Turned down slab

[cad0211]

We're working on putting new equipment on an existing foundation. I have attached a picture of the existing foundation.

When evaluating bearing pressure on this type of foundation, what area can you use for the bearing area? We assumed it's just the turned down area (in this case the 1'-6" length) but wasn't sure if the diagonal area (2'-6" in length) from the thickened edge to the center 12" slab could be counted as well.

Thanks for any help.

 

[BridgeSmith]

I would think you could use a width twice the distance from the center of the load to the edge of the slab (equal distribution about the center of the load). If the backfill under the slab and the diagonal taper is truly "well-compacted", then the extra 5' of fill and the angle should make little difference. I'm no geotech, so I could be completely off base, too.

 

[Ron]

Since the soil within the truncated pyramid of the foundation is so confined and well compacted, for bearing purposes it will act as a block as if it were all concrete....so the bearing area is the same as the top area of the foundation.


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[KutEng]

Ron, if that was the case then what is the purpose of the thickened edge here?

 

[Mad Mike]

I agree with Ron- if you were to only consider bearing on the 1'-6" base tips, it would give you a very high bearing pressure with theoretically high settlement; however, any settlement there would rapidly engage the "well-compacted backfill" wedge, which because it is confined, acts as a rigid block.

Bearing area is the top area of the foundation.

All the best,
Mike

 

[patswfc]

In my opinion it would be difficult to compact that material. Unless the foundation is very long I dont understand why anyone would choose this shape over a rectangular cross section. Anybody have an explanation for this?

 

[cad0211]

The drawings are about 20 years old so we don't have an explanation but we thought maybe it was a frost depth issue? Or because of the soft clay soil. No way we can be sure though.

 

[jayrod12]

It possibly is to bypass the frost depth minimum requirements, while at the same time reducing the amount of concrete.

I find nowadays that it is significantly easier to just pour a majorly thick slab, within reason. I don't think however that pouring a 6ft thick slab is appropriate. That would require a lot of additional reinforcing and concrete that is likely not required.

 

[cvg]

the drawing does not show excavation down to the foundation level and then placement and compaction back up to the top, so I don't think you can count on the "well-compacted backfill" to be as firm as concrete.

 

[charliealphabravo]

I think the sloped side of the footing indicates that the concrete is cast against the subgrade without a form.

 

[MotorCity]

The projected bearing area on a horizontal plane is the same with or without the thickened edge. I see equipment foundations like this quite often (sometimes with vertical faces instead of sloped faces at the thickened edge). It is done for frost protection, although in this case it seems that the thickened edge is also retaining soil.

 

[darthsoilsguy2]

Not that it matters... but i'm curious what the "Insulated Rebar Detail" is that this details apparently sends you to go see. Do you mind sharing that? I've never put those 2 words together.

There is a lot of means/methods questions to the history here that make me wonder what is under the slab. This thing looks weird to build and i would think the contractor approached this differently than the detail. if i had to actually build this, i would have looked at the cost difference (+materials; -labor) of building a full vertical donut with a 12" slab on top ... and i would have tried to negotiate the donut width down to 3' or at most 3'-6" with the engineer instead of 4' total.

if it is critical to your new loading to count more than the 1'-6", i think there is some due diligence to get a snapshot of that core construction. a 6" core drill could cut a hole that a hand auger and cone penetrometer could enter and get some info below the slab.

 

[cad0211]

Here is the rebar detail. It is necessary for the equipment sitting on the foundation. It's a reactor that requires magnetic clearance so no closed loops for metallic parts.

 

[jayrod12]

Well that's interesting, not sure I've ever come across that before, but I'll keep it in my back pocket should the need ever arise.

 

[Guest262653]

How old is the foundation and what kind of equipment was sitting on top of it? I tend to agree that the full width of the foundation should be counted for the evaluation of bearing pressure, especially if it's an old foundation that has already seen a lot of load.

 

[darthsoilsguy2]

oh... 'insulated' in the terms of stray electrical currents... interesting. thanks.