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Generator foundation slab sizing

Generator foundation slab sizing

Generator foundation slab sizing

Hi all,

I need to do a preliminary sizing for a foundation slab for a residential building generator. Does anyone have any rules of thumb to apply here? My guess is that this thing is going to be on the order of 10k pounds - without getting mathematical, how would you recommend to size the slab for vibration control? What kind of reinforcing for crack control? (For right now, I have an 8" slab with 4's at 12" o.c. top and bottom, area of slab is about 3x that of the generator footprint.

RE: Generator foundation slab sizing

I always size these things for either 3 times the equipment weight or 10 times the rotating portion weight. Since it's very hard to extract the rotating portion weight, it's almost always the first one.
I've put in minimum temperature and shrinkage reinforcing, minimum bending reinforcing, #5's at 12 inches each way top and bottom and .0052 rho. My conclusion is that it doesn't make any difference. But I wouldn't use an 8 inch slab. I'd want the generator built up some above grade (six inches?) and down some (twelve inches?) meaning at least an 18 inch foundation, at least local to the generator.

RE: Generator foundation slab sizing

10k is quite a residential generator.

RE: Generator foundation slab sizing

multi-story residential, I should have said.

RE: Generator foundation slab sizing

Is it all hard mounted?
I'm guessing the OEM has some usefully detailed recommendations, probably right in the installation manual.
What engine configuration? Reciprocating equipment inertia forces run from benign to violent.

Have you looked at API and ASHRAE and old ACI standards?
Their concerns about soil bearing strength and slab width relative to CG height, especially in the plane of rotation, can quickly render mass ratio thumb rules as a little too generic.

RE: Generator foundation slab sizing

Lol at 8inch slab. I just saw one done in the office by another engineer like 3 feet thick.
Rule of thumb is 3' weight and frequency/resonance check

RE: Generator foundation slab sizing

As the others have indicated, there are a lot of rules of thumb out there. (I.e. weights, dimensions, etc.) Here are some I've used over the years for trial sizing:

-Block weight should be 5 times the weight of the machine or 10 times the weight of the moving parts of the machine.
-Width of the foundation should be (at least) 2 times the vertical distance from the bottom of the foundation to the center line of the shaft/rotor.
-Minimum thickness of the mat should be 24" or 1/5th of least foundation dimensions or 1/10th of largest foundation dimensions, whichever is greater.
-For flexural reinforcement, I never use less than 200/Fy. And even if I do not do a dynamic analysis, I may do a (crude) fatigue check for the re-bar.
-Limit dynamic soil pressures to about 10% of the allowable. (Geotech input on this would be good.)
- To limit settlement: specify ground compaction to 98% proctor. (Or whatever it takes to get to about 75% relative density.)

Keep in mind this is for trial purposes and guarantee nothing. You just have to weigh the risks and decide if a dynamic analysis is worth it.

RE: Generator foundation slab sizing

You guys/gals are in luck!.......

I put together a memo of bullet points for generator pad design.

I cut & pasted rules directly out of ACI 351 Dynamic Equipment.

I also cut & pasted guidelines out of the American Petroleum Institute's recommended practices.

See attached......

If you do not want to do a true dynamic analysis, ACI has a 'rule of thumb' method. (yes, that is the official name)

RE: Generator foundation slab sizing


I don't want to hijack this thread, but do you base your 5-times weight on the DRY weight of a generator or the WET weight?

On larger diesel generators that have ~36 hour fuel tanks, that can make a big difference.

The fuel adds mass, which serves to dampen, but I'm not sure that matters under the 'rule of thumb' design rules.

RE: Generator foundation slab sizing



I don't want to hijack this thread, but do you base your 5-times weight on the DRY weight of a generator or the WET weight?

Usually dry. (If I have that info.)

What always worries me (as much as anything) with these scenarios in a residential setting is what it does to the adjacent building. The vibration might be ok for the generator foundation itself......but a lot of materials used in home construction aren't particularly forgiving when it comes to vibration. You don't get away with the stuff you get away with in (for example) a industrial setting. (That's why I made the comment I made about risks.)

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