## 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

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)

## 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)

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

Subsequent to my post, I found out the concentrated load is not nearly as high as I was first lead to believe when I originally made this post.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)

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

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

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)

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