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# Plain Concrete Punching Shear (Slab-on-grade)

## Plain Concrete Punching Shear (Slab-on-grade)

(OP)
I am trying to analyze an existing 6" slab-on-grade to determine whether it can support an equipment rack with a high concentrated point load.

Since the reinforcement in the slab is only minimal W.W.F., I am using the equations in ACI Chapter 22 (Structural Plain Concrete) to determine the allowable punching shear of the slab.  The equation for punching shear is Vn = 2.66*SQRT(f'c)*bo*h. (Equation 22-10)

My question is how do you determine bo.  I know for reinforced concrete, it is equal to 4*(baseplate width + d).  For plain concrete, I assume it would be 4*(baseplate width + h)?  The way I read section 22.7.6, my assumtpion would appear to be correct, but I was looking for some confirmation that I am understanding the code correctly.

Thanks in advance for any help anyone can provide.

Dave

### RE: Plain Concrete Punching Shear (Slab-on-grade)

That is my interpretation.

### RE: Plain Concrete Punching Shear (Slab-on-grade)

You may want to advise the client that this is a little 'iffy' but that he is benefiting from the reduced cost of reconstructing a slab.  Unless tilting is an issue, failure should not be catestrophic (sp?) and it may be necessary to remedy (even if unlikely just to CYA).

### RE: Plain Concrete Punching Shear (Slab-on-grade)

I also used the similar way for the problem. In my opinion it is quite conservative in this situation. Our slab-on-grade is alway support by high modulus of subgrade, so the slab will be OK if the ACI equation is satisfied.

### RE: Plain Concrete Punching Shear (Slab-on-grade)

(OP)
Thanks to everyone for their responses.

Subsequent to my post, I found out the concentrated load is not nearly as high as I was first lead to believe when I originally made this post.  Using the equation in ACI Chapter 22 with the actual load, I am at 20% capacity for punching shear.

### RE: Plain Concrete Punching Shear (Slab-on-grade)

Beware in using a large value of modulus of subgrade reaction (k) for sustained loads - if the underlying soils are compressible, the value of "k" will decrease over time.  But dik is right - I wouldnt expect a catastrophic failure, either.

What is going on the racks?  How tall will the racks be?

### RE: Plain Concrete Punching Shear (Slab-on-grade)

Focht3:
Thanks for the 'heads up' on the reduced Modulus of Subgrade Reaction... was aware of it, but should have noted it... and thanks for the correction on 'catastrophic'...

wubwub:
Just as a minor caution, punching shear may not be the only issue, but if the loads are small then the loading pattern and the flexural stress shouldn't be an issue.

### RE: Plain Concrete Punching Shear (Slab-on-grade)

dik:

You might try IESPELL - "free" software.  It's an IE plug-in that checks for spelling errors in Internet Explorer dialog boxes.  I think it's great stuff -

www.iespell.com

### RE: Plain Concrete Punching Shear (Slab-on-grade)

(OP)
Focht:

I was originally told over the phone that the rack system was 5 pallets high with each pallet weighing 2000#.  Turns out the person telling me on the phone was incorrect and the system is only two pallets high and will actually only need to support one pallet with a second pallet sitting on the floor.  Big difference.

dik:

Thanks, I did indeed check the slab for flexural and bearing as well to ensure they were designed adequately.

Thanks to both

Dave

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