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]

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

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?

 

[driftLimiter]

Simpson portal frame kits, and strong walls are the wood alternative to using a moment frame. These guys are tested with extreme aspect ratios and designed to fit within the width of typical walls.

 

[80PercentTruth]

@phamENG

This link shows a conventional shear wall almost as a contrast to the continuous tension rod system:

https://www.strongtie.com/products/lateral-systems/strong-rod-systems/ats

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."

 

[phamENG]

No, it shows a continuous rod in a conventional shear wall as a contrast to a hold down in a conventional shear wall.

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.

 

[jerseyshore]

I try to avoid 3" o/c nails at all costs, but have done it for a project or two. Gets messy with the sheathing on both sides and the code requirements.

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.

 

[80PercentTruth]

@jerseyshore

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."

 

[Craig_H]

In my neck of the woods, the shear walls bear on the main floor, which is then in turn fastened to the foundation. The rim joist transfers the shear from the wall to the sill plate, which is then fastened to the concrete (standard platform construction).

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.

 

[phamENG]

Craig - interesting approach. Do you require a special lap detail or horizontal strapping of the rim joist to ensure continuity, or just rely on splices in the sill and rim not being aligned?

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....

 

[Craig_H]

Do you require a special lap detail or horizontal strapping of the rim joist to ensure continuity, or just rely on splices in the sill and rim not being aligned?

Pham, that depends on the demand and how much I am trying to butter out the base shear. I have specified straps for the rim joist on one project where the layout was about as gymnastic as I think possible, and I didn't want anything left to chance. There were multiple interior shear walls point loading the main floor diaphragm, and all sorts of other mess. Most times, though, I just indicate a separation and fastening between splices of the rim and the sill.