ACSEngineeringFL
Structural
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.
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.
