Location of control joint in CMU elevator core wall (special reinf. shear wall)

[Labs763]

I have seen several threads that agree that you should avoid designing CMU elevator core walls assuming you get coupling action from the lintel, since it is difficult to get the loads to work. This follows guidance from NEHRP Seismic Design Technical Brief No. 9, where testing showed that even when adding a control joint on both sides of an opening, coupling action in the lintel was still observed.

I am modeling an elevator core as a C shape w/ lips, but am having trouble understanding where to put a wall control joint and how to verify the lintel will be ok and protected against coupling forces. Since this is an exterior elevator core that is being located adjacent to an existing building, the walls are spanning horizontally and I need the returns/lips of the C shape to provide support for out of plane forces; this is why I do not want to add a CJ on each side of the opening.

The control joint is only being provided to break up the shear walls. Since I have so much horiz reinf., my crack control req'mts should be met without adding CJ's.

How have you handled this situation in the past? See attached for hand sketch of scenario.

Thanks.

 

[JAE]

Why are you providing a CJ in the wall?

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[Labs763]

My thought was that this is similar to a concrete wall with openings. If your intent is to design a concrete wall with an opening as a shear wall with a coupling beam, that coupling beam must meet code req'mts which very often force you into using stringent/expensive reinforcing like the "x" diagonal rebar. Contractors around here have little to no experience doing this kind of thing, so we usually avoid it. My office will usually design a cantilever lintel over the opening. The control joint is provided at the face of the opposite end of the opening so that no coupling action can occur.

Since this is masonry, I am not comfortable cantilevering anything. If I add the control joint 16" off the face of the opening, at least the wall piers on each side will have a lower stiffness compared to the rest of the wall oriented in that direction.

If you do not add any control joint in the CMU core, does that mean you are designing the beam as a coupling beam?

 

[JAE]

With one CJ on one side of the openings - you'd still have partial coupling action on the non-CJ side - sort of a cantilevered leg reaching out to the opposite "pinned" side as you suggest.

With that - you'd have to design the link/coupler under deformation compatibility conditions I would think....ignoring the stiffness effect of the partial coupler and seeing what drift occurs, then calculating the deformation of the link from that and designing based on its self-stiffness.



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[Labs763]

In my example of cantilevering beam over the opening (with concrete walls only), there is no attachment on the other side - my control joint terminology was incorrect, I should have said expansion joint. There is no reinforcement crossing the plane, thus preventing all coupling forces.

For this masonry core, I will end up having CMU piers on each side of the opening, so I guess there is no way to say that it is not a coupling beam. The control joint should isolate the wall/prevent it from acting as a full box shape and in turn reduce the forces, but not eliminate them. But your guidance still applies I think: I will design the core as a "C" shape with no coupling beams to determine deformations, and then verify the lintel can work as req'd.

Out of curiosity, are there any papers or guidance on this? I have read a decent amount about designing core walls in various shapes, but the discussions are almost always geared towards concrete. I know people do them out of CMU as well, I just wish I knew what the "typical" design looked like (always nice to know I won't get tons of complaints of over designing things by the GC and architect).

Thanks for the help.

 

[JAE]

I don't have much familiarity with masonry shafts with coupling beams....as you suggest they would be difficult to detail in the same fashion as cast concrete coupling beams with the diagonal bars, etc.

I guess I'd be inclined to go with concrete vs. masonry if the seismic was high.

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