IRC 2018 Shear wall Limits (can we ignore if...)

[EngStuff]

In the IRC There are limits that we are required to meet for wood shear walls.

One of the limits listed on Figure R602.10.2.2 "Location of Braced Wall Panels" requires a 10'-0" Max from corners and a 20'-0" max between shear walls.

Another limit is R602.10.2.3. which is to have 2 braced walls.

Now from my understanding, this is prescriptive.

I have a custom home, this home doesn't meet any of those limits. For example, one exterior wall has all openings besides only 1 small shear wall and it doesn't meet either limit. I assume there won't be limits if we were to engineer the shear wall and make sure the chord forces transfer into the wall. Also, to make sure part of the load from half of the diaphragm transfers into that one wall.

I am about 95 percent sure the answer is that none of those limits mater if we are engineering it? As long as we follow a few things about chord and diaphragm requirements on IBC and ASCE7-10 but want to see what the consensus is?

Also, from my understanding, This would be considered a flexible diaphragm with a ridged wall?

Thanks!

 

[phamENG]

You're essentially correct. A few things:

The information in the IRC is, as you pointed out, for "Braced Walls"....NOT shear walls. Shear walls refer to a specifically engineered lateral force resisting system designed and detailed in accordance with the building code (typically the IBC with local modifications) and the NDS/Special Design Provisions for Wind and Seismic.

So if you are going to design the lateral force resisting system based on shear walls, you don't need to follow those limits. But you do need to follow your load paths and design everything in it. That means your collectors, diaphragm chords, diaphragm nailing, lateral transfer through the diaphragm, stability of chords and collectors, etc. all need to be analyzed. It's quite a bit of work, really.

 

[Enable]

phamENG: What is the likelihood of getting a residential contractor to even semi-correctly follow a unique design in such situations? I have to imagine oversight being a pretty significant concern (or cost to the owner) to make sure the contractor doesn't just do their usual thing. What is your experience in such circumstances? Genuinely curious because if it was me I would try to go prescriptive if at all possible, even if only just to keep it standard fare for the contractor (I dont do much residential as you know though).

 

[phamENG]

Runs the whole spectrum. Fairly modest but just complex 'enough' house to require shear walls that the contractor followed perfectly to 7,000sf custom ocean front home where, after all framing was up, asked: What's a shear wall?

They had built the entire house without looking at the structural drawings. No shear wall nailing. No shear wall hold downs. The steel portal frame at the garage wasn't built (or it's foundations!). What a mess.

 

[Aesur]

Wow, yall make me glad the contractors here at least ask questions and look at structural plans when building custom houses. We almost always engineer the entire house for residential projects; the only time I have used the IRC was when dealing with strawbale type houses.

I agree with phamENG's response on the use of shear walls versus braced walls; make sure you engineer the load path and you are good to go using shear walls where needed and ignoring IRC requirements for braced walls. Please note that I would not mix the two codes for lateral design however, I would either use braced walls everywhere or shear walls everywhere, not mix and match.

Personally I try to avoid the IRC as I have almost never gotten the IRC sizes in their tables to calc out when doing some spot checks, I'm assuming it's all based on "testing".

@phamENG - I am curious, what did you tell the contractor in regards to taht 7000 sq custom home?

 

[EngStuff]

Thanks guys I appreciate it!

I Was going to make a moment frame at all the window openings, Shear wall at the center of the house and the other side is another a mess. Out to Out offsets, and each wall has a large opening. I am thinking portal frames at each wall. trying to get away from steel moment frames all over.

Also, One thing I want to be clear in, the "braced wall line" on the IRC. I've always assumed Strictly for the IRC, they lumped the terms shear panel, let-in brace and portal frames into "braced wall Line" but only what's limited to the Charts they provide?

Thanks

 

[phamENG]

Aesur: "Fix it." They had to epoxy in anchors where they were two widely spaced and for hold downs, strip all of the Tyvek and fix the nailing, rip the garage door apart and install the portal frame. Not sure if the framer or GC survived that one, but if they did they'll probably look at the structural drawings next time.

The wood structural panel braced wall, let in bracing, portal frame, and all the others are just different kinds of wall bracing. There's a white paper floating around that gives you values for the the IRC portal frame to use in engineered applications (gives capacity and deflection).

Braced wall lines really don't apply to shear walls. I've seen it used (and have used it myself) for some fairly straight forward designs where I assume that the shear walls within a certain lateral distance will share the load without significant modification to the diaphragm, but it's probably best to leave that aside the first few times until you really get a feel for what that distance is and why. Look at the individual load paths and how the shear is transferred through the diaphragm from each shear wall.

And if you're doing a house with a lot of windows, FTAO shear walls are your friend. They can seem intimidating and some popular software packages don't cover them, but they're easy once you get the hang of them and APA has some really useful design tools for them.

 

[EngStuff]

PhamENG,

At the one wall, The house has full openings from top to bottom. I wouldn't be able to use FTAO on that location. I just have enough to add a beam above to support the roof, and use that as a chord member to transfer it into the moment frame.

The other side with the out to out offsets, I can see if I'm able to use it there. I might have to mix different kinds of lateral resistance systems, portal frames, steel Moment frames and shear walls. Because some locations on that wall it's full openings from ceiling down to the floor, then that exterior wall offsets another 4 feet and has regular depth windows, but span really close to the corners, then it offsets back to the original wall line and has another issue with the openings. That wall goes in and out every 7 to 12 feet. Not sure if this makes sense without seeing a sketch

Thanks