3. Gypsum board shaft wall assemblies are designed to extend very tall heights, but I doubt they can extend up to five stories unsupported. Embedded structural steel may be used per your structural engineer's recommendation to achieve the height, but will require individual protection of the steel and cannot rely on non-rated construction for lateral support. Unsupported exit enclosures of this height generally are of masonry construction. Otherwise, floors will need to be rated to support the shaft construction (which requires the construction supporting the floors to be rated), or you go to Type IB construction (for 2-hour floors with 2-hour shafts) or IIA (for 1-hour floors with 1-hour shafts).
 
The exit stairway is designed as a shaft and is not technically building structure. Therefore, it isn;t always required to be independent of the building structure. You can shaft walls from floor to floor each level although I do not know how feasible this may be. The supporting construction of the shaft must also be fire resistance rated (always a bit difficult in Type IIB construction). Refer to Section 713 which requires the supporting construction for the shaft.

https://up.codes/viewer/general/int...pter/7/fire-and-smoke-protection-features#713
 
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