Transition CMU Wall Thickness a intermediate point of wall height

[MJC6125]

Is it acceptable to transition CMU wall thickness (i.e. 12" to 8") at an intermediate point along the height of a wall? See image below for reference. Assuming you can get the wall to work as 8" CMU with #5 vertical at 48" o.c., which option below do you think would apply:

1. This is not acceptable because the compression face of the upper block will not bear on the compression face of the lower block.​

2. This is acceptable only if the wider CMU below is fully grouted.​

3. This could be acceptable even if the wider CMU below isn't fully grouted.​

Obviously the reinforcement would need to be continuous through the joint. This has come up because someone wants to get rid of a 2nd level in a building with CMU walls constructed this way to support 2nd level wood framing.

 

[KootK]

1) Good on you for spotting this as a potential problem.

2) You probably do not want to do this if the wall will see appreciable moments. See this thread for the reason why and report back if you have any further questions.

3) I'd only be game for this if I had confidence that:

a) the rebar was centered on the 8" all the way up and/or;

b) the rebar was sloped very gently across the transition.

 

[North2South]

You also need to check bearing at the intersection of the different sizes of block. The 8” block only bears on the ends and web of the 12” block. You will likely want to fill the blocks above and below the intersection.

 

[r13]

Looks like this is an addition job. You might not be able to insert reinforcing into a fully grouted masonry wall. How about remove a few layers of the 12" CMU, and provide a bond beam at the transition.

 

[hardbutmild]

Is this an existing structure or a change in design phase of a not yet built structure?

I'm not familiar with US codes, but do you not have a requirement for a bond beam at every n meters (in europe it's every 4 meters)? This would probably mean that you need to have a bond beam mid height anyway to satisfy that demand (since two stories w/o a beam is probably not permitted).

If it's a new structure I'd put a bond beam at the op of 12" wall and run continuous vertical reinforcement in the 8" part of the whole wall you should be good.

 

[MJC6125]

This is an existing structure. It's a somewhat older building (80s I'm thinking) in a low seismic region, so I doubt there are intermediate bond beams. There may be one at the top of the 12" CMU since that was a floor line, but it didn't actually look like it from the pictures I've seen.


KootK,

If the reinforcement was centered on the 8" wall all the way up or sloped gently at the transition, would you still be worried about the compression portion of the flexure at that joint? I.E. check bearing only on the ends and the web like Fletch10 mentioned or confirm the 12" CMU wall is solid grouted at the joint?

Also, I read through the thread that you linked and have come across some of your other concrete detailing posts on the site before. I'm curious if you have any recommendations for a concrete design textbook? I plan to get one, and I'm not sure if there's any books out there that stand above the rest. I find myself somewhat lost when I try to read through some of the real detailed discussions on concrete joint detailing, strut-and-tie, anchorage vs. development, etc.

 

[KootK]

If the reinforcement was centered on the 8" wall all the way up or sloped gently at the transition, would you still be worried about the compression portion of the flexure at that joint? I.E. check bearing only on the ends and the web like Fletch10 mentioned or confirm the 12" CMU wall is solid grouted at the joint?

I'd definitely give the compression situation some attention. If your bars are off center in the 12" block (unlikely), one interesting consequence of that is that your bending strength will be greater for outward wind pressure than inward wind pressure. That, because your flexural resistance would be working over a larger lever arm. This affect may well negate the impact of delivering your compression force to the webs of the 12" block rather than the inside face shells.

With regard to the top course(s) of the 12" block being grouted, I struggle with deciding how much I personally would want to rely upon that. There are numerous instances in masonry construction where we implicitly rely on a loaded chunk of grout to not slide through the open cores below when loaded in compression. At the same time, there's no explicit check available for that to my knowledge. So I acknowledge that there's a mechanism there but am personally reluctant to rely upon it.

I'm curious if you have any recommendations for a concrete design textbook?

Do I ever. I fear that modern textbooks will be the death of us because few to none of the popular ones seem to adequately deal with the all important subject of detailing well. For the most part, I think that they're geared towards being used as college textbooks and, thus, sales. Undergraduate coursework usually doesn't touch this stuff and graduate coursework rarely does either, at least not in any detail. Here's what I recommend:

Toward a Consistent Design of Structural Concrete - Schlaich Freebie!

Detailing of Reinforcement in Concrete Structures - Gilbert Freebie!

Reinforced Concrete Structures - Park & Paulay

I know, it sounds nuts to buy and read an antique book from 1975 written by New Zealanders. It's not your code, it's outdated by half a century, and it was written before anybody was even using the terminology "strut and tie" regularly. You'll just have to trust me on this. To my knowledge, this book has no meaningful competition with respect to setting out the fundamentals of first principle concrete design. To read it is to feel as though you are walking a path set out for you by gods, placing your tiny mortal feet inside each size 36 footprint as you go. It took me a solid nine months to read the whole thing. If Alzheimer's has a silver lining for me, it will be that perhaps I can read the book over again with fresh eyes and once again know the rapture that was those nine months.

Lastly, if there's anything that I can personally help you with in this regard, feel free to contact me directly. It's pretty much my favorite topic in all the world. A peak at my Eng-Tips profile and a little resourcefulness would facilitate contact.

 

[Compositepro]

Why be concerned about removing load from an existing structure. I would only be concerned about any loss of bracing of the wall.

 

[r13]

Compositepro,

Yeah, I wouldn't worry either, as there won't be any chemical reaction to expect. But he's adding a new thinner wall to the existing thicker wall below, which has many unknown elements. Who knows what will happen before the mystery is dissolved.

 

[MJC6125]

For what it's worth, there is no addition happening here. It is an existing two story structure where they want to remove the 2nd level to create one big open building. The 12" and 8" CMU are both existing. The loss of bracing is specifically what I'm worried about, and the fact that there is a transition from one type of CMU to another at that location seems pretty problematic.

 

[r13]

Compositpro,

I apologize for my joking around, turns out you were correct, fool is here.

 

[XR250]

There are numerous instances in masonry construction where we implicitly rely on a loaded chunk of grout to not slide through the open cores below when loaded in compression. At the same time, there's no explicit check available for that to my knowledge

No kidding! I always wonder that myself- especially when they use some wet mortar mix that shrinks instead of C476 grout.
Sounds like an opportunity to patent some contraption to make this actually work.

 

[r13]

The problem isn't bearing at the transition. Without the existing floor, the wall becomes a simple span beam, or propped cantilever, with geometry change in the middle. You may end up with redesign and rebuilding the wall if there is no way to get reinforcing steel and grout into the 12" wall.

 

[KootK]

Sounds like an opportunity to patent some contraption to make this actually work.

I feel as though it's probably a research "need". Something akin to a development length for a grout core inside a block. It always strikes me as odd when you see those typical details showing steel beam bearing on block with two or three courses grouted below and a zillion pounds coming down. Even without the quality control issues, that's seems pretty sketchy to me. Yeah, you get load spread, but how much and how fast?

Consider playing chess with me on the Social Chess app at iTunes. Same handle. Fear not, I suck.

 

[r13]

I remember CMU construction requires inspection on certain type of walls, but don't recall the specific.

The paper linked below introduces the limits in grout height.

Link

Picture of CMU bond beam.

Link