CMU Out of Plane Shear

[phamENG]

I'm curious - is anyone out there checking out of plane shear in masonry walls? Seems like it would make sense to. I found an Australian standard that looks at out of plane shear capacity of CMU retaining walls. But US codes are oddly silent. All of the shear equations in ACI 530 reference shear walls or beam shear and are dependent upon moment. If the moment is zero, f'm actually drops out of the equation (multiplied by zero). For a simply supported wall, moment will be zero where shear is at a max, so that doesn't feel appropriate. Then when you look at all the examples out there in the world...nobody seems to address the out-of-plane shear. For a retaining wall, moment and shear are maximum at the same place, so maybe I could use that equation there...but again, nobody seems to care about it. Perhaps it's been shown to not really matter in normal applications?

I did find this paper that discusses blast loading on masonry walls and the resulting shear failures. They put forward an out of plane dynamic shear strength of V_m=2*√f'_m*A_n. This is similar to the URM shear wall shear stress value of 1.5*√f'_m given in ACI 530 for ASD. So maybe the 1.5√f'_m would be appropriate here?

But I circle back to the first question...is anyone checking it? If not, why not?

 

[bones206]

For what it's worth, I include an out-of-plane shear check in my CMU design spreadsheet. I believe RISA includes this check as well in their software.

 

[phamENG]

Thanks, bones. Are you running ASD or LRFD in your spreadsheet? The variation in the two in the code is infuriating. I found the provision I was looking for for LRFD (9.3.5.3 of the 2013 version), but it doesn't appear to be there for ASD. RISA's documentation mentions it, but then refers to "Equation 8-25, above" but doesn't have an equation 8-25 above...

 

[WARose]

I'm curious - is anyone out there checking out of plane shear in masonry walls?

Every time.

 

[phamENG]

Good to know, WARose. It seems odd that so much of the literature is silent on it.

For the record, I have, too. But I was taught to use ASD for masonry design so I've always felt like I was winging it with all of the references pointing specifically to in plane shear.

 

[WARose]

It seems odd that so much of the literature is silent on it.

Granted a lot of my references tend to be out of date.....but I cannot think of any that don't check it. (And that includes my MDGs.)

 

[phamENG]

If you find yourself with a spare few minutes, do you mind uploading a shot of an ASD example that does it? I'm curious to see which provision they use. Though I may go ahead and buy the latest MDG...we're switching to the 2018 IBC this summer...

(I'm guessing equation 8-29 from the 2013 version is what they use.)

 

[WARose]

If you find yourself with a spare few minutes, do you mind uploading a shot of an ASD example that does it?

I can, but it's probably just simpler to say:

From the MDG (4th ed (yeah, a old one)), p.17-72, citing ACI 530-02, Sect. 2.3.5.2.1, it gives the allowable shear as:

F_v=√f'_m≤ 50 psi

Let me know if you still want a screen shot.

 

[phamENG]

Thanks, WARose. No need for the screen shot. Looks like it has evolved a bit over the years.

 

[bones206]

Here's a screenshot from RISA using TMS 402-16 (ASD):

When I changed the code to TMS 402-16 (LRFD), the OOP shear check disappeared from the results.

 

[kipfoot]

I think you'll find what you're looking for in ACI 530-13, 8.3.5. The commentary explains that the equations are basically the strength design equations with an additional safety factor and that the term M/Vd may be taken as 1.0.

 

[phamENG]

Thanks, kipfoot. That's what I'd found as well. I also found a CE presentation about out of plane masonry design - the notes indicate that you should check it...but it will never control. Which is probably why most of the stuff I'd found on it ignores it altogether.

bones - odd that it disappears since, as kipfoot mentioned, the ASD shear equation is a modified version of the strength equation.

Thanks, everyone, for affirming the OOP shear requirement and helping to clarify the the TMS requirements. Maybe it's just me, but that code always feels really cryptic...until you unravel it and then it seems so simple that it makes me feel dumb for asking the question....

 

[kipfoot]

fwiw, a screenshot from the IES masonry wall software: