Isolated PAD footing, Weight of soil

[palk7 EIT]

Hi,

For an isolated pad footing, when it needs to resist horizontal force from the column, is it normal industry practice to take into the account the weight of soil above the footing (lets say 3 feet of soil, if the footing thickness is 1 feet and the bottom of footing is 4 feet below grade). This weight of soil helps in the resistance when we add it, however my Seniors suggestion was to ignore this soil weight that helps in resistance. Any thoughts from the people who have done so many foundations and might have ran into this? also is there a code clause or something that could be referenced to whether include it or not?

Thank you!!

 

[HDStructural]

I have often seen people ignore the top 6" of soil to be conservative, so they would consider 30" of soil in your case. There would also potentially be a 0.6 load factor on the weight of the soil which adds to the factor of safety.

 

[HTURKAK]

Yes, you may take into the account the weight of soil above the footing to resist OT, Uplift and sliding . You may also introduce shear key to increase sliding resistance . I preferred the use of tie rods in past.
I will suggest you to look ,

-CRSI Design Handbook,
- METAL BUILDING SYSTEMS by A.NEWMAN ( You can find worked examples in this book)
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[JoshPlumSE]

I think it is normal to consider this soil.... Some caveats:

1) Think about the site and what it's being used for. If, for example, this is a footing that the underneath a level floor slab, paved surface that you know is going to be maintained for the life of the structure / footing, then I would consider ALL of it.

2) If you are unsure what they will use for the top surface of the facility. It might be soil, concrete, asphalt, gravel, sand, wood chips, then I think it often makes sense to discount the top 6 inches of "soil" because it may not be there or it may weigh less than you're counting on.

3) I've seen cases where an outdoor footing was exposed all the way to the footing. Meaning that the site was re-graded after the fact. That might have been temporary, I'm not sure. But, it DOES happen. I think it's rare enough that we shouldn't be thinking about that case during our design.... It's like thinking that the Earthquake it's at the exact moment when maximum live loading occurs. It's just too rare of a case for use to consider happening concurrently with our worst case lateral loads.

4) Sometimes (for retaining walls), I might discount more of that stabilizing weight on the toe side. But, that's because the lateral pressures are pretty constant from the retained soil. And, I'm more concerned about stability under these long term active pressures.