Wood shear wall question 1

[nr123]

Wondering what others thoughts are on this one. I have a 15' tall x 15'-8" wide wood sheathed wall for a commercial building that has 2 large openings in the middle (door + window above) essentially leaving only 3'-10" wide legs on either side. The height exceeds 10' to address as a conventional portal frame wall and the aspect ratio of the legs is greater than 3.5:1. The in plane loading is relatively light.

Would seem that an engineered solution is certainly possible as a portal frame (2 stacked on top of one another) or as (2) individual shear walls but that code restraints may not allow such proportions. Is there something in the code that I am unaware of that would allow this to be engineered.

If not, I fear I need to approach the client to ask them to make the legs of sufficient width so that 3.5:1 aspect ratio is met. Unfortunately this could be an awkward conversation as "another" engineer/firm provided drawings on a previous project of the same dimensions and provided what I believe was something that may not have technically worked. Their design treated the legs as individual walls but provided hold downs only at the boundary element of the entire wall (at say 1' and 14'-8" marks), and provided no detailing for strapping or nail patterns of the headers to the walls.

The project is SDC "D" in WA.

 

[KootK]

1) Rationally, I favor your stacked portal frame concept and feel that would satisfy the spirit of the code limitations if not the letter of them. Whether or not a code reviewer would agree, however, remains to be seen.

2) Minor point but I'd be inclined to consider the upper tier as two cantilevered, segmented walls rather than a portal frame owing to the limited beam depth available.

3) It sounds as though the previous engineer employed something like the perforated shear wall method.

 

[nr123]

Thanks for the reply. I like your thought on the upper tier as cantilevered segmented walls as you're right, there is limited depth above to the truss bearing.

It is unclear to me what the previous engineer employed. I tend to agree perforated due to the hold down locations except they also extended the low lintel to the boundary of walls similar to a portal. Due to the lack of detailing, it is unclear.

I am going to look into this some more but I believe my approach may be a discussion with the client to explain that we could provide an engineered design that we feel satisfies the spirit of the code but that to we could be at the mercy of what a plans reviewer will want to see or how they view the scenario. This would then let them make the decision to give it a shot or circumvent the potential revision later and just widen the legs now.

 

[phamENG]

nr123 - have you tried Force Transfer Around Openings? I only recently discovered it myself (my old firm's design software only handled segmented and perforated, and I didn't have a lot of time to explore beyond that), and haven't had a chance to use it yet. But it sounds like it's made for your sort of scenario.

FTAO Presentation

 

[KootK]

I tend to agree perforated due to the hold down locations except they also extended the low lintel to the boundary of walls similar to a portal.

If they went segmented:

1) They may have done it as a two tier thing, for the same reason that you may.

2) To call it two tiered, with any validity, they'd need to run the low beam end to end to serve the function that the top plates normally would.

Just spitballin'...

 

[jdgengineer]

You could potentially find a way to calculate this with the FTAO method. The code is fairly vague on the requirements for FTAO so it is left to the engineer's discretion on the method chosen. The code requires that it be based on a "rational analysis". The most common method is likely the Diekmann method but I believe this requires wall available above and below the opening (i.e. no doors). There are other methods (drag strut, cantilever beam, etc.) which may work for your condition but are less commonly used and I'm not sure how well they align with test results.

Personally, I'd put in two strong walls and call it a day (one strong wall calcs, but I'd prefer two for redundancy). But I'm in a market where that wouldn't be an issue.

 

[msquared48]

I agree with the Simpson Strong walls if they make one with that configuration.

Expensive, but saves a lot of construction headaches.

Mike McCann, PE, SE (WA, HI)

 

[DM2222]

To consider them "stacked" I would think something needs to brace them like a typical floor/roof diaphragm would. I am dealing with something similar now, and I think I 've talked them into a 2' deep "shelf" that I can sheath like a floor diaphragm to brace an intermediate top plate so I can meet the 3.5:1 ratio for the short wall segments on each side based on two 10' tall walls instead of 20'. (the width of this taller section is only about 13 feet- so I'm thinking 2' deep should be sufficient to span that width) Although I'm not sure if there is any guidance on what that force is. (kind of like what force is required to consider a column braced - I know there is some guidance on that, but I'm not aware of any for shearwalls)Obviously it will have to be sufficient to support the out of plane forces also. I've had to consider this for my situation, but it looks like a Force Transfer shearwall may be able to work for yours. APA has a downloadable spreadsheet for FTAO that is very helpful - although I don't think it has the ability to accommodate stacked openings like this. You could use it if you ignored the sheathing between the door and window, but I'm doubtful the forces to transfer would be reasonable simplifying in that way.