Residential seismic upgrade to URM

[ELREng]

Client needs to do seismic upgrade (Seismic design category = D) to existing wood framed house. Basement story is half in the ground with wood pony walls on top of URM walls along the perimeter of the basement. I am considering adding full-height plywood shear walls installed along the inside face of the URM basement walls, but I am concerned about the stiffness compatibility between the wood shear walls and the URM. Is the stiffness compatibility issue adequately addressed by designing the new basement sheathed wood shear walls using the lower of the R values between the masonry and the wood (1.5 vs. 6.5)?

 

[Labs763]

I don't think designing for a lower R value necessarily resolves your question. Sure, you are designing for a larger force, which is good and something I agree with since you are dealing with URM, but it has nothing to do with stiffness compatibility.

I have never tried to reinf. URM w/ wood shear walls - I have always added partial reinf. and then shotcrete along the inside. That being said, you can calculate the stiffness of your plywood shearwalls and see how it works out. I would think you could justify - I have seen a lot of engineers retrofit URM with pinned base moment frames, so I know they would be fine with it.

Keep in mind you still need to support the wall out of plane, including seismic forces induced by the soil. That sounds like a difficult task to accomplish with wood studs.