Exst CMU walls - min embed into fnd for new long. reinf?

Question

Exst CMU walls - min embed into fnd for new long. reinf?

Labs763 (Structural)

(OP)

30 Oct 17 14:18

First post, apologies if this is answered elsewhere. My situation is I have an exst. structure with unreinforced CMU bearing walls (with brick veneer). This will be a "change of use", where they are converting what is currently a warehouse into a wing of a new school building, so it has to be brought up to current code. I am in a high wind/high seismic zone. I have seen a lot of posts for retrofit of existing CMU walls; one method is to sawcut the faceshell and add reinforcing (shown in FEMA 547).

I am trying to wrap my head around the requirement that wall reinforcing must be fully developed into the foundation below per ACI 18.13.2.1. The building has a 12" continuous footing below the CMU wall. If the minimum development length of a bar is 12", does this mean there is no way to properly retrofit it with this method? Does everyone else that is using this method have foundations that are 16" deep?

Is there some way to bypass this? For example, could I provide a shotcrete shear wall for lateral resistance, and then just provide the new longitudinal wall reinforcement for out of plane reinforcement only? I am not clear if I would be bound by the same constraints, or if I could then determine the max tension in the reinforcement and design it as an anchor per Chapter 17 (or old App D). Are there any conditions where these wall starter bars do not have to be fully developed - for example, what if I had a new CMU partition wall that is "not participating" per ACI 530 - do these require fully developed longitudinal reinforcement as well, regardless of As,req'd vs As,prov?

Thanks for the advice.

KootK (Structural) · Answer 1 · 2017-10-30 21:17:11

I don't know of a code clause that would explicitly get you out of that requirement but, certainly, one could make a rational argument for relaxing it. If your footings aren't deep enough to develop your wall vertical bars then it's rather unlikely that you're relying on the footings to resist overturning in your wall anyhow. I guess one could argue that you're using the footings for shear resistance and, therefore, you should develop the bars for shear friction across the joint. That seems a bit of a stretch though.

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.

Labs763 (Structural) · Answer 2 · 2017-10-30 21:49:11

Do you think ACI 18.13.2.1 is directed at shear wall/moment frame designs and not towards simply supported out of plane wall design? For example, I could provide strongbacks at some regular interval along the wall, and they would be considered pinned base - I would only need to design the baseplate conx's for shear. It seems logical I should be able to do something similar for post installed rebar in grouted cells, which I think is what you are saying - just design the top and bottom reinf. dowels for shear.

KootK (Structural) · Answer 3 · 2017-10-30 23:11:58

That's exactly what I was getting at. For out of plane loads, one argument that I've considered is this:

- The bottom of your block walls are pretty well fixed right up until they're not.

- If the dowels are not developed, demand placed on them may cause them to pull loose from the concrete en route to hing formation.

- Now you left with what is truly a "dowel" connection for shear. All slippy longitudinally but still keyed in for shear. And that too is probably okay. We don't really have provisions for that but I think Europe does.

I'm not proposing this at all as a design strategy as I think that it takes things too far. I'm just throwing it out there to play devil's advocate. Truth be told, even when you do develop your dowels, that doesn't mean that they might still not rip out a failure cone Appendix D style. It's unlikely given common spacings, but not not impossible. And we give no heed to that.

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.

kipfoot (Structural) · Answer 4 · 2017-11-02 15:14:55

I would say that you're not bound to ACI 318 because it's masonry. I'm not well versed in seismic provisions, but it seems that ACI 530 chapter 7 provides some allowance for Ordinary Plain (unreinforced) Shear Walls.

[for others looking at an older ACI 318 as I did, the provision corresponds to ACI 318-11 21.12.2.1 - Longitudinal reinforcement of columns and structural walls resisting forces induced by earthquake effects shall extend into the footing...and be fully developed for tension at the interface]

Labs763 (Structural) · Answer 5 · 2017-11-02 22:17:54

Well, that is an interesting point. I was interpreting this req'mt to apply since was provided in the foundation section of earthquake resistant structures, rather than in the wall reinforcing section (I believe there are further requirements in the wall section, i.e. develop to 1.25 req'd development length for yielding regions).

I have been looking through some existing building plans I have received over the years that other engineers have designed, and some will allow rebar to be epoxied into a thickened slab (8" embed) and they provide it as a typical detail. These are likely all partition walls, but I guess it is safe to say I can get away with it for pinned base connections; I will avoid for any lateral force resisting system type req'mts.

sandman21 (Structural) · Answer 6 · 2017-11-22 19:54:13

High seismic zone is not going to allow for ordinary shear walls. As you mention you are required to comply with the current building code. Would you allow boundary element reinforcing to not be properly developed into the foundation? No, then the retrofit program you have to come up with needs to comply with the current building code. An epoxy detail would need to comply with 1.25Fy or shows that the rebar development is met.

Labs763 (Structural) · Answer 7 · 2017-12-06 20:15:23

Thanks sandman.

Just to close this out, for those who may be interested:
I ended up creating a pricing set of drawings for comparison with a new rebuild. I strengthened the CMU walls for out of plane capacity and added shotcrete shear walls and new foundations throughout the bldg. The cost came back high enough that the owner has elected to rebuild.

I am not surprised by this, as I have run into this kind of thing several times. I am however still confused as to how other engineers in my area are able to retrofit buildings at some significant scale and have it be cost effective. I guess without knowing their exact situation (i.e. maybe no change of use), there is no way to know for sure.

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Exst CMU walls - min embed into fnd for new long. reinf?