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CMU control joints and shearwalls

CMU control joints and shearwalls

CMU control joints and shearwalls

I am curious to get a few opinions on an issue I have run into:
I like to provide control joints in CMU per the NCMA recommendations, and am typically able to do so with most buildings. I have run into a residential application with lots of openings. I am in a Seismic Category "C" with high winds as well. The shearwalls have been a challenge, and there just flat isn't a suitable place for a control joint in a few directions. I had to coax the architect into some reduced openings in several locations to make the lateral design work. The lintel reinforcement is required to extend 24" past the openings, so this means a control joint adjacent to an opening would reduce the shearwall length by 24" (on each end in many cases), and I just can't give up the shearwall.

What I have decided to do, is to heavily reinforce the wall horizontally in accordance with the NCMA TEK 10-3 which prescribes a minimum horizontal reinforcement that allows for control joints to be omitted entirely. Unfortunately, I don't see many other options, but I just hate to vary from what I am used to. Structurally, I am okay with this, but I do fear that there may be objectionable cracking in the finish stucco coat that will cause fingers to be pointed at me. At the end of the day, I don't know if we will have a major earthquake, but I KNOW the wall will shrink. So a couple questions:

1. Has anyone had success omitting CJ's by reinforcing heavily? (requires area of horizontal steel = .002*An)
2. I have seen many people that still put a CJ, but run all the horizontal reinforcing through. This seems like it would not be effective as a CJ, but does anyone follow this practice with success?
3. I also hear of folks not reinforcing per TEK 10-3, and ignoring the need for control joints (with purported success). Anbody in this category?
4. At the end of the day, do you say: small shrinkage cracks aren't structural and structural concerns trump aesthetic shrinkage cracks and roll on?
5. Anybody have a magic bullet, awesome structural note, or waiver that will let the shearwalls work, eliminate all cracking and help me sleep well?
6. What would you do?

Thanks for anyone's time and input!

RE: CMU control joints and shearwalls

At a previous company, I came up with a detail for a client that really wanted the control joints to line up with the edge of the windows. The client really didn't like seeing the jog in the control joint around the steel lintel. We came up with a detail that provided a slip connection for the lintel beam, so that the joint could go straight up from the edge of the opening. Fair warning: the masons hated it, and I got yelled at a lot about this detail. But the client and the architects were happy.

RE: CMU control joints and shearwalls

Keep in mind that with birneys detail, you are going to really struggle to transfer out of plane wall forces back into the wall piers with a single bolt to the bottom of the lintel if you are in a high wind/seismic region.

I have used the .002An a few times. That is not to say I removed all control joints, but I did space them out a lot farther than the usual 25ft. I actually walked by the building the other day and could not find any evidence of cracking (it was constructed several years ago), but that is not to say there will never be any. I would say to use it if you need to, TEK should not have published guidance on it if it was not allowed. ACI has a reinf. requirements for crack control in containment walls as well when control joints are spaced farther, so that provides a little comfort on the design philosophy.

RE: CMU control joints and shearwalls

Thanks for the inputs! Yes that lintel detail looks to introduce some complication/difficulty as well. Most of the openings are small with conventional masonry lintels. I suppose the guidance in the TEK for crack control is clear and uncomplicated, so I have confidence in that respect, but just not accustomed to not providing joints.

Here is another question:
I typically specify CJ locations on the structural plan because it is typically obvious enough to accomplish this way, but with so many aesthetic considerations, it is difficult in this case. (I am talking about the joints I AM able to provide) How do you typically specify joint locations? Has anyone ever taken the route of requiring the contractor and architect to submit a proposed CJ joint layout for approval, and just provide structural input as needed?

Note to self.....I didn't charge enough for this project.

Thanks again.

RE: CMU control joints and shearwalls

We never (rarely) let the contractor select joint locations. Typically its a joint effort between the architect and engineer. The architect wants them based on aesthetics, the engineer wants them based on function. Its a dance back and forth, but we let the architect take a swing at it first by providing him/her the max spacing

RE: CMU control joints and shearwalls

The NCMA TEK and their recommendations for extending the location of control joints in horizontally reinforced walls (joint reinforcement or bond beams) is a totally acceptable way of designing a masonry wall (or they wouldn't have published it). Our nature as engineers is to be conservative unless we can see it for ourselves. There are plenty of buildings that have used the "new" engineered method for locating control joints and are performing fine.

The archaic ways of putting control joints at the ends of openings, while still acceptable doesn't mesh very well with the type of reinforcing we are seeing on projects today which typically have vertical rebars at each side of the opening and a masonry lintel above. When doing reinforcing this way, it really does make sense to move the control joint away from the window and into the masonry between the openings. Extending the location of control joints may also help with the design of shear walls. Remember, like reinforced concrete design, we are not totally eliminating cracking, but controlling it and keeping the cracks small enough that they don't matter.

Detailing the opening like in birneys can make things so complicated they can't get built (hence the masons complaining). While not wrong, there may be easier ways to skin the cat (sorry cats). Using a reinforced masonry lintel may avoid the awkward details at the ends of the lintel and actually bring down the cost of the building since you are allowing a single trade to build the wall. Here is a good read on the use of steel lintels vs masonry lintels in a masonry wall: http://www.forsei.com/wp-content/uploads/2016/12/M....

Finally, the ACI 530/ASCE 5/TMS 402 and its companion ACI 530.1/ASCE 6/TMS 602 (soon to be just TMS 402/602) requires that designers "indicate the type and location of movement joints on the project documents" (Mandatory Reqts Checklist 3.3.D.6 and Section 3.3.D.6). Therefore, it is a code requirement to "design and detail" any movement joints including brick expansion joints for a project. While common practice is to let the mason locate the control joints on a project, would you trust a guy (or gal) who hasn't taken any engineering or materials courses to design your building? I know, I know, we don't get paid enough to design every little thing on a project, but refer back to DM2222's comment...

RE: CMU control joints and shearwalls

My two cents...I recently did the Slab on grade detail of 0.5% rebar with NO control joints. Lot of steel...and the slab cracked up a lot (client didn't want the joints). Now, it's safe and likely won't be much of an issue because the cracks are stitched together really well, but point is that it cracked up because there were not joints and it shrank. This is masonry, and I get that, but don't necessarily count on the extra rebar to prevent cracking.

As for a suitable solution for you and to shed liability...design something that numbers work for and WARN the client and the architect, IN WRITING, about your concerns for cracks. Say it won't be a life safety issue but could be a serviceability issue. Ask them how they would like you to proceed. Save their email response for future questions.

I am of the opinion that the purpose of the building code is life safety. Anything the code requires that isn't related to life safety are good guidelines but ultimately boil down to preference (don't even get me started on the energy code). Similarly, the NCMA control joint recommendations aren't even codified (at least not in my areas of practice), but are good guidelines. If you present your client with the best practices and they say to ignore it and move on, you don't need to own that liability. They chose to ignore it, and it isn't life safety.

RE: CMU control joints and shearwalls

Thank you all for the input. I am thankful for this sounding board. My plan is to put joints in walls where shearwalls allow for them, and trust the TEK guidance for the remainder of walls that can't tolerate them.

In my discussion with some other engineers, it seems many do not account for Control joints in the lateral analysis. And even in many published examples, they seem to be ignored. (they use the entire building length) I have always calculated a 50 foot long wall with a control joint in the middle as two individual shearwalls coupled with a bond beam collector at top of wall as opposed to one 50 foot long shearwall. Of course this doesn't often make or break the design, but when I am counting on a 5 foot shearwall- I dont want to split it in half. How do you masonry experts handle/analyze this?

RE: CMU control joints and shearwalls

Use joint reinforcing at 16" on center and horizontals at at least 48" on center. Explain to the client that the number of openings is prohibitive to reducing the risk of cracking. Tell them you are designing to the latest best practices in the industry, but the risk of cracking, albeit small cracks, is still higher due to the number of openings and lack of shear walls.

How are you analyzing the wall segments? I would suggest FEA models with some hand verifications in order to better capture the contribution of the coupling affect of the masonry lintel.

To stiffen up your short masonry lengths, you can fully grout the segments or specify a higher masonry strength.

Another note about reinforcing for controlling cracks versus control joints. I believe control joints also protect the contractor from poor construction methods and improperly compacted subgrades. If it helps the contractor, it'll help the engineer, too. If a crack opens in the foundation due to poor subgrade prep, it is less likely to create a crack in the supported masonry when the masonry is divided into smaller panels before the panels can move independently. If only reinforcing is provided to control cracks, the wall seems to be more susceptible to projecting cracks from the foundation, even if held tight.

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