We are looking at a one-hour occupancy separation between an S-1 occupancy on the first floor and an R-2 occupancy on the second floor of a dormitory Building. Supporting construction of this one hour horizontal assembly is required to be protected by the same one hour rating per CBC 712.4. The floor joists in this assembly are supported by wood framed bearing walls at the exterior of the building. Are the exterior bearing walls required to be rated for exposure to fire from the exterior as well as the interior or can they just be rated for exposure from the interior side? Just rating these walls from the interior side seems logical since the occupancy separation is separating occupancies located at the interior of the building but we are unable to locate any provisions in the code that address this question. The Construction type is V-B and we have sufficient distance from property lines to allow non-rated construction at exterior walls per CBC table 602.
I would say that the protection is from the inside. You are protecting the structural system of the building from fire exposure on the inside of the building. If the exterior wall has a fire separation distance of 10 feet or less, then protection from exposure on both sides would be required.
The justification for this is Table 601 and Section 705.5. The fire ratings in that table are essentially for the structural stability of the building. With the exception of Type IIIB construction, exterior bearing walls, which support fire-resistance-rated horizontal assemblies, have a fire-resistance rating equal to or greater than the horizontal assemblies; and per Section 705.5, it states that exterior walls per Table 601 (and 602) only need to be protected from an interior fire exposure and from both interior and exterior if the fire separation distance is 10 feet or less.
Therefore, your condition, although not driven by Table 601, is nearly identical, so the same logic for exterior walls would apply.
Thank you, I appreciate your participation in this forum.