continuity of rated walls in wood framed buildings

[Hyrax4978]

have a two story wood framed building with two stairs required from second floor due to travel distances.
1009.2.2 enclosure points to 1022 and 1022.2 points to 707. 707.5 continuity and 707.5.1 continuity of the shaft seem to require a supporting structure to be rated for 1 hr. how is this best achieved in a wood framed building?

 

[steveray]

Oversize the stair holes and frame or shaftwall inside of it without bearing on the wood floor...

 

[Hyrax4978]

how does one support the top of the wall? the top of wall needs to be supported by something to keep it from falling over.
If the wood framing at the top is not rated you are not meeting the 707.5.1 requirement.

 

[steveray]

Meaning at the roof? The roof would not be required to be rated.

 

[Hyrax4978]

But the roof is part of the supporting structure, and the supporting structure is required to be rated per 707.5.1.

 

[steveray]

But the roof is part of the supporting structure, and the supporting structure is required to be rated per 707.5.1.

OSBI will disagree with that.....otherwise we would be rating every roof structure in a building with rated walls....I don't have great backup on that, but I believe the intent of "supporting" is gravity loads....

 

[Hyrax4978]

That would be great if true, but based on the commentary of 707.5 it seems to suggest you need to rate the top also based on the diagram 707.5.
If you had a 2 story rated wall and it was only supported at the slab on grade and roof purlins above; if those roof purlins were removed it would not stand on its own very well. If the purlins were subject to fire, they could fail and thus bring the wall down prior to its rated hourly intent. I would love some back up or additional information. I have also seen other project designs call out for spray fireproofing of structures at the top of these rated walls.

 

[steveray]

The left side pic, right side of pic, (commentary)show a termination at a nonrated roof deck....I am pretty sure if the roof and floors start falling down, all bets are off anyway...Thats why the stairs (exit enclosures) are pretty much built as independent shafts so that the exit is protected longer than the building...

Firewalls have the "don't fall down with the building" staus, not fire barriers...

 

[Hyrax4978]

Understood, but that is why i believe they have this rating of the supporting structure. Even though stairs are shafts, they are not fully independent from the structure(no expansion joint between the structure and stair shaft). So the structure that holds the top of the stair shaft is afforded the same rating as the enclosure, so it holds for that length of time.

 

[steveray]

And when you terminate the shaft at a rated lid and then the unrated roof collapses on it, does the rating stay intact? Look at the right side of the lefthand pic in the commentary....It shows the termination to an unrated deck.

 

[Hyrax4978]

why are you putting a rated top to the stair shaft? running the rated walls to underside of roof deck and providing a UL joint between the two I have never provided a rated horizontal assembly at the top to the stairs.

 

[steveray]

why are you putting a rated top to the stair shaft? running the rated walls to underside of roof deck and providing a UL joint between the two I have never provided a rated horizontal assembly at the top to the stairs.

If someone wanted to get out of rating the roof, the lid would be cheaper....Not that I think rating the roof would be required....

 

[RLGA]

If you're a considering a completely enclosed exit stairway, won't the two adjacent fire-resistance-rated walls provide lateral support?

 

[Hyrax4978]

Response from ICC in which OSBI agrees with.

I’ve always viewed the supporting construction provisions as addressing gravity loads only, not requiring lateral support members to be rated as ‘supporting construction’.


This is consistent with the provision in 705.5 which notes that the top of the wall needs to be “securely attached thereto”…..no mention of rating here nor is there a mention of rating lateral restraint in 707.5.1, only “supporting construction”