IRC Townhouse Braced Wall Line Requirements 1

[L_Bey]

We have been having a discussion in my office lately on the requirements for designing townhouses per the IRC. The main question is do we need to comply with the braced wall requirements per unit, or is the entire group of townhouses to be designed as a single structure. Often these structures are 4 or more units, maybe a garage door on one end and entry/windows on the other, leading to pretty limited available wall in one direction and plenty in the other (with the demising walls between the units).

The IRC defines a townhouse as "A single-family dwelling unit constructed in a group of three or more attached units in which each unit extends from foundation to roof and with a yard or public way on not less than two sides."

The section on wall bracing reads as follows: "R602.10 Wall bracing. Buildings shall be braced in accordance with this section or, when applicable, Section R602.12. Where a building, or portion thereof, does not comply with one or more of the bracing requirements in this section, those portions shall be designed and constructed in accordance with Section R301.1." (Section R301.1 allows an engineered solution in accordance with the IBC where the prescriptive requirements are not met)

Table R602.10.1.3 lists "Townhouse" as a building type for the maximum braced wall line spacing requirements for wind and seismic. Section R602.10.3 item 3 notes that townhouses in seismic design category C se the greater of the wind or seismic braced wall line lengths.

Both of these requirements lead me to believe that the intent of the code is to design the townhouse (a single unit in a group of 3 or more, with exterior walls on 2 sides) per the braced wall requirements, meaning that each unit in the group will have the full braced wall requirement met, and if one unit burns down (or isn't built at all) the individual is still acceptable. The alternative would be to design the entire structure per the IBC and not use the prescriptive requirements at all. My boss has argued that if the entire group of townhouses (since R602.10 states that a Building shall be braced) can meet the braced wall panel length requirements. This would mean that each individual townhouse unit would not need to meet the full braced wall line requirements by itself.

What say you?

 

[jayrod12]

Lately in these buildings we've been having a real push from architects to have the floor sheathing discontinuous across units. That pretty much then requires each individual unit to be stable on it's own.

 

[L_Bey]

That's a really good point. That hasn't come up at the moment, but it's definitely something to keep in mind.

 

[jayrod12]

Sorry I can't speak directly to the IBC requirements because I only practice north of the border.

 

[Harbringer]

Why don't you actually design the lateral system? Then you will know if it works or not. Its not very hard to check shear walls on a single line.

 

[Aesur]

Can you provide a sketch? I fail to see how this matters, if you use the whole building, I assume you have a party wall between units, which would create a braced wall; if the units are too wide (say greater than 60' for wind design), then you would have perpendicular walls to brace the wall at distances inside the unit to create braced walls and then you would have the party walls. I'm struggling to think of an occurrence that the individual units don't meet the requirements yet the whole building does.

 

[Arp32]

Architect here...

Somewhere in the IRC (maybe in the Definitions?) there is language that each unit shall be structurally independent. The ones I've worked on treated each unit like it got wind from all sides, so they ended up being pretty redundant.

The reason the architects are pushing for discontinuous floors is that in the Townhome chapter, it does say that there cannot be any horizontal penetrations thru to adjoining units above the slab. So sewer lines can connect below grade, but once above the slab, nothing can cross between units except for waterproofing and cladding. There's a couple of options for the roof sheathing (you don't have to carry "fins" above the roof plane anymore), most of what I see is PT plywood within I think 4' of the line between units.

I think there's one party wall assembly that uses a single CMU party wall, but all the other prescriptive assemblies are totally independent (interior gyp board, stud wall, rated gyp board, then an air gap between units, rated gyp board, stud wall, then interior gyp board). The one I've used has stud walls with GP or LP 1" gyp board "shaftliner" facing the adjoining unit on either side and breakaway clips. In theory, one unit should be able to burn to the ground without affecting the neighbor. One assembly requires floor joist hangers *on top* of gyp board when the joists bear on the party walls, and Simpson makes a special hanger for that. I try to keep my framing parallel with the party walls (makes it easier to run HVAC thru to the exterior windows).

The braced wall thing is a distraction. Personally, I have never worked with an SE who thought the braced wall tables were worth reading, they all just did shear wall calcs. Braced wall is intended mostly for non-registrants who can stumble through a a table but can't do math that has exponents or Greek letters. Plus, if you have 3 story townhomes with a 20'-24' module and 16' or 18' garage doors on one end, the portal walls are hard (impossible?) to make work with braced walls anyway. Easier with masonry at the first floor, or maybe Simpson SSW panels.

Good luck. I did 30 or 40 townhomes a few years back but stay away from them now. Touched one condo last year (a kitchen remodel) and my insurance went up $4,000.

-Jason, Architect

 

[L_Bey]

Thanks Jason - That was my reading but it's good to have someone more familiar with that part of the code weigh in.

Regarding the why don't we just do the engineering for the shear walls - generally that's where we end up, particularly for anything that's taller than 1 story (or 1 1/2). Sometimes it's fastest to run a braced wall line check first (Simpson has a neat little tool for that) and see whether it's even in the realm of possibility, and adjust our design fee accordingly. End of the day complying with the IRC braced wall line tables tends to be more conservative than an engineered solution so if it's faster and the client doesn't want to pay for more thorough design I don't have a problem with giving them something they're familiar with.

However that doesn't get away from the question of does each unit need to be independent or can the whole group be designed as one. And it sounds like the answer is if it's designed as a townhouse per the IRC each unit must be independently braced (regardless of how redundant it is), but if it's an IBC building (and the sheathing is continuous etc) we could design the full group as a single structure.

Aesur see the attached sketch - basically we might have 5 units, each of which have one 4 ft panel between the front door and the window on one side, and maybe (2) 3 ft panels on each side of the garage door at the back. By itself each unit does not have enough bracing to meet the requirements either for braced walls or for engineered shear walls. But the whole structure has 20 ft of wall on the front and several locations for strong walls or garage portal frames on the back. (I'm not worried in the north-south direction, since as you note we have plenty of demising wall to rely on there) If I can consider the whole building as one structure I might not need any strong walls on the front, and maybe just one on each end on the back. But if each individual unit needs to meet the shear requirements than the 4 ft of wall on the front isn't enough, nor is 1 portal frame on the back. So I'll have to use some interior walls (if there are any) or add more strong walls to make the system work, for each unit. Definitely redundant, but if the building needs to be IRC for other reasons according to the architect then I'm stuck.

 

[phamENG]

This is from the Virginia Residential Code (IRC with Virginia amendments/revisions), so yours will be similar if not identical, and least in the same place:

Each individual townhouse shall be structurally independent.

Exceptions:​

1. Foundations supporting exterior walls or common walls.​

2. Structural roof and wall sheathing from each unit fastend to the common wall framing.​

3. Nonstructural wall and roof coverings.​

4. Flashing at termination of roof covering over common wall.​

5. Townhouses separated by a common wall as provided in Section R302.2.2, Item 1 or 2.​

6. Townhouses protected by a fire sprinkler system complying with Section P2904, NFPA 13, NFPA 13R, or NFPA 13D.​

[/indent]

Item 5 discusses common wall fire rating requirements, so if you have a 2-hour rated wall you don't have to comply with the structural independence.

And in case somebody wants to argue that 'townhouse' refers to the group of units (the whole building), they're wrong:

[RB] TOWNHOUSE. A single-family dwelling unit constructed in a group of three or more attached units in which each unit extends from foundation to roof and with a yard or public way on not less than two sides.

Arp32 - I used to work at a firm that held that belief, but since getting deeper into the single family residential, custom home market a lot more I've found braced walls to be a wonderful thing. In the rare instances where a homeowner wants to spend the extra money to ensure a certain level of performance I go with a fully engineered lateral system and require a robust inspection regimen, but if not I try to use braced walls where I can. A lot of the framers in my area work mostly with large developers who specialize in tract homes that never see a DOR, so you're lucky if they get the braced walls right. Shear walls are usually a nightmare in the field around here.

 

[Arp32]

EDIT oops, Where I said PT roof sheathing I meant fire rated. Not the whole roof, just on either side of where units adjoin.

I took over a townhome project from a guy who did really sloppy drawings and somehow got a permit - he had no detail of how the party wall met the roof sheathing. No engineer, the diaphragm wasn't tied together, and no detail to keep fire from jumping between units. Inspector made them hire someone new to fix it (me).

One part of the solution was fire rated roof sheathing. I remember the GC being mad about it, he had never heard of it and I gather that's not something sold at Home Depot. That would be something to make sure you account for in your GSNs and/or make bold on the framing plans if that's the method the architect uses.


-Jason, Architect

 

[phamENG]

Fire treated ply on the roof is common around here. There's a development of townhouses going in a few miles up the road from me with that stripe of red plywood between each unit. And no...I don't think Home Depot stocks it.

 

[Aesur]

@L_Bey, I assume the issue is the need for (2) 4' wall segments along the front because you don't meet the requirements for the minimum wall length? I apologize as I am not very familiar with the IRC, used it maybe once of twice and didn't agree with the results it gave for sizes. Now I only use it for strawbale and similar construction.

Well said @Arp32, very informative post. I believe you are referring to the exceptions of IBC 705.11 to avoid parapets, hence the 4' of FRT. I have had similar experiences with contractors doing residential work having no idea what FRT or even shear walls were.

@phamEGN you hit the nail on the head, many architects don't seem to understand that designing a custom home is actually more work that a much larger commercial project and the ONLY way I can understand how some of these other engineers managed to do that work for such little fee is to use "short cuts" such as a the IRC. For residential projects, I tend to only go after high end custom homes (2.5 million+) and give a fee that allows me to actually engineer the project, not to mention rarely do these type of homes meet the requirements for using the IRC in the first place. I have given a fee for a few smaller homes and immediately get push back on the fee being too high when in reality it's probably half of what it should be many times, but is still 2x the going market rate around here.

About a year ago we had an architect wanting us to design a 3 story "artsy" residence, on a tiny lot resulting in a building where nothing stacked; I gave him a lower than normal fee to see how it was to work with him and try to use this as the "in" to get the commercial work. He immediately came back saying he wanted to use us because we provide more detail and better service than his other engineers, but he needs us to cut out fee in half to match the other engineers; we walked away. That kind of response tells me exactly what it will be like working with that kind of architect.