Resultant outside the middle third of a footing

[XR250]

I have a residential job where the footings were not placed too accurately. Is there any reason that I cant just find the location of the load resultant and assume that it aligns with the centroid of a triangular soil bearing pressure wedge with a max pressure less than my allowable soil bearing pressure? See attached pic.

Thanks.

 

[KootK]

Works for me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

Any thoughts on differential settlement?
I guess you have the same issue even if you keep it within the middle third of a footing. In my case we are simply not using the entire footing with the remainder acting as a counterweight to pull the resultant inwards a slight amount.

 

[KootK]

Meh.

A) Usually only interested in average settlement.

B) I've very little confidence in this, or anything, being the correct soil stress profile anyhow.

C) Maybe the counterweight bit cracks in negative flexure and you open up a vector for rebar corrosion. Unlikely though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

With a few arcane exceptions not worth discussing here, I hold fast to my belief that adding more material to a thing that works always results in another thing that works.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lomarandil]

I'll agree with KootK's points here.

I've certainly justified a resultant outside the kern in the same way you've described before.

 

[BUGGAR]

On recommendations from a geotech, I have used a pressure distribution not unlike that in concrete ultimate stress design. I think we simplified it to a rectangular block at the outer edge of the footing. This was one of those desperate situations and settlement was not an issue.

 

[MrHershey]

Don't see why not. Hopefully a geotechnical engineer will weigh in.

Don't know that it's really disallowed anywhere either. The very first section of the Foundation chapter in the SERM has an equation for calculating max pressure when you're outside the kern and the pressure distribution looks exactly like what you're doing.

 

[msquared48]

I have used it many times before outside the kern area. Just far more likely to overstress the soil due to the increased leverage. Will get more rotation in the wall too.

Mike McCann, PE, SE (WA)

 

[DaveAtkins]

I wonder if the codes will eventually require ultimate strength design when it comes to soil bearing pressure--everything else in structural engineering has moved in that direction. Then, your assumption of a rectangular stress block will be what we all use.

DaveAtkins

 

[XR250]

Why would USD change the shape of a soil stress block?

 

[jayrod12]

Canada has gone to LSD design in foundations Dave.

Not that it has really changed much, all we have now is two checks to make for the foundation. Where I am it is always the SLS seeming to govern design anyway.

 

[emmgjld]

Be sure of the 'toe' pressure. Also assuming that expansive soils are not part of the problem. As long as Soil Shear or Excessive Settlement does not occur at the toe, this should not be a problem. Exactly the same issue as retaining wall base bearing.

 

[XR250]

Don't know if excessive toe settlement will occur. The site has a geotech report and plastic clay soils but supposedly undercut and added flowable fill.