CMU Bearing Walls with Wood Shear Walls

[CBSE]

I'm doing a building review for change of occupancy and additional openings. There was a remodel completed about 25 years ago and it appears the lateral system was drastically changed. The designer used wood shear walls on the inside of the CMU. The CMU walls are only acting as bearing walls and I believe it is unreinforced CMU (built in 1972 and the original drawings don't show any vertical rebar, just bond beam bars). The remodel had new openings placed in the CMU as well...not many though, there is a lot of wall left.

Is using wood shear walls acceptable with CMU? I haven't ever seen it, I thought there was something in the code that prevented it, but I can't find the reference.

 

[KootK]

I worry about strain compatibility. Do you have to destroy the CMU bearing walls before the wood shear walls kick in laterally? Quite possibly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[txeng91]

Not sure why you would ever use wood shear walls in conjunction with CMU, CMU will be much stiffer and stronger. I see what you are hinting at KootK with the discrepancies in stiffness, but wouldn't the CMU, even if it is reinforced, have a higher shear capacity that the wood shear wall anyway? You would need to make sure you are not putting excessive uplift forces into the CMU whether it is from overturning or uplift.

 

[CBSE]

I agree with both. I'm just trying to figure out if it is acceptable or not. I will have to look at all the loads and connections and see what happens. It certainly is odd.

 

[jdgengineer]

I didn't not like it but NDS SDPWS allow it for 2 stories per 4.1.5.

 

[atrizzy]

Not a good idea to mix lateral stiffness systems, especially in seismic zones. I would separate the systems completely unless seismic is negligible.

Wood shrinkage is also an issue since your cmu walls wont shrink. You coyld mitigate this by putting your bearing surface on an amount of sill plates equalling the number of flat playes total for the wood bearing portion.

 

[MotorCity]

I am not aware of a code provision that prohibits the use of cmu and wood shear walls in the same story. I think it could work if the two were not coupled together, in other words you had independent wood walls and independent cmu walls. But if you have cmu walls with a stud wall backing, I am not convinced that the wood walls would ever see lateral loads. Can you post a detail? If the trusses are part of a diaphragm and are connected to the cmu walls, the wood walls would not resist the lateral loads.

 

[CBSE]

I will try to post a detail.

Just to clarify, I'm not trying to do it, I'm just wondering if it is justifiable because it appears that is what is there. At this point, there is a CMU bearing wall for the roof joists, and on the inside, there is a wood wall with OSB and blocking between the joists with toe-nails to the double top plate and anchor bolts into the slab. It really looks like they tried to do wood shear walls inside the CMU walls. I'm not convinced either that it would ever work for the reasons stated about the stiffness/strength differences.

 

[Nor Cal SE]

Where is your structure location? In addition to the concerns already mentioned, you could run into difficulty trying to justify the CMU walls per current code if they are largely unreinforced. In particular, ACI 530-13 contains specific requirements for minimum reinforcing in walls, varying depending on your ASCE Seismic Design Category, but note that many of the ACI 530 requirements apply to all walls, even if not considering them as part of your lateral force resisting system.

 

[CBSE]

I'm having the client test the existing CMU for rebar and grout filled cells. This will give me a much better idea of what I have to work with for the additional modifications. The site is in SDC "D"...so it's going to be a bit of a pain.