Is stud size in T-721 minimum, or absolute?

[Yikes]

I want to use this assembly, but with 2x6 studs instead of 2x4. I can't find any footnotes or other text in chapter 7 that indicates the stud size is a minimum rather than an absolute.
I know the GA manual states their stud sizes are minimums, but they don't have an equivalent assembly to this.

 

[RLGA]

I would treat it as a minimum, even if it doesn't state it. Logic dictates that the further the two surfaces are apart, the longer it takes for thermal transmission to reach the same level as an assembly with 2x4s, which is why both GA-600 and UL allow larger framing studs in their assemblies.

 

[Yikes]

I would treat it as a minimum, even if it doesn't state it. Logic dictates that the further the two surfaces are apart, the longer it takes for thermal transmission to reach the same level as an assembly with 2x4s, which is why both GA-600 and UL allow larger framing studs in their assemblies.

Thanks RGLA - - is there anything in Chapter 7 that would help our plan checker to interpret the assemblies as a minimum stud size?

 

[RLGA]

Thanks RGLA - - is there anything in Chapter 7 that would help our plan checker to interpret the assemblies as a minimum stud size?

Not really--even Table 722.6.2(2) for the calculated method states in footnote 'b' that "All studs shall be nominal 2 x 4..."

However, I would fall back on Section 721.1 which states in part, "Where materials that change the capacity for heat dissipation are incorporated into a fire-resistance-rated assembly, fire test results or other substantiating data shall be made available to the building official to show that the required fire-resistance-rating time period is not reduced." For your "substantiating data," provide copies of UL's "Guide Information for Fire-resistance Ratings" and GA-600's "General Explanatory Notes - Fire Performance of Systems" and highlight those areas where they state the size of studs indicated is a minimum (for UL, it's Section VI; for GA-600 23rd Edition, it's item 18). Since these are blanket statements applicable to ALL assemblies in their directories, it would be difficult to refute that increasing the depth of a stud would reduce the fire-resistance-rating time period.

 

[Yikes]

RGLA, that seems like the best approach to utilizing table 721.
This afternoon I found that UL Design U356 will solve my particular needs, so for now, I'm abandoning Table 721.

Another problematic thing with the T721.15-1.3 assembly is that it requires 2"x4" studs, not nominal 2x4 studs.
This can be a dealbreaker for a by-the-books plan checker.