CBC / IBC Table 721.1 Wood Stud Sizing

[Brainhurts]

Hi Folks,
I'm trying to use Table 721.1(2), Assembly 14-1.5 for a 2-hour fire wall assembly. I had planned for a 2x4 wood stud wall, however due to some plumbing issues, the wall now must be change to a 2x12 wood stud wall, or a double-stud wall. I can't find any "solid" info about maintaining the rating when I increase the stud size?

The table says "minimum finish thickness face-to-face" I don't know if that's just referring to finish thickness variation only, or stud size variation as well. I don't know if what I'm asking is just "common knowledge" or something missing from the code.

Thanks for the help!

 

[Brainhurts]

clear up my question:
Are the stud sizes referenced in Table 721.1(2) minimum stud sizes OR prescribed stud sizes?

 

[ADAguy]

Not exact if that is what you are asking? A 2 x 4 is not actually, it nets at 1.5 x 3.5.

 

[Brainhurts]

Not exact if that is what you are asking? A 2 x 4 is not actually, it nets at 1.5 x 3.5.

No - see the table. the question is Minimum Stud Size, or Prescribed (Mandatory) Stud Sizes. Can I use a larger 2x8 or a 2x12 in-lieu of a 2x4 and still meet the "prescribed" fire resistance rating. I think the answer is "yes" but there isn't any specific allowance for it in the footnotes.

 

[Brainhurts]

Can't find it in the CBC, however from the Gypsum Association Fire Resistance and Sound Control Manual, page 20:
"18. STUD SIZES AND DEPTHS - Greater stud sizes (depths) shall be permitted to be used in metal- or wood-stud systems. Metal studs of greater mil thickness than those tested for fire performance shall be permitted. The assigned fire rating of any load-bearing system shall also apply to the same system when used as a nonload-bearing system. Indicated stud spacings are maximums in regard to the fire performance of the system only. Greater mil thickness studs or closer stud spacing may reduce the STC. Greater stud depth may improve the STC."

I suppose I should stick with GA File numbers - however seems to defeat the purpose of Table 721. to not have this info in there.

 

[Yikes]

I want to resurrect this thread, as no one gave a straightforward answer in 2019.
When you see this (below) in a table), is there anything written in the code that allows the 2"x4" studs to be 2x6 instead?

 

[Paul Sweet]

A lot of Table 721.1 is based on archaic materials. Several constructions in item 15 Exterior or Interior walls list 2x4 and 2x6 studs separately. I don't think I see anything in the IBC that specifically states that a larger stud may be used. Some jurisdictions will deny it based on that, wile others will allow it based on verbiage in UL and GA fire ratings.