CMU Shearwall ACI 530

[DLong]

I have a reinforced CMU shearwall. I am using ACI 530-05. In calculation of shear stress in masonry fomula (2-19) found in paragraph 2.3.5.2.1 the formula reads
f(sub)v= V/bd
What is the "b" dimension I use, is it two times the face shell or is it related to the width of the masonry or possibly the number of reinforced cells? I have run across a couple of conflicting reference calculations. Thanks for any help.

 

[JAE]

b is the "effective" thickness of the masonry. For totally ungrouted block, yes, you'd use 2 times the face shell thickness.

For grouted masonry, there are tables (see NCMA TEK manual) that offer effective thicknesses of masonry with grouted cells at various spacings.

When a particular cell is grouted, you can use the grout area and the webs on either side of the grouted cell as part of the shear area. Stand-alone webs (i.e. webs with air on either side) shouldn't be included.

See the attached copy of a text-book sheet that offers some values.

 

[UcfSE]

As a reference, the table provided by JAE is also included in Amrhein's masonry text, page 300.

I agree with JAE about which width to use. Concerning the NCMA tek, properties are listed in TEK 14-1B which has been recently updated. While in the copy I have the equivalent thickness is not noted, you could find the equivalent thickness by dividing the unit area by the unit length (12 inches). This will give you slightly lower equivalent thickness values than referenced above. The other option would be to not worry about finding an equivalent thickness since the TEK provides you with both the net and average area. You could use that value directy. Note typically we use net values for strength and average for stiffness.

You may also have reason to want to use the face shell only. That could be called a matter of judgment. My MDG-04 examples are worked using only the face shell. I think either way would be valid.

 

[DLong]

Thanks for the input. What I was running into was that if only face shell dimension is used in the shear capacity calculation, this becomes the governing factor in the design of a masonry shearwall. The amount of reinforcements becomes a non-factor because the masonry shear comes into play with only minimal shear reinforcement provided. Therefore it seems that the grouted cells have to come into play in the determination of the "b" dimension. Any other thoughts would be greatly appreciated.

 

[UcfSE]

You can always solid grout the wall if you don't have enough area. The rebar spacing doesn't necessarily have to change.

 

[faromic]

Which publication is MDG-04?

 

[DLong]

Faromic,
I was assuming that UcfSE was refering to the "Masonry Designer's Guide" (4th edition) as published by The Masonry Society. They now have a 5th edition available, released this year.