Double Fire Walls

[Noob]

I have a Type I, S-2 concrete garage, and a Type VA R-2 residential building directly against each other. there's a seismic gap between them. I need to connect the garage to the residential building at each level, but require a Fire Wall between the two separate buildings. IBC tells me to build a Fire Wall per NFPA 221. NFPA 221 tells me I can build a "Double Fire Wall" - so here's where I get confused:
1. NFPA 221 6.10.3 indicates that a Double Firewall needs to have a door in each wall. For obvious reasons, this doesn't work wall for a residential apartment up against a parking garage.
2. Alternatively, IBC 705.3 Exception 2 say that the door should be in the S-2 (garage) wall, and not in the R-2 wall.

Which is right?

Additionally - I need a 2-hour fire wall rating. my assumption is that I need two 1-hour rated (exposure from both sides) walls to get the 2-hour rating on the double wall. A type I garage, I don't think, would collapse in a fire very quickly, so I'm not sure why that is even a concern for Fire Walls in Type I construction (unless I'm missing something). Of course, if a concrete garage did actually collapse next to a light framed building, then good luck to your light-framed firewalls. So what am I missing?

 

[Noob]

Ugh - it appears North Carolina adopted language to clarify this exact issue: https://www.oah.nc.gov/documents/rules/rrc/42309-building-code-technical-changes/open Unfortunately this isn't in NC, think I need to contact the city on this one?

 

[Jay Smith]

I’ve never considered this condition to be a double fire wall. It’s either back-to-back exterior walls as in buildings on the same lot or it’s a two-hour wall that serves as the fire wall and the garage exterior wall.

 

[Noob]

I might be confused.
If I have two buildings with a specific Fire Separation Distance (let's say it's 2 inches, imaginary line, not a PL). I can just follow Table 705.5 which indicates that S-2 needs a 1-hour exterior wall, and R-2 needs a 1-hour exterior wall. Firewalls don't even enter the equation. But if it's a "party wall" then it needs to be a firewall, correct? If this is the correct interpretation, what is the reasoning - is it because of fire insurance, the need to protect your neighbors property essentially where you wouldn't need that protection on a single lot? Where else are "Fire Walls" required?

 

[classicT]

With two exterior rated walls, the fire ratings are additive - meaning the fire must burn through one wall and then the next.

With a fire wall, you typically have a single wall that can be shared between two buildings/tenants.

 

[Noob]

Ah, okay - this is starting to make sense. So for the Concrete Garage S-2, even though likelihood of collapse is almost zero, 706.2 would still require the concrete garage to collapse without affecting the firewall or the adjacent wood-framed R-2 building? Are there any exceptions that allow the Type I or II garage to have the 2-hour "fire wall" as it's exterior wall?