Residential Design - Limits of Wood Shear Walls
Residential Design - Limits of Wood Shear Walls
(OP)
In residential, often times wood shear walls are designed using segmented, perforated, or FTAO methods in custom homes where prescriptive wall bracing cannot be applied. I frequently run into situations where in-plane shear demand creeps around 1000 plf (allowable) in which very tight edge nailing is required to transfer the demands down to the foundation. As we all know, typical anchor bolt spacing per the IRC is 1/2" bolts at 4' or 6' centers, however, at highly loaded shear wall locations, there is no way to get this to work and as engineers, we have to tighten the spacing to transfer the load (wood bearing controls on the sill plate). Usually, I find myself using 5/8" bolts every 18" - 24" to take the shear (my comfort level) but am finding that contractors simply do not adhere to these requirements when the home is ultimately built.
Curious what you all have seen and the limit of using wood for these types of designs before handling with a steel moment frame, pre-engineered strong wall / strong frame or even continuous tension rod take up devices? Seems like architects are more frequently coming up with huge homes, no interior shear walls and the rear face of the home absolutely peppered with windows!
Curious what you all have seen and the limit of using wood for these types of designs before handling with a steel moment frame, pre-engineered strong wall / strong frame or even continuous tension rod take up devices? Seems like architects are more frequently coming up with huge homes, no interior shear walls and the rear face of the home absolutely peppered with windows!
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
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RE: Residential Design - Limits of Wood Shear Walls
If I know the contractor and trust them to order the right materials, I'll allow it to go up to 1860plf with Structural 1. But the stuff is special order, and more headache than it's worth if they order the wrong stuff. Getting more bolts or hold downs is usually just a trip to Home Depot.
I try not to worry about whether or not a contractor is following my plans. My job is to design the structure. Their job is to build it in accordance with those plans. If the owner or architect want to pay me to inspect it, I'll do it and then I'll ensure they're following the plans. Whether I'm inspecting it or not, I don't let them make it my problem if they don't follow the plans. If there's a way to salvage the situation, I'll charge accordingly to redesign it, but I'm not opposed to telling the contractor to build it to the plans - rip out what you have to to get it done.
RE: Residential Design - Limits of Wood Shear Walls
RE: Residential Design - Limits of Wood Shear Walls
Regarding shear capacity, my limit is 2040 plf with 7/16" S1 w/ 8d nails at 2" spacing, but think about sill anchor bolts in the foundation wall. With a double-sheathed wall, I don't think you could develop 2000 plf of shear before the wood fails in bearing parallel to the grain. For a 5/8" anchor bolt spaced at 12" on center, you can only take about 1000 plf. Sure we can design the shear wall to resist it, but are we wasting our time if the anchorage is disproportional to the load?
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
The mudsill would be at 3x thickness for very high shear, we can definitely get more than 1000 plf out of that but you're correct that it becomes a lot of bolts.
I am commonly finding the allowable story drift is the controlling factor and not in the in plane shear.
Generally if I am on a fancy home with many windows, I will just say moment frame or similar very early on. Then get into the calcs and if I can actually avoid it I'm the hero. Spoiler (this hasn't happened yet lol)
RE: Residential Design - Limits of Wood Shear Walls
RE: Residential Design - Limits of Wood Shear Walls
With the 3x mudsill, the anchorage requirement would become 5/8" anchor bolts at 12" on center. I'd like to believe that contractors adhere to the tight requirements, but I simply have not seen such anchorage actually implemented with the many custom homes built around here.
Regarding drift, this is something I typically don't worry about for residential, but I can see merit in taller commercial buildings.
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
When you start getting very few shearwalls that are also slender and in seismic country drift becomes something to look at.
The nice thing is that if your wall is drift controlled it might not have as much shear as the capacity of the plywood/nailing and you could back off on the anchorage a bit.
RE: Residential Design - Limits of Wood Shear Walls
Most windows/doors I run into would have custom jamb extensions made by the trim carpenter in oddball wall thickensses. No special ordering required.
Does not take that long to fabricate.
Zip-R sheathing has made this commonplace.
PSA, if you see an outswing door with Zip-R, specify the sheathing to be cut back and solid wood to be installed around the opening. Otherwise, the hinge screws end up in the foam! (ask me how I know)
RE: Residential Design - Limits of Wood Shear Walls
If you have a trim carpenter good enough, then great. I've seen some hack jobs trying to accommodate it, though, at it wasn't pretty.
RE: Residential Design - Limits of Wood Shear Walls
Link -- https://www.strongtie.com/products/lateral-systems...
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
These systems are pretty cool imho, but I only see them being used on 3 story plus wood construction.
Not clear to me how exactly using these would alleviate moment frames. I guess if hold down forces are your limiting factor you can get more out of them. With a single rod you can do what two HD19's can do.
If hold downs getting over 19kips on a residence I would look to use an HSS at the shear wall boundary that has multiple anchors down into the footing. Rather than try to use Cont. Thrd. Rod.
RE: Residential Design - Limits of Wood Shear Walls
The system resists moment by forming a coupled force between the tension rod on one side and a gang stud on the other which resolves the compression load. This provides stability via overturning and shear coupled with the fact that the cables are thin... so you wouldn't need much space to implement between windows. Slick if you ask me and a good alternative to the costly steel moment frame.
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
Simpson Strong-Rod Webpage
RE: Residential Design - Limits of Wood Shear Walls
I've become quite versed in these systems as I am currently in the process of designing a 4 story light frame building that uses continuous threaded rods solely for hold downs.
RE: Residential Design - Limits of Wood Shear Walls
Thank you both for your input -- much appreciated. To your point, if one did indeed try and use these systems to resist in-plane shear through the tension side effectively "pulling" at the end as the wall rotates, you wouldn't have much stiffness at all and the drift would become astronomical. Certainly a "too-good to be true" type of application and the literature is slightly misleading.
@driftLimiter very cool you are using these systems frequently...I understand Simpson offers free design consultation to help with the calculations and details if specified on a project.
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
Are you saying conventional shear wall design doesn't work? I'm confused.
RE: Residential Design - Limits of Wood Shear Walls
The through rod system does nothing to effect in-plane shear strength. The rods are only good to resist forces along their longitudinal axis. You do not pretension these guys or anything (not that that would contribute to shear).
To get equilibrium on the shear wall you need to resist both horizontal and vertical shear. The hold down/compression stud is taking care of vertical shear, the shear panel is doing all the work for horizontal shear.
RE: Residential Design - Limits of Wood Shear Walls
No, I am discussing the effective behavior of these continuous rod systems if one tried to idealize them to take in-plane shear through pulling on the tension end rod as the wall rotates. I am in agreement that these should only be used for uplift in conjunction with conventional shear walls. Thanks for the participation.
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
Anyway to your other statement about ATS Rod System. The can be designed by the manufacturer if you specify the loading correctly. I prefer to do it myself but that is a decent option. And both MiTek and Simpson have a good set of details to drop on your sheets.
RE: Residential Design - Limits of Wood Shear Walls
Right. But that's exactly how shear walls work. The wall rotates, and the hold down and tension chord studs go into tension. Cumulative tension if there is more than one floor.
You also say they should only be used for uplift. Which is what we're talking about. They can't be used for anything else, and the literature doesn't bill them as anything else? So what is misleading?
RE: Residential Design - Limits of Wood Shear Walls
RE: Residential Design - Limits of Wood Shear Walls
This link shows a conventional shear wall almost as a contrast to the continuous tension rod system:
https://www.strongtie.com/products/lateral-systems...
The phenomenon I am referring to is not something that is commonly considered, but as the wall rotates, the angle of the rod on the tension side changes proportional to the amount of drift. In this way the rod has a vertical and horizontal resultant -- but this is hard to quantify. Also the compression studs bear on the sill plate and you see some friction there to resist the lateral loads depending on the magnitude.
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
If designed properly, the drift never reaches a level where that becomes a concern. And some theoretical friction under the compression chord will pale in comparison to the continuous shear connection to the foundation through the sill.
RE: Residential Design - Limits of Wood Shear Walls
Most framers don't give a shit what nailing pattern you specify when it comes to residential construction so I try to make things as simple as possible. I'll go down to 4" o/c nailing, but try to use that everywhere where I have shear walls so that it is uniform.
The Simpson strong walls are expensive but very convenient for the tight aspect ratio walls where you absolutely need a SW but don't have the length. The Simpson steel frames are way more expensive than just designing the steel yourself and having a fabricator make it.
RE: Residential Design - Limits of Wood Shear Walls
I've done 2" OC nailing for a few jobs, one of which was double-sheathed with FTAO straps around openings. So far in residential, I haven't had the need to use a strong wall yet given the capacity you can get out of an FTAO strapped shear wall. One of my clients always prefers a single-sheathed shear wall with tighter nailing versus a double-sheathed shear wall with greater spaced nails.
"Engineers only know about 80% of the truth, the next 10% is very difficult to achieve, and the last 10% impossible. If we are bound to be wrong, we may as well be wrong simply and conservatively."
RE: Residential Design - Limits of Wood Shear Walls
In this case, you have a floor diaphragm and rim joist capable of spreading your shear demand over more sill plate than just the portion resting below your shear wall. I often utilize this to keep the sill plate anchorage reasonable.
RE: Residential Design - Limits of Wood Shear Walls
I design exclusively for wind in residential (all the houses here are SDC A, so the seismic loads are laughably small), so for houses on a crawl I could use the rigid diaphragm assumption for wind load at the first floor and redistribute the shear to more of the foundation walls....I'll have to think on that....
RE: Residential Design - Limits of Wood Shear Walls