CMU "moment frame" 1

[milkshakelake]

I'm designing a CMU wall as a moment frame. It's an existing 8" reinforced CMU wall (with 4" brick face) that needs to be upgraded for lateral. I'm going to replace some of the horizontal "CMU beams" with concrete beams. Anyway, do I need to check diagonal shear around the window openings? And if so, anyone know how to do it in ETABS or by hand?

I do need to do other things, like adding rebar and grouting cells. I know how to do all that stuff, but I'm just not sure about diagonal shear. Cracks around windows are pretty common in brittle materials, and moreso when it's part of a lateral system.

 

[lexpatrie]

There are provisions for coupled shear walls in the masonry code, I'd start there. That's what this really is, if you ask me.

 

[KootK]

1) I would check diagonal shear cracking. I'd expect that to be the governing failure mode in most of your pier elements.

2) I believe that you'd check diagonal shear cracking in your wall bits the same as you would for any CMU shear wall / beam thing per masonry code so long as you're not high seismic (NYC).

3) How do you plan to construct viable moment frame joints between the existing CMU piers and your new concrete beams? That strikes me as a pretty big ask. Given that the joints will be more critical than the beams, I'd be concerned that you'll wind up something worse than what you started off with.

4) Is it accurate to assume that this walls has been performing as a VLFRS element for a while now? If so, what has triggered the need to reevaluate it? Are you modifying the ground floor piers as suggested by your ETABs model?

5) In general, I feel that this will be a pretty difficult thing to accomplish robustly.

 

[milkshakelake]

@lexpatrie Thanks! I'll start looking into that, hopefully ACI 530 has something on that.


@KootK Thanks for the response! Some clarification below:

1) I would check diagonal shear cracking.

Is that regular shear? Because regular shear cracks are diagonal. I was wondering if I had to check a Von-Mises type of shear.

3) How do you plan to construct viable moment frame joints between the existing CMU piers and your new concrete beams?

A. I was thinking of fully grouting the CMU, adding more vertical rebar, and epoxying new beam rebar with the correct development length.
B. Or possibly take out one face of the CMU and cast the connection in place. If I use expansive repair mortar, it'll bond nicely to the existing CMU that was chipped out. Then I'd grout afterward, with the rebar already in place.

4) Is it accurate to assume that this walls has been performing as a VLFRS element for a while now?

Yes, it is. It wasn't designed to.

If so, what has triggered the need to reevaluate it?

Long story short, this building got a violation. Building department looked into the structural drawings and found that there's no lateral system. This building is fully finished, so it's extremely hard to add new shear walls inside. It would mean ripping up tiles, cabinets, toilets, etc. The most feasible solution is doing it from outside. The owner is pushing hard to use the facade for lateral. I told him that it's extremely janky to do it this way, and might be rejected. He's willing to pay for my time to try to make it work, so that's what I'll do. If the building department rejects my proposal, he'll have to start doing the hard thing and rip up his finished building to add sane shear walls.

Are you modifying the ground floor piers as suggested by your ETABs model?

I'll reinforce them, but not expand them. The model was made before I did real measurements, and I have to adjust the dimensions a bit. But in general, no.

5) In general, I feel that this will be a pretty difficult thing to accomplish robustly.

I agree. It's super wonky. It's kind of a moonshot, and has all kinds of things that need to be done to a higher standard than usual. I don't feel too bad about it though. There are much taller buildings in this area that have far less of a lateral system (open storefront with brick on top). It doesn't make this particular situation palatable, but at least it'll be a real design.

 

[KootK]

Is that regular shear?

Yeah, regular shear.

I don't feel too bad about it though.

I wouldn't either. I've done plenty of equally aggressive things in the name of client service.

 

[KootK]

My gut feel is to see it like this out of the gate.

 

[KootK]

Just a concept to ponder. Probably impossible to do the CIP pier replacement without disrupting of the finishes behind those piers. Given the contributing seismic weight that would be taxing the low piers, I really can see those being tough to make a go of. I feel as though the upper levels could be justified.

It's a shame one is not supposed to allow an infill to lean on its neighbors...

 

[milkshakelake]

Interesting. I was just planning to just make the horizontal pieces between piers as concrete.

I wasn't considering doing it to the whole thing, which might be a better idea. Definitely works better as outriggers.

Yeah, the lower piers will have a hell of a time in shear. I think full concrete replacement might be impossible, but I might be able to put 4" concrete epoxied w/ rebar on the side of it, similar to FRP reinforcement. I did mention that I plan to use the exterior piers as outriggers, and they accepted it preliminarily. It'll be a lot of pages of calculations.

It's a shame one is not supposed to allow an infill to lean on its neighbors...

I have this fantasy where a city has entire blocks of buildings connected laterally by code. The overturning moment and shear would be shared by an entire block, allowing tall, cheap buildings to be made. If there's an undeveloped lot, you put an outrigger/link beam over it. If you add a gigantic mat foundation under the entire block, you also save on SOE. Just a fancy.