Fire Wall between Type I and Type V - Combustible framing ok on one side of double fire wall?

[Lorenbb]

Is this a case when the fire wall between a Type I and a Type V building can include wood framing?

The situation:

1. We need a 3-hour fire wall between R-2 occupancy Type V and Type I buildings per IBC 706.4.
2. Code edition is Washington State 2015 IBC.
3. The 3-hour rating is being achieved with two separate 2-hour walls following the double wall approach outlined in IBC 706.2 and NFPA 221-2015 Table 4.5. In this double wall approach, 2-hour walls are located at the opposing faces of each building, and those two walls provide a combined assembly fire-resistance rating of 3-hours.
4. This double wall approach contrasts with a conventional fire wall, where the single wall assembly is attached to, and supported by, both buildings. Reasonably, a conventional fire wall must comply with the requirements of both buildings, and because of the Type I building, wood framing would not be allowed.
5. In this case, the two 2-hour walls in this are physically separated by a 16” seismic gap between the Type V building and the Type I building. The 2-hour walls are each exclusively located in, and supported by, only one building or the other.

Would the exception to IBC 706.3, which allows combustible construction in Type V buildings, apply to the side of the double wall that is in the Type V building? The 2-hour wall in the Type V building Is not a part of the Type I building, so I would think that the exception does apply.

Does anyone agree?

 

[Lorenbb]

I asked ICC, and they agreed.
The local AHJ reconsidered, but then said "No, we do not agree. It is still a fire wall at a Type IA building: no wood."

 

[steveray]

I'd have to see the details on the double wall, but I could support that each wall belonging to each building and constructed accordingly....

 

[Paul Sweet]

If there aren't any openings, could they be considered as two exterior walls (IBC 705) with 0 fire separation distance?

 

[steveray]

Do you have an assembly picked out? I could argue that a shaftwall/ firewall is non-combustible even though the wood wall holds it up....

And it is only through exception that drywall is non-combustible now...

703.3.1 Noncombustible materials. Materials required
to be noncombustible shall be tested in accordance with
ASTM E136. Alternately, materials required to be
noncombustible shall be tested in accordance with ASTM
E2652 using the acceptance criteria prescribed by ASTM
E136.
Exception: Materials having a structural base of
noncombustible material as determined in accordance
with ASTM E136, or with ASTM E2652 using the
acceptance criteria prescribed by ASTM E136, with a
surfacing of not more than 0.125 inch (3.18 mm) in
thickness having a flame spread index not greater than
50 when tested in accordance with ASTM E84 or UL
723 shall be acceptable as noncombustible.

 

[Builder Bob]

I disagree, the code always defaults to the most restrictive - therefore the three hour wall would have to meet Type I constuction IMHO.

 

[steveray]

And the 3 hr wall does....

 

[Lorenbb]

If there aren't any openings, could they be considered as two exterior walls (IBC 705) with 0 fire separation distance?

That's the path we are taking!
It's what the jurisdiction recommends.
At this point, I really want to keep them happy. Also, it replaces two 2-hour walls with 1-hour walls, and also removes most of the fire blankets from around the perimeter of the seismic joint covers.

 

[Lorenbb]

Do you have an assembly picked out? I could argue that a shaftwall/ firewall is non-combustible even though the wood wall holds it up....

And it is only through exception that drywall is non-combustible now...

703.3.1 Noncombustible materials. Materials required
to be noncombustible shall be tested in accordance with
ASTM E136. Alternately, materials required to be
noncombustible shall be tested in accordance with ASTM
E2652 using the acceptance criteria prescribed by ASTM
E136.
Exception: Materials having a structural base of
noncombustible material as determined in accordance
with ASTM E136, or with ASTM E2652 using the
acceptance criteria prescribed by ASTM E136, with a
surfacing of not more than 0.125 inch (3.18 mm) in
thickness having a flame spread index not greater than
50 when tested in accordance with ASTM E84 or UL
723 shall be acceptable as noncombustible.

I was getting ready to make that argument, but instead we are treating the walls as exterior walls, facing each other across an imaginary line.
I wish we had thought of that back in 2020 when we designed the building.