Considering a Generator Pad as a One Way Slab

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.


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”. 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.

Additionally, 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.

Above is a snippet.

Some generators have the complete set mounted on vibration isolaters.
Some sets have the engine and generator end mounted to the base by resilient motor mounts and the base set directly on the concrete base.
In this case I have used the weight of the engine and generator end as the generator weight and added the weight of the base to the weight of the concrete.
Is this acceptable or do I need a re-think.