Wood framed rated stair shaft - Horizontal Assemblies / Supporting structure

[Hyrax4978]

5B, new construction 2 stories. stair shafts at exits that have offset walls from floor to floor.
What i have is a stair shaft for an exit with a 1 hour rated wall siting on an unrated wood framed floor assembly and is offset from the rated wall at the floor below creating about a 5' horizontal area that is wood framed. There is not a UL that will maintain a 1 hr horizontal rating for that area that would allow for only 3/4" subfloor on the top. If i hang a shaftwall below, its supported by the floor above that is the supporting structure that is not 1 hour rated which loops me back around to my initial issue.
Does anyone have any bright ideas or details on how to achieve this, without having to frame everything completely independent.

 

[jeffc]

This proposal does not meet your criteria (only one layer of 3/4" subfloor), but is close: 2015 IBC, Table 721.1(3) assembly 13. Joist 16" oc, Footnote m provides 15/32" plywood with 19/32" plywood on top of joist.

 

[Hyrax4978]

Yes, there are a bunch of ways to achieve it with thicker wood on top of joists, but that would necessitate framing that area differently than the rest of the building because it would be cost prohibitive to provide that over the entire structure. I would assume there are a few wood framed 2 story structures out there with rated offset walls, there has to be a better solution that framing this all completely independent.

I get why stair shafts are rated, but if the entire structure around the stair shaft is combustible, if the wood floor your standing on isn't 1 hour rated, it won't even support you to get to the stair shaft.

 

[mtlogcabin]

The floor needs to be protected from below so maybe a 1-hour floor ceiling assembly across the area supporting the shaft above. One-hour is not hard to meet for the walls supporting the one-hour floor ceiling area

 

[Paul Sweet]

Try Gypsum Association FC5406. It's basically a double layer of 5/8" type X for the ceiling, which provides the 1-hour fire protection for the framing. This is from the Gypsum Association Fire Resistance Design Manual GA-600.

 

[steveray]

CT project Frame it all independent....

(Amd) 1023.5 Penetrations. Penetrations into or through interior exit stairways and ramps are
prohibited except for equipment and ductwork necessary for independent ventilation or
pressurization, sprinkler piping, standpipes, electrical raceway for fire department communication
and security systems and electrical raceway serving the interior exit stairway and ramp and
terminating at a steel box not exceeding 16 square inches (0.010 m²). Such penetrations shall be
protected in accordance with Section 714. There shall be no penetrations or communicating
openings, whether protected or not, between adjacent interior exit stairways and ramps.

Structure not listed as a penetrant....

 

[IJHumberson]

Steveray hit the nail on the head - the rated envelope of the stair enclosure has to be continuous. The situation described by the OP creates a non-allowed penetration of the exit enclosure. Many architects haven't yet figured this out - it's very difficult to maintain exit enclosure integrity in any Type V or Type III construction unless the stairs enclosure walls are in the same place from slab to roof with no offsets.