4" CMU with 4" Brick Composite Wall 1

[RFreund]

A few questions about 4" CMU and brick composite wall.

In general I'm not so comfortable with this. Probably because I would normally not use 4" CMU for anything. However in this case if it is a composite wall, I might feel OK if the wall works as unreinforced fully grouted. However even if it does work as fully grouted/unreinforced, I would still be inclined to add #3 rebar. However, from reading other threads it sounds like the rebar makes grouting difficult, so I may infact be making things worse by adding the rebar.

Thoughts on 4" block/brick composite load bearing walls?
Thoughts on adding reinforcement?

EIT

http://www.HowToEngineer.com

 

[JAE]

If the two wythes are interlocked together then conceivably they could be considered as an 8" wall, with slightly different stiffnesses and stress capacities of course.

However, brick over time will expand while concrete masonry will contract causing some possible distress and loss of interlock between the wythes, which then would imply that you loose that composite action that you need.



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[PUEngineer]

I believe there are different ways to create a composite wall. I don't think just grouting the collar is sufficient, although I could be mistaken. BIA or NCMA are probably your best bets for resources. I've done it in the distant past, but I think I added bars to the collar as it was large enough and this gets it in the middle of the wall.

 

[RFreund]

JAE -> Interesting, I don't doubt you but I'm surprised I can't find more on this being a problem (meaning brick / cmu composite with grouted collar joint). Although maybe it is not done often and that is why.

PUEEng -> Yes you are right you must be able to transfer shear forces. However if the wall works as unreinforced then I'm not sure it is required. Although I would still have joint reinforcement that extended across the collar joint.

EIT

http://www.HowToEngineer.com

 

[XR250]

I never have designed those as composite. I alway use a 6" or 8" CMU as the backup and consider the brick non-structural. The architects hate me.
On one screenwall job, we did 8" CMU with bonded thin brick. It kept it pretty svelte.

 

[RFreund]

It's good to hear that I'm not crazy for being hesitant to use this. I'm going to push for 6".

Some follow up info from MSJC's ACI 530-11

Multiwythe composite walls must be connected by either headers or filled with mortar/grout and connected by wall ties. With a grouted collar joint being the recommended method.
Limiting shear stress:
Mortar = 7psi
Collar joint = 13psi
Headers 1.3xsqrt(compressive unit strength) / net area of header
Tests indicates a vary sporadic range of allowable shear stress values. From 5-100 psi.
Type S Mortar had better shear bond characteristics than Type N.
Joint thickness / unit absorption and reinforcement had negligible effect on shear bond strength.
The show a diagram Fig CC-2.1-1, which to me is a bit misleading. They show a tension / compression stress diagram which is a normal stress diagram. The stresses you would want to know are the shear stresses, which would be VQ/I. Or am I missing something here?

Thanks again for the input!


EIT

http://www.HowToEngineer.com

 

[RFreund]

Not that I was doubting JAE, but just to reinforce his point:

NCMA Tek 16-1A states:
"While not prohibited by the code(ref. 2), wythes of composite masonry walls should not be constructed with dissimilar materials, such as clay and concrete masonry, as rigidly bonding such materials together does not permit differential movement between wythes."

So I guess if you are using concrete brick and concrete block you would be OK.

EIT

http://www.HowToEngineer.com

 

[RareBugTX]

In my view, Full grout and properly spaced wall ties take care of differential. I have seen quite a few older buildings mixing these types of masonry without any problems. Maybe its just a commonplace myth.