IRC Braced Wall Lines on Townhomes/Rowhomes

[MJC6125]

I'm working on IRC townhomes and trying to implement IRC braced wall line methods for checking lateral. I'm not a pro with the braced wall line methodology, and I'm coming across some nuances with how townhomes/rowhomes are built that relates back to lateral resistance.

Refer to example plan view below. Assume we have (4) rowhomes stacked back-to-back. There is a fire separation/common wall between each unit, so the diaphragms are technically discontinuous. You can however tie the exterior walls that are perpendicular to the common wall together, and drag load from one unit to the other at those edges (at least in the jurisdiction I'm working in you can). Is it acceptable to assume interior braced wall lines of these units in this plan N/S loading direction can also share load and be designed like one large single family home for braced wall line requirements?

Not sure if anyone is implementing IRC braced wall line methodology for townhomes like this and has any insight.

 

[lexpatrie]

I don't think so, look at the provisions for structural independence (R302.2.6). Each unit is supposed to stand on it's own without shielding/support from the end units. From a fire standpoint, I think the sheathing can go across because it's less than 1.5" thick and wouldn't act as a fireblock (SEAOC had a paper on it).

I suppose that's under the IRC, the decision might be more debatable under the IBC, but switching may have other more complex issues relating to construction that the Architect (if there even is one) may not quite catch on to.

Each townhome is on it's own "imaginary lot line" last time I looked. So hypothetically the one in the middle can be demolished, or burn down, etc.

Regards,
Brian

 

[MJC6125]

I did notice the structural independence provision R302.2.6. I believe these townnhomes would meet the exception #5 of that part of the IRC (Townhouse units separated by a common wall as provided in Section R302.2.2, Item 1 or 2), so I don't think that code section needs to be met in this case.

 

[lexpatrie]

By adding a two hour firewall? I'm not convinced this is a solution, because as I mentioned in the SEOSC paper, then they are fighting to keep the floor sheathing continuous. Whenever I see a firewall, I think structural independence anyway, and then we're back where we started, needing a valid lateral system for each townhome. Plus there's all the fun and excitement with parapets that I never see done correctly (pointing at the architect here).

The plan looks repetitive, if you get it working as a braced wall construction it's basically the same analysis for each unit, further, if you are actually doing braced wall, there's no engineering calculations in there, just establishing enough wall length in the right places. I did this once and it backfired nicely in-house. Boss was asking where my lateral design was when the Arch wanted an IRC set of townhomes. Came up while I was moving all my things for the new job.

 

[MJC6125]

Sounds good. I think we are in agreement. I was just hoping a residential engineer on here may have a trick that they have used or a code section that allows an entire townhome building to always be designed as one big IRC braced wall building. But I'm guessing no such thing exists.

For your reference, there is an exception in section R302.2.2 of the 2021 version of the IRC that states Common walls are permitted to extend to and be tight against the inside of the exterior walls if the cavity between the end of the common wall and the exterior sheathing is filled with a minimum of two 2-inch nominal thickness wood studs. I'm guessing this exception was added so shearwalls and lateral loads can be dragged from one unit to the next through a drag/collector element, i.e. the exterior wall continuous double top plate. Obviously not every engineer may be comfortable with this design approach and the construction of the exterior wall across the unit joint becomes more critical. But there does seem to be some ways to get load from one unit to the next at the diaphragm edges even if the diaphragms themselves are discontinuous.

 

[Celt83]

You don't have a common wall you have a double wall per R302.2.1 so exception 5 of R302.2.6 does not apply and each townhome must be structurally independent.

Most jurisdictions in my area force R302.2.6, and the front walls of these things are usually all glass so have never been able to do prescriptive bracing per IRC.

 

[MJC6125]

Interesting. I was under the assumption that the typical party wall details I see on townhomes are IRC 302.2.2 common walls, but not my area of expertise so I'm probably wrong. I typically see the detail shown below. Do you know if that is a "double wall" or a "common wall"? I just tried searching online for an answer, but I couldn't find anything definitive.