Cut CMU opening at existing control joints?

[ACSEngineeringFL]

Good afternoon,
I'm not quite sure why I haven't run up against this question before, but I guess I have been just lucky so far. More likely, most builders around here don't use as many control joints as they ought to. I've been trying to track down any answers to this question, but have thus far not been successful.

I am involved in a renovation project for an existing big-box store. I think it was originally a Walmart or similar structure. The architect involved in the project is subdividing this into 4-6 tenant spaces. Each of these tenant spaces will get their own new storefront glass entry that will be cut out of what is currently a solid CMU wall.
For what it's worth, the walls are 12' CMU, approximately 22' high, with a bar joist roof that bears over some proposed openings and not over others. The building is roughly 10 years old. We expect to find filled cells and tie beams.

It turns out that there are control joints in the CMU where several openings are supposed to go.

My question is: does this matter?

It is my understanding that the control joints in a CMU wall are there predominantly for the same reason as control joints in a slab: as the block and mortar cure, they shrink somewhat, so the control joint prevents cracking. Therefore, most of the movement that will ever occur at these joints has already occurred.

I had come up with a cast-in-place concrete header design for these new openings and also an alternative pre-cast header design. For both methods, I have designed the header to be the sole source of support for the wall. Though there is > 10' of CMU above the new header, I have assumed that it does not contribute any strength or stiffness to the span over the opening. Weight, yes, strength, no.

Now if this is constructed, there will be a control joint that comes down and dead-ends into the top of the new concrete header. This will probably create a crack from the end of the control joint down into the concrete thanks to thermal effects and so on, but again I ask, does this matter? Concrete cracks. That's why we put steel in it. There's quite a lot of steel in these headers, of both the longitudinal and stirrup variety, so it seems to me like every possible way that this could fail is fully constrained.

What are your thoughts on this? I have to admit that it does make me nervous to do this, but I can't really see how it could be a problem. Am I missing something fundamental here? The architect doesn't want to have to redesign the entire renovation to avoid the control joints if he can help it.

Has anyone seen this issue addressed in literature someplace? Where?

Thanks for your thoughts.

 

[jayrod12]

I feel if you've designed the headers to take the entire load above and successfully transferred the load from the header to the foundation then everything should be alright.

I don't see how having a control joint in the masonry above the header should cause any headache to your design. Maybe it will cause some issues for the contractor to temporarily shore it while installing your header but that comes with the territory.

 

[concretemasonry]

You certainly have plenty of "beef" in the headers since you took the entire weight and loading over your opening and neglected the arch action of the masonry over the new opening. That is also the reason for the amount of steel. The control joints will certainly form a structural break in the wall so they usually act independently. There will probably extra cores with reinforcement grouted on each side of the existing control joints.

Is the structure load bearing masonry or is it just a lightly loaded exterior wall? Some of these buildings have steel columns and bar steel roofing, while others are load bearing masonry to speed up the total construction schedule. Usually, most are running bond masonry without excessive arbitrary grouting that is not needed.

If the contractor is good, he will probably recognize the available arch action for the temporary support that you may have to approve in some way.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

ACSengineeringFL.....I'm curious. Where is the project? Not generally, but actually. The reason I ask is that if this is located in the Volusia County, FL or Seminole County, FL area I might be able to steer you in the direction of original plans if my guess is right about the developer.

If not, your approach is conservative and reasonable. I'm familiar with those structures in particular...in fact, I know of one of them that was built under the original threshold inspection law in Florida in the mid-80's.

The crack that will develop in your header as a reflection of the restraint on either side of the control joint should not be an issue. It will likely happen, but is structurally irrelevant. Even though you are discounting the effect, you will get arching as Dick noted.

 

[ACSEngineeringFL]

I don't have the address in front of me, but I think it is 1988 SR 44, New Smyrna Beach, FL. It's on the south side of the road. Original plans are always a help to have, though I've found plenty of situations where field inspection showed something different than what the plans indicated. At least in this case, there are not too many hidden parts of the structure.

In answer to the question above, yes, the roof bears on the exterior walls. Some of these openings then will carry bar joists and a large tributary area of the roof, while others will carry only a narrow tributary strip of roof.

Another reason I opted to shy away from the arching action of the CMU was that a few of those bar joists land inside the triangular arching area. There should be a tie beam up there, but I did find a number of voids in other areas that were supposed to be filled. I therefore thought it prudent not to assume much more about the existing structure than I could prove.

Thanks for your insights on this.

 

[Ron]

I believe it is one of the ones I'm thinking of. If so, the developer was Sofran Development out of Jacksonville (they are still around). They have an office in Orlando. They developed numerous strip centers in the '80's in that area, most of which had "Regional" attached to their name such as "West Volusia Regional Shopping Center" located in South Deland, or Apopka Regional, etc. I believe it was built by Keene Construction Company. Their office is in Maitland. I doubt the contractor has anything since this one was built in about 1985 or 1986. It was required by the Building Department of NSB to be a threshold project (their prerogative), though I don't believe any of the others in the area were so required. I believe it was a WalMart store. If I remember correctly, the exterior is split-face masonry.

Good luck and post back with what you find out.

 

[ACSEngineeringFL]

Yes to split-face block, yes to Walmart. The original building is that old, but the section I am working on was a much later addition.
Thanks to Google Street View, I see a sign out front saying both "NSB Regional" and "Walmart" so it looks like you are right on the money.