Generator Foundation "standard" detail 2

[LUGuy]

I was just given one of those "standard details" for a generator foundation to be blessed. Nobody knows where it came from, but it has been used on this MEP's drawings since the dawn of time.

To support a 2000 lb generator, the detail has an 18" slab with rebar at 12" on center top and bottom in the long direction and 6" on center top and bottom in the short dimension. The edges of the slab extend to 4' deep with hairpin-type bars. The entire structure is monolithic.

The rebar size is not specified and the slab is not dimensioned, except for the depths. This is completely generic, so there is no site information to consider.

- The slab is exposed to freezing temperatures - wouldn't frost heave the slab anyway because it is only 18" below grade? Shouldn't there be some sort of compressible insulating material under the slab portion to isolate it from frost heave.

- Is it necessary to drop the sides to 4'? Couldn't a lightly loaded slab like this be permitted to "float", provided all of the connections were flexible?

Miscmetals

 

[Focht3]

As you noted, the design is very generic. You have raised good questions - and the answer to each is generally, "it depends." It depends on the subsurface conditions, the seasonal weather patterns, and the client's tolerance for both movement and risk. While we can offer suggestion and opinions, all of these factors must be evaluated for each client, generator and site.

Think about it: would a "generic" answer in [green]Eng-Tips[/green] really be any better than the "generic" design you are reviewing?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[LUGuy]

With all due respect, I am looking for an understanding of why this detail is the way it is, not an answer as to whether it is acceptable for any particular (or generic) loading/soil/environmental condition. That step will come later. I just see too many questionable things in this "electrician's standard", and what's worse than an electrician doing civil work that was designed (cut-and-pasted) by an electrical drafter?

In any case, I am reluctant to accept any so-called standard, whether it is copied from an electrician or from the civil guy sitting in the next office. I have worked with too many contractors and engineers doing shoddy work with the justification that it hasn't failed yet, so it must be good.

And in this case, I don't like and can't understand what I see, even if my gut tells me it appears strong enough to launch a small rocket from. If I can gain some understanding, albeit generic, then I can better figure what step to take next.

TIA,
Jim

 

[Focht3]

Fair enough.

I will leave the frost heave issue to others -

The "design" appears to create a stiff, but shallow, foundation for a generator. The layout appears to take advantage of geometric damping - wide, shallow footings get much higher damping than smaller, deeper ones. The shallow vs deep issue has always seemed counterintuitive to me; but it is based on measured performance.

The 4 ft perimeter beams help to insure that the foundation remains in contact with the ground at the perimeter. Could it be reduced? Probably. Eliminated? No - over time, a gap between the soil and concrete can form at the perimeter, reducing the effective size of the foundation - decreasing the geometric damping. And that's bad news -

The foundation needs to be much stiffer than would be needed for a static load. The bar spacing sounds right to accomplish that; but of course it will need to be sized (i.e. designed)!

I hope this helps.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Hookem]

Review API 686, Chapter 4. If reciprocating, the weight o of the concrete should be at least 5 times the weight of the rotating equipment; 3 times if turbine. They have other suggestions as well.