Fireproofing Light-frame Structural Members

[Jacob Goldman]

I'm designing a light-frame structure, and I can't figure out how to fireproof the exterior walls studs. Does it need to be 2 hour fire-rated on each side of the stud, or is it enough to have a 2-hour fire rated wall-assembly?

Note my question pertains the "secondary member" requirement of Table 601. IBC 704.1.1 requires light-frame construction secondary members to be individually encased by a fire-rated membrane, but I'm unclear how that membrane protection works. (Is UL U425 meant to address that?)

 

[RLGA]

The rating of an exterior wall is based on fire separation distance, and if that fire separation distance is greater than 10 feet, then the rating needs only be based on an interior fire exposure only.

I assume you meant IBC Section 704.4.1. Metal studs are not subject to individual protection per that section, but the framed assembly as a whole can be protected using membrane protection consisting of one or more layers of gypsum board or other tested materials. UL U425 would be an acceptable assembly for either condition (fire exposure on both sides and fire exposure from the inside only).

 

[Jacob Goldman]

Thanks for the insightful response. Yes indeed I was referring to IBC Section 704.4.1 (my bad).

If I understand your response correctly, you're addressing fire-rating requirement for "Exterior Wall", because it would be irrelevant to address fire-rating requirement for the metal studs since they're not subject to individual protection.

However, the text in that same section can be seen as stating that the metal studs do need individual protection (and I can relate to it too).

704.4 Protection of secondary members. Secondary members that are required to have a fire-resistance rating shall be protected by individual encasement protection, by the membrane or ceiling of a horizontal assembly in accordance with Section 712, or by a combination of both.

704.4.1 Light-frame construction. King studs and boundary elements that are integral elements in load-bearing walls of light-frame construction shall be permitted to have required fire-resistance ratings provided by the membrane protection provided for the load-bearing wall.​

Apparently the code seems to view "king studs and boundary elements" as "secondary members", however it gives it special treatment exempting it from individual encasement protection, and instead it would only need membrane protection, but it does not imply that we can disregard the member protection entirely, and instead just rely on the wall-assembly as a system.

Am I correct that the difference between individual encasement and membrane protection is whether each member has to be individually encased, vs. just applying a facing membrane to protect all such members in aggregate? And if I'm correct, then wouldn't the facing membrane by itself need to provide the full 1- or 2-hour fire-protection for the members, aside from the wall-assembly's fire-rating as a whole?

 

[RLGA]

Am I correct that the difference between individual encasement and membrane protection is whether each member has to be individually encased, vs. just applying a facing membrane to protect all such members in aggregate? And if I'm correct, then wouldn't the facing membrane by itself need to provide the full 1- or 2-hour fire-protection for the members, aside from the wall-assembly's fire-rating as a whole?

For the first question, the answer is yes.

For the second question, the answer is also yes, but not in the way you assume. Each tested wall assembly is tested for the duration for exposure on both sides. A wall with a 2-hour rating will provide protection for that duration from one side only. If the assembly is symmetrical (i.e., same construction on both sides), then it needs to only be tested from one side since it is assumed the other side with the same construction will perform identically. For asymmetrical construction, the wall must be tested for exposure on both sides and the rating is based on lowest performing side.

Therefore, the membrane of a 2-hour wall will also provide the 2-hour protection for secondary members framed into that wall per Section 704.4.1.

 

[Jacob Goldman]

For the first question, the answer is yes.

For the second question, the answer is also yes, but not in the way you assume. Each tested wall assembly is tested for the duration for exposure on both sides. A wall with a 2-hour rating will provide protection for that duration from one side only. If the assembly is symmetrical (i.e., same construction on both sides), then it needs to only be tested from one side since it is assumed the other side with the same construction will perform identically. For asymmetrical construction, the wall must be tested for exposure on both sides and the rating is based on lowest performing side.

Therefore, the membrane of a 2-hour wall will also provide the 2-hour protection for secondary members framed into that wall per Section 704.4.1.

Thanks a lot. This is enlightening.

 

[ADAguy]

Thanks for the insightful response. Yes indeed I was referring to IBC Section 704.4.1 (my bad).

If I understand your response correctly, you're addressing fire-rating requirement for "Exterior Wall", because it would be irrelevant to address fire-rating requirement for the metal studs since they're not subject to individual protection.

However, the text in that same section can be seen as stating that the metal studs do need individual protection (and I can relate to it too).


Apparently the code seems to view "king studs and boundary elements" as "secondary members", however it gives it special treatment exempting it from individual encasement protection, and instead it would only need membrane protection, but it does not imply that we can disregard the member protection entirely, and instead just rely on the wall-assembly as a system.

Am I correct that the difference between individual encasement and membrane protection is whether each member has to be individually encased, vs. just applying a facing membrane to protect all such members in aggregate? And if I'm correct, then wouldn't the facing membrane by itself need to provide the full 1- or 2-hour fire-protection for the members, aside from the wall-assembly's fire-rating as a whole?

The words "or" and membrane are both offered as options.