Shaft Wall Rating in Individual Residential Units

[klarenbeek]

That exception applies only to condition #3, which is only applicable to non-combustible floor-ceiling assemblies.

With the way it's indented I agree and stand corrected.
That being said there's still no need for shafts or dampers if none of the assemblies are rated. Based on the IBC definition of "floor", the attic is very clearly considered part of the second story:
STORY. That portion of a building included between the upper surface of a floor and the upper surface of the floor or roof next above (see "Basement," "Building height," "Grade plane" and "Mezzanine"). A story is measured as the vertical distance from top to top of two successive tiers of beams or finished floor surfaces and, for the topmost story, from the top of the floor finish to the top of the ceiling joists or, where there is not a ceiling, to the top of the roof rafters.

The attic is by definition part of the second story so no more than two stories are connected.

 

[redeyedfly]

No, in that case it's not a story. But it's still outside of (beyond) the second story. It would then be a roof-ceiling assembly. With the equipment in the attic, penetrating the ceiling portion would me a membrane penetration. The applicable criterion from 717.6.3 is:

Again, the attic is literally defined as part of the story directly below it. You continue to insist something that is black and white in the code. The attic is certainly NOT "beyond" the second story.

"STORY. That portion of a building included between the upper surface of a floor and the upper surface of the floor or roof next above."

How is the described duct "more than two stories"?

The code is what is written, not what you imagine.

 

[Yankee Chronicler]

The attic is by definition part of the second story so no more than two stories are connected.

The way I read it the attic is NOT part of the second story. 2021 IBC definition:

[BG] ATTIC. The space between the ceiling framing of the top story and the underside of the roof.

 

[walker.t]

You continue to insist something that is black and white in the code.

I don’t think ICC would agree that everything in the code is black and white, otherwise they wouldn’t sell commentaries!

Again, the attic is literally defined as part of the story directly below it.…The attic is certainly NOT "beyond" the second story.

ICC seems to disagree…

2018 IBC Illustrated Handbook Commentary on “Story” (partial quote)
In the case of the topmost story, the height of a story is measured from the floor surface to the top of the ceiling joists, or to the top of the roof rafters where a ceiling is not present.

We have a ceiling so the story stops there.

 

[redeyedfly]

I don’t think ICC would agree that everything in the code is black and white, otherwise they wouldn’t sell commentaries!


ICC seems to disagree…

We have a ceiling so the story stops there.

Commentary is not code. And this one is black and white. A story goes to the upper surface of the roof above. I don't know how it could be more clear than that.

 

[klarenbeek]

So.... based on the commentary, the second story stops at the top of the joist, not the bottom. It basically stops in mid air (or mid insulation) between the joists???

 

[walker.t]

So.... based on the commentary

Technically it’s not “the” commentary, it’s from commentary in the 2018 IBC Illustrated Handbook which is copyrighted by ICC but lists three authors in the front matter. The above quote from that book is very close to the IBC definition of “Story”:

2021 IBC Definition of “Story” (partial quote)
A story is measured as the vertical distance from top to top of two successive tiers of beams or finished floor surfaces and, for the topmost story, from the top of the floor finish to the top of the ceiling joists or, where there is not a ceiling, to the top of the roof rafters.

The Illustrated Handbook isn’t interpreting the definition, it’s restating it with minor change to the language.

It basically stops in mid air (or mid insulation) between the joists???

I’m a little confused when you say “between the joists,” the top of the story in this case is the imaginary plane resting on top of and running across the top of the ceiling joists, it probably would cut through insulation.

 

[klarenbeek]

Literally the space between one joist and the next. So if the air handler was located on second floor with a duct system that ended up going to first floor, and the duct went into the attic but stayed below the top level of the joists before turning back down, say to get to the chase without building a soffit below the lid, its still in the second story. Have the duct go above the top plane of the joist before turning back down and suddenly now we need a rated shaft?
And it is still commentary, not the actual code book....

 

[walker.t]

Literally the space between one joist and the next.

OK, thank you for the clarification.

Have the duct go above the top plane of the joist before turning back down and suddenly now we need a rated shaft?

Like with many things in the code, a line has to be drawn somewhere. A person is building an office building, wants to save money and not install sprinklers, exit access travel distance is limited to 200’ (Table 1017.2.) If they exceed that by 1’ they have to add sprinklers or redesign to shorten the distance to 200’ or add an exit stairway or exit passageway. Does that extra 1’ travel distance really make a difference? Maybe not, but that’s where the line has been drawn.