Footing depth for exterior footings

[skimboard20]

I am working on a very simple structure for a generator enclosure. It consists of a couple retaining walls that retain 3 ft max, with a slab-on-grade to support a back-up generator for the client's house. The proposed location of the generator pad is very close to the property line, so we've designed a few soldier piles for the contractor to install temporary shoring. Of course, the contractor would rather not install the soldier piles, so we are currently looking at ways to reduce the excavation cut so that the excavation stays away from the property line.

Currently, the bottom of the footing for the retaining wall is 16" below the slab-on-grade. This is standard for the frost depth in our area. The contractor would like to pour the slab directly on top of the footing (t.o. footing = b.o. slab) or even at the same height as the footing (t.o. footing = t.o. slab), which would reduce their excavation by about 7 inches. At a 1:1 cut, they've now saved 7 inches of horizontal cut as well (we're really reaching here).

The only rebuttal I have is that the bottom of the footing needs to be below the frost depth, measured from the top of the grade. But I'm wondering if this is only for heated structures? Does this still apply to a couple concrete walls that just sit outside and enclose a generator? I haven't been able to find any code language that allows you to not place the footing below the frost depth. What's the worst that would happen if the footing is not below frost depth? Their generator slab cracks a bit?

This has become long-winded, but the short of it is I'm trying to figure out how to respond to this contractor. Can we specify the bottom of the footing above the frost depth?

Any insight here would be appreciated.

Thanks!

 

[Enable]

For me it really depends on the particulars. We can obviously design certain things to experience heaving (i.e. decks on grade blocks or sheds, etc).

If it can heave / jack you might have connections to the generator that cannot accommodate such movement, and that would be a bad thing. If the generator is self contained and/or the connections are not of concern, then maybe you dont care.

Even in the case of not caring so much, you'll want to declare the risks of such an approach to the owner. It is their structure and if they are okay with the prospect of heaving (which could lead to slabs cracking, could lead to voids underneath footings come melt, etc, etc) then I would be okay with it. But a disclaimer would need to be signed up front IMHO.

 

[BAretired]

I live in an area where the frost depth is six to eight feet under an unheated space. Minor retaining walls or pads such as you are describing can be placed at or near the surface. If they heave, it really doesn't matter.

BA

 

[dauwerda]

As Enable said, as long as connections to the generator (electrical and possibly fuel) can facilitate some movement there is no reason that the foundation need to extend to frost depth. Many electrical equipment foundations (pad-mount transformers, breakers in substations, a/c condensers, etc.) do not extend to frost depth.

If you are dealing with a local jurisdiction for permitting you may still have to convince them.

 

[dhengr]

Skimboard20:
Why not cast the slab with thick enough edges, and maybe some small soil shear keys, and then cast the 3’ high walls on top of the slab. This might be a two or three sided, open topped box, something akin to an shoe box with one or two sides cut out. The slab will act as the footing for the walls. Slope the slab to drain. Do you need some handrails around the high side of this? As mentioned above, the connections must be flexible, not unlike those on your A/C unit.

 

[KingPileMan]

For me I would give the contractor a choice, they can go shallow but I would reinforce the slab to make it stiff enough to withstand the seasonal differential movement and require all fittings/connections to the generator be flexible (no hard pipe). Or put it at a depth you are comfortable that it won't move enough to matter (little is okay). I know we have to work with contractors, but they have to work with us too. I have had good results explaining the trades of the optional approaches and coming to an mutually acceptable solution.

Ultimately, its often been my experience that at the end of the day if something in your design fails the contractors will point the finger at you faster than you would believe. I've always told my engineers contractors won't lose their license doing your way, but you could lose yours doing it their way.

Just something to think about. With smaller slabs/mats that are at risk of differential movement I throw in some twisted steel Micro rebar, like helix steel, into the mix design on top of my conventional reinforcement. Its great for shrinkage versus WWF, and unlike normal steel the concrete doesn't have to crack for it to engage. Also works much better than fiber because it doesn't clump and the filament's increase the tensile strain capacity.

Hope this helps.