Fire separation distance based on occupancy?

[Sifu]

Given a FSD of 1' for a non-separated, non-sprinklered, VB building containing a B and M occupancy. The B occupancy is adjacent to the exterior wall with the FSD of 1. T602 would require a 1-hr exterior wall rated from both sides (705.5). The M is on the other side, approximately 18' away from the FSD. M would require a 2-hr exterior wall. My thought was that in a non-separated building, the M would be the driver, however as an alternative they could provide a fire barrier to separate the occupancies which would let the B be the driver for t602. I thought it was a viable alternative, but when I look for the rating of that fire barrier to create separated occupancies, t508.4 does not require any separation. So t602 contends that an M is a higher hazard for exterior walls, but t508.4 contends that the hazard is equal.

Or, do I get to use B from t602 since it is the occupancy that is within the FSD? I guess I have never considered how a mixed use building fits in t602.

 

[Yankee Chronicler]

In a building of non-separated mixed uses, the type of construction, building height, building area, and fire separation distance are governed by the most restrictive of the mixed uses. (In your example, the M occupancy.) What is controlled by the occupancy is things such as the exit access travel distance and allowable common path of exit travel within each occupancy.

If you erect a rated fire barrier between the B and the M occupancies, then you no longer have non-separated mixed uses.

 

[RLGA]

Section 508.3.1 states that the requirements of the code "apply to each portion of the building based on the occupancy classification of that space." Only the requirements of Chapter 9 apply to the entire building based on the most restrictive occupancy.

Thus, whether using the separated or nonseparated occupancy method, the fire resistance rating of the exterior wall would be based on the occupancy group immediately adjacent to the exterior wall.

 

[Sifu]

Thus, whether using the separated or nonseparated occupancy method, the fire resistance rating of the exterior wall would be based on the occupancy group immediately adjacent to the exterior wall.

Thank you for the clear answer, it is what I was hoping would be the case.