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Considering a Generator Pad as a One Way Slab

Considering a Generator Pad as a One Way Slab

Considering a Generator Pad as a One Way Slab

Hi All,

I am working on a generator pad design. I always used L>2B as my criteria for one way slab vs 2 way. The slab that I am designing has a 43"x76" footprint. This does not meet my normal criteria but I do have a continuous support on the long side on both ends. So there is flexure only in one direction. Per ACI 318-14 7.1.1 it DOES meet the criteria for one way slab as it only flexes in one direction. Would you all agree that this would work under a one way slab condition?

See my attached sketch.


RE: Considering a Generator Pad as a One Way Slab

Yes, with that configuration almost all of your moment will be one-way. There will be a bit of moment in the other direction (some twisting moment as well)......but min. steel usually knocks that out with an aspect ratio like that.

RE: Considering a Generator Pad as a One Way Slab

With only support on two sides, the slab cannot span to air. Therefore, a one-way slab supported by the two parallel support beams.

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RE: Considering a Generator Pad as a One Way Slab

Is this generator pad on the ground supported by soil? Or, is the slab in the air and just supported by the 2 beams?

Also, related issue......Are you doing a dynamic design? You can either do a true dynamic design or use ACI's "rule of thumb" method, which requires you have 5:1 ratio of concrete weight to generator weight. I did a short memo covering the 'rule of thumb method' and can post here if you need.

RE: Considering a Generator Pad as a One Way Slab

If the pad is on grade then although it will span both directions you can always design it as one way and be on the conservative side. IF it's elevated on beams then for sure one way slab must be used. Either way you can model it as one way and get a safe design.

RE: Considering a Generator Pad as a One Way Slab

JoelTXCive - I`d be very interested in the memo, if you're willing to share it. I do a lot of foundations for equipment and its never clear to me when I need to compare the mass of the fndn to the mass of the equipment. Generators seems to be the one clear cut equipment where this is advisable.

RE: Considering a Generator Pad as a One Way Slab

Here you go!

In the attached document, I literally just did screen snips directly out of the ACI and API design documents. There is no commentary from me.

I did my screen snips from ACI 351.3R-04, but ACI has just released a new version of the Dynamic Equipment Foundations guide. The new one is ACI 351.3R-18.


RE: Considering a Generator Pad as a One Way Slab

I forgot......

For a while I was using the fully fueled generator weight and multiplying by five to get a required concrete weight. On the larger generators with 48hour or 72hour tanks, the amount of concrete required can get quite high. This did not go over well with other members of my office that are more on the client side of things.

After further thought and discussion, we feel that the extra fuel weight will drop the natural frequency and provide more mass to dissipate vibrations.

We now use the 'wet weight' of the generator, which is the weight of the generator and enclosure ready to run. The 'wet weight' has oil, coolant, etc.., but no gas.

RE: Considering a Generator Pad as a One Way Slab

Can you tolerate movement? Can you use a stiffened edge slab?


RE: Considering a Generator Pad as a One Way Slab

Thank you JoelTXCive.

Yes the design calls for compacted fill in the void under the slab.
We also call for elastomeric isolators at the base of the generator to minimize vibrations on the slab.

I have been using this as a check which calls for 125% of the generator mass for concrete mass. http://www.woltergroupllc.com/generator-set-founda... My calculation currently brings me to a factor of 2 with the weight of concrete to weight of equipment. I could go with a heavier weight concrete. My overturning calc is satisfied.

This a monolithic pour without the use of concrete block and the foundation is 6" underground. I am assuming 2ksf soil bearing capacity, its essentially a sandy silty soil.

Thank you!

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