Plain Concrete Punching Shear (Slab-on-grade)

[wubwub]

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

 

[redhead]

That is my interpretation.

 

[dik]

You may want to check with the Portland Cement Association. They have a computer program for the design of airport pavements and point loadings on slabs. The loading pattern is required because one leg load may add or diminish the slab flexural stresses. The program relies on the loaded area, the loading (psi) at the base of the leg (wheel), the concrete Ec value and the modulus of subgrade reaction. It does some calcs and 'spits' out the flexural tensile stress. Depending on the strength of the plain concrete, repetitions of load, etc. you can determine a rough safety factor and decide if this is adequate.

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).

 

[npthao121]

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.

 

[wubwub]

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Subsequent to my post, I found out the concentrated load is not nearly as high as I was first lead to believe when I originally made this post.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

 

[Focht3]

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 [blue]dik[/blue] is right - I wouldnt expect a catastrophic failure, either.

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

 

[dik]

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.

 

[Focht3]

[blue]dik[/blue]:

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 -

http://www.iespell.com

 

[wubwub]

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