Placement of Single Reinforcement Layer in CMU Wall

[oengineer]

I know that reinforcement is used in concrete & masonry materials due to them being weak in tension.

I have seen in some reinforced concrete text books that the main/primary layer of reinforcement to be placed on the face of the wall that the load is acting on.

(See Example image)

In the image shown above the main/primary reinforcement are the #6 @ 12"o.c. and they are shown on the side that the soil is acting, which is the forces the wall is deigned to resist.

But another wall for a basement has the reinforcement shown on the interior side of the building not adjacent to the soil, which is the load being resisted.

(See Example image below)

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Based on engineering principles, it would be because the basement wall is designed as a simply supported beam, with tension on the interior face of the wall.

Likewise, the retaining wall is designed as a cantilever beam with tension on the exterior face of the wall. Which is where the reinforcement is located.

I am trying to make a point to a colleague.

Comments are appreciated.

 

[r13]

Yes, both of your statements are correct. One been simply supported (basement wall), the other is cantilever (retaining wall). The reinforcing steel in the front face of the retaining wall is temperature and shrinkage steel for crack control.

BTW, both wall shown are concrete, why use "CMU - concrete masonry unit" in the subject title?

 

[phamENG]

I'm confused. Your title is talking about CMU, but your images are for cast in place concrete.

In CMU, it depends on the wall thickness. Reinforcement in anything less than 8" is rare. In 8" block, centered is about all you can do. 10" can be reliably offset with a good mason, but if you really need to have offset bars 12" is probably the way to go.

CIP concrete is a bit more flexible as you can put it more or less wherever you want in the form. But yes, tension reinforcement belongs in the tension zone.

Just remember, the tension will find the reinforcement. So if you have bars in the middle of the wall (as is typical for CMU), you do your effective depth calculations and compression block calculations just as you always would, but your d' is only going to be about 3.8125".

 

[oengineer]

Yes, both of your statements are correct. One been simply supported (basement wall), the other is cantilever (retaining wall). The reinforcing steel in the front face of the retaining wall is temperature and shrinkage steel for crack control.

BTW, both wall shown are concrete, why use "CMU - concrete masonry unit" in the subject title?

The reason I mentioned CMU wall design is because it is in reference to a CMU Wall I am designing to resist a distributed load on one face of the wall. I am only using one layer of reinforcing to resist the forces. I should have mention this initially, my mistake. I just used the reinforced concrete images because those were the main references I had on the subject.

I'm confused. Your title is talking about CMU, but your images are for cast in place concrete.

In CMU, it depends on the wall thickness. Reinforcement in anything less than 8" is rare. In 8" block, centered is about all you can do. 10" can be reliably offset with a good mason, but if you really need to have offset bars 12" is probably the way to go.

CIP concrete is a bit more flexible as you can put it more or less wherever you want in the form. But yes, tension reinforcement belongs in the tension zone.

Just remember, the tension will find the reinforcement. So if you have bars in the middle of the wall (as is typical for CMU), you do your effective depth calculations and compression block calculations just as you always would, but your d' is only going to be about 3.8125".

In the TEK 14-19A " Allowable Stress Design tables for Reinforced Concrete Masonry Walls", it provides a d=4.625 inches for 8" CMU Block Walls & d=6.625 inches for 10" CMU Block Walls. Wouldn't this mean that it is structurally acceptable to offset the reinforcing layer inside the blocks to build the CMU Wall?

 

[oengineer]

I would imagine that the principle holds true for CMU wall designs.

 

[r13]

Yes. Always place rebars on the tension side of walls, if required.

 

[hokie66]

You can certainly offset the reinforcement in an 8" CMU wall toward the tension face. It just requires inspection of the bar placement, including access at the bottom for tieing the verticals to the starters. That access is also required for cleanout of mortar droppings. Now if all this is left to the contractor without inspection, you won't know where the bars are.

 

[JAE]

I have seen in some reinforced concrete text books that the main/primary layer of reinforcement to be placed on the face of the wall that the load is acting on.

This is not always correct. The load direction doesn't matter really...what matters, and what others here have suggested, is that the reinforcement should be placed where there is tension in the concrete.

Depending on the nature of the wall, that could be independent of the face of loading....as in whether the wall is cantilevering or a simple span.

 

[phamENG]

Yes, you can technically offset the bars to one side - even in 8". But it's rarely practical. Quality of construction in masonry is highly variable. I'm typically a proponent of just specifying what it needs to be and letting the tradesman work his trade, but there are limits (especially in masonry). It's too easy to end up with a house foundation contractor accustomed to building 3 courses of CMU and walking off a job on a light commercial job when the market conditions are right. And it's too hard to expect somebody with a really high degree of skill to work on any residential project. That's largely regional, too - in Florida where 80% of the houses are CMU the low end masons could give the top tier guys where I am a run for their money on some projects.

Anyway, back to my original point: unless you know that a high quality mason will be building your wall, expecting perfect bar placement 1" off center is unlikely. For one thing, bar positioners are typically manufactured to center the bar, so they require field modification of oversized positioners or custom ordered pieces (if anyone knows otherwise, please straighten me out). So, you're counting on an awful lot of precision in a generally imprecise trade to achieve minimal gains (1" increase in depth to rebar).

 

[oengineer]

Yes, you can technically offset the bars to one side - even in 8". But it's rarely practical. Quality of construction in masonry is highly variable. I'm typically a proponent of just specifying what it needs to be and letting the tradesman work his trade, but there are limits (especially in masonry). It's too easy to end up with a house foundation contractor accustomed to building 3 courses of CMU and walking off a job on a light commercial job when the market conditions are right. And it's too hard to expect somebody with a really high degree of skill to work on any residential project. That's largely regional, too - in Florida where 80% of the houses are CMU the low end masons could give the top tier guys where I am a run for their money on some projects.

Anyway, back to my original point: unless you know that a high quality mason will be building your wall, expecting perfect bar placement 1" off center is unlikely. For one thing, bar positioners are typically manufactured to center the bar, so they require field modification of oversized positioners or custom ordered pieces (if anyone knows otherwise, please straighten me out). So, you're counting on an awful lot of precision in a generally imprecise trade to achieve minimal gains (1" increase in depth to rebar).

My initial concerned was that would the contractor be able to offset the rebar properly in the CMU wall, but I discussed this what a another engineer and he ensured me that it would be possible.

 

[phamENG]

Possible and practical are two very different things. But yes, it is possible specify offset bars in 8" CMU and not run afoul of any code requirements. Just be prepared for a fight at your inspections. (For your sake I hope I'm wrong and the masons in your area are awesome.)

 

[BridgeSmith]

Rather than specifying the bars to be offset, the more practical way, and the one more likely to actually be achieved, is to specify more reinforcing in order to meet the bending capacity requirements with the smaller effective depth.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[XR250]

For 8" CMU, I always design for the center and draw it on the edge. Gives me a little belt and suspenders when I find out they used mortar for grout