Residential Design - Limits of Wood Shear Walls 4

[80PercentTruth]

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!

"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."

 

[phamENG]

1,680plf allowable shear is usually my limit. That's 2x framing with 15/32" APA rated sheathing (not Structural 1) on both faces, 10d commons at 3" on center.

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.

 

[phamENG]

Oh, I usually try to do a preliminary lateral analysis early. If I need more than 840plf anywhere, I give the architect a heads up that double sided shear walls are coming since it impacts their finish schedules, and all the doors and windows become special order in that wall (stock frames are usually built to fit in 4.5" or 6.5" walls). Sometimes they go along with it, sometimes it starts a conversation to find a more 'traditional' load path somewhere in the house. I'll also warn them if I need strong walls or steel frames since these can drive cost up and sometimes, to keep things in budget, it's better to lose a big window early before the owner is too attached to the idea rather than 2 days before permit submission.

 

[80PercentTruth]

Absolutely, I hear you regarding the contractor following the plans. On a recent job I actually specified a 7"x7" EWP to carry a 25k axial load from a W16x57 beam over a garage. The construction team went ahead and swapped it with a 3.5"x14" LVL column and the beam actually beared to one side (huge eccentricity)! Needless to say we added the 7"x7" in front of the makeshift LVL column and to do it we cut out the slab and installed down to the footing.

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."

 

[driftLimiter]

@80%

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)

 

[phamENG]

A 3/4" diameter anchor bolt in a 2x sill plate can get you to to 1897lbs. So there are options.

 

[80PercentTruth]

@DriftLimiter

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."

 

[driftLimiter]

I hear what you're saying, I'd like to believe contractors are building what we show on the plans too. If I put bolts @ 8" o.c. then I expect to see them in the field hard stop. I see this alot in my area but we have high seismic loads so contractors are used to doing things that might feel a bit extreme.

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.

 

[XR250]

doors and windows become special order in that wall (stock frames are usually built to fit in 4.5" or 6.5" 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)

 

[phamENG]

I usually convince my clients not to use Zip-R. The stuff is awful for medium and high wind regions.

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.

 

[80PercentTruth]

Have any of you used continuous tiedown rod systems as an alternative to steel moment frames in residential design? Seems cleaner, cheaper and easier to construct in general as opposed to steel moment frames. I've considered this on a couple jobs, but decided against it due to the specialty nature of the system and my confidence in the contractor actually being able to install it properly. Simpson actually has a general notes sheet you can copy over to your project and they can size it if needed...pretty nifty

Link --

https://www.strongtie.com/products/lateral-systems/strong-rod-systems/general-information/ats-vs-urs

"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."

 

[driftLimiter]

I've never seen continuous tiedowns on a residence before. However I know they have an application for high wind uplift resistance.

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.

 

[80PercentTruth]

@driftLimiter

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."

 

[phamENG]

They do the tension, but not the shear and it won't replace a moment frame. It's just a shear wall hold down system. You have a threaded rod that runs up with a bearing plate and nut with a tension take up device at each level. That way you don't have to rely on a bunch of straps or doubled hold downs at each floor.

Simpson Strong-Rod Webpage

 

[driftLimiter]

Your shear is always going to be limited by the shear ply, nailing, and anchorage. Using a continuous tie system (with products on the market) you can get a hold down around 40 kips ASD capacity.

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.

 

[80PercentTruth]

@driftLimiter and @phamENG

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."

 

[phamENG]

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

Are you saying conventional shear wall design doesn't work? I'm confused.

 

[driftLimiter]

I am also confused.

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.

 

[80PercentTruth]

@phamENG

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."

 

[driftLimiter]

@80% Gonna be straight with you here. I'm not buying that as a possibility the statics don't make sense to me. The tension side is pulling already (tension?)

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.