When is a masonry shear wall not a shear wall? 1

[tlen]

Greetings-
I am working on a 1950's precast-frame concrete building. Many of the exterior walls have partially grouted 8" cmu infill. The typical wall detail shows 2 #4 vertical bars at 32" oc and bond beams 48" oc with 2 #4s. There is also a single #4 embed 6" into the slab at each vertical rebar location. Considering the age of the building, I might consider these walls shearwalls as did a previous engineer who looked at retrofitting this building.

Here's the problem, I see absolutely no connection between these walls and the frames. At some locations there are windows between the top of the walls and the bottom of the frames for the full length of the wall. The details show the reinforcing stopping immediately below the top row of cmu. So, even at those locations where the walls are in contact with the frame, there is no connection. I recognize the walls will add stiffness to the frames and when the frames displace the walls could become engaged but I without any sort of direct load transfer mechanism, I am hesitant to consider these shear walls.

Any thoughts? Any one familiar with how they did things in the 50's?

Thanks for any thing!

 

[JAE]

Keep the concept of stiffness and strength separate.

When thinking of strength, the side column on the windward side will compress the vertical side of the masonry while the far column will pull away from the wall since there's no positive connection.

So you'd have a sort of concentrated load at the top of the wall, from the side column, or you could go a little less conservative and assume a triangular distribution of stress on the wall, with 0 at the base and a maximum at the top.

This would then be a classic case of a shearwall with a horiz. concentrated load at the top...and I guess I'm assuming you have a wall that IS fixed at its base. If not, then I think it gets complicated.

This should work for strength - if the wall can take the load.

For stiffness...the wall creates a rigidity against the column, but can slip slightly and allow more frame sidesway than if it was doweled into the frame. But the wall is so stiff that I have a hard time seeing how the slight slippage would be resolved into a higher delta.

In the 50's!!! I "grew up" as a young engineer working for engineers who started their careers in the 1940's and I know they didn't get real caught up in lateral design much.

 

[tlen]

Thanks JAE-
I really appreciate your thoughtful response.

Was it pretty common to not connect the cmu walls to the frames in the 50's?

I should have been more specific as I wish wind was my concern. I am in a seismic zone 4. I also don't think I can assume fixity at the base as all I have is 1 #4 at 32 inches on center embedded 6 " into either a 10" or 12" slab, depending on wall location. At least the #4 does have a 6" hook on it.

I am figuring that my best bet would be to determine the capacity of the walls for their current reinforcing and, if I need them, create a load path from the roof diaphragm and then also improve the shear transfer at the base. Otherwise, I won't include their capacity in the lateral design but include their effects on the frame stiffness. I think this sounds reasonable but maybe too conservative?

tlen