equipment pad design

[kngpenn]

I am an EIT with a question about concrete pad design for heavy equipment such as generators, HVAC units, etc.

What is an accepted method for the design approach? I know that a turned down edge footing is typical, but what drives the dimensions of it?

 

[dougantholz]

The pad should be bigger than the equipment and thick enough to anchor into. Beyond that, some areas may require looking at the equipment in an overturning load case (very high winds, very high seismic). Other than that, you will need to ask another engineer in your office about frost depth. It varies by the PE.
One other thing - its a footing - so look at it that way and you will see the above is all you really need to look at unless you are talking about fuel tanks or something tall and slender.

 

[kngpenn]

Thanks,

I figured that the footing should be the main concern.

The equipment does not sit directly on the pad. It bears on the pad via two skids or channels so to speak. Since this is not a distributed load case, it introduces bending at the center of the pad. Thus putting the top of the pad in tension and the bottom in compression. Therefore in this situation, I felt that another design approach should be considered.

 

[dougantholz]

If you're worried about concrete in bending, then I suggest visiting the plain concrete section of your ACI. Look at modulus of rupture and design a pad (say 8" thick) that can bend without the need for rebar. Put rebar in the pad, but center it. Corrosion is probably a bigger problem/headache than an extra inch or two of concrete.