Square Footings combined with Stemwall Foundations: A rational approach
Square Footings combined with Stemwall Foundations: A rational approach
(OP)
Occasionally I encounter a point load from posts that exceed the bearing capacity of a stemwall foundation footing. I then typically call out a square footing (ie. 24"x24"x12" w/ (3) #4 bars each way) to increase the bearing capacity at this location. However, in certain situations I've noticed that the number don't quite work for the stemwall footing alone or the square footing alone. It would seem logical that one could combine the bearing areas of both the continuous footing and the square footing and the total bearing area would account for this overlap as shown in the diagram below:

Does this seem like a rational and reasonable assumption to make?
Up until now I have simply ignored the contribution from the stemwall footing and sized the square footing as if it were an isolated footing.

Does this seem like a rational and reasonable assumption to make?
Up until now I have simply ignored the contribution from the stemwall footing and sized the square footing as if it were an isolated footing.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com






RE: Square Footings combined with Stemwall Foundations: A rational approach
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.
RE: Square Footings combined with Stemwall Foundations: A rational approach
For the distribution of the load in the first part of the sketch, I think you can spread the load more than 1.5xH. I would spread it at 45 deg from the top of the wall all the way to the underside of the footing for a total width of 2x(Stem Wall + thickness of footing). I would expect this would take care of 90% of point loads in a residential application.
RE: Square Footings combined with Stemwall Foundations: A rational approach
Mike McCann, PE, SE (WA)
RE: Square Footings combined with Stemwall Foundations: A rational approach
I don't think there is a problem with your rationale. However if faced with a similar situation I would probably put a footing under the point load such that I wouldn't need to rely on the strip footing.
RE: Square Footings combined with Stemwall Foundations: A rational approach
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.
RE: Square Footings combined with Stemwall Foundations: A rational approach
with the length along the footing being about 1.5 times the stemwall height.
The extra footing/pad is enough to take the excess load to the strip footing.
I find the concreters are appreciative of only cutting up 1 side of the footing formwork.
Of course, being a geotech, I insist of the subgrade being good and I am quite
confident of where the Allowable Bearing is really at.
RE: Square Footings combined with Stemwall Foundations: A rational approach
emmgjld:
That is a fantastic approach for only doing a bump out on one side. I never contemplated that but it makes a whole lot of sense...I have a new trick in my hat!!! Thanks.
RE: Square Footings combined with Stemwall Foundations: A rational approach
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.
RE: Square Footings combined with Stemwall Foundations: A rational approach
For large residential point loads, anything that requires a square footing 30"x30"x12" or larger I automatically just consider the square footing bearing area and neglect the stemwall footing.
The grey area is situations requiring a 24x24 or even 18x18 footing.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Square Footings combined with Stemwall Foundations: A rational approach
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Square Footings combined with Stemwall Foundations: A rational approach
A lot of times if I only need 1 sqft more I'll re-examine the loading to see if I can trim it down somewhere, maybe over assumed dead loads, or conservative rounding.
RE: Square Footings combined with Stemwall Foundations: A rational approach
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.
RE: Square Footings combined with Stemwall Foundations: A rational approach
RE: Square Footings combined with Stemwall Foundations: A rational approach
RE: Square Footings combined with Stemwall Foundations: A rational approach
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Square Footings combined with Stemwall Foundations: A rational approach
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.
RE: Square Footings combined with Stemwall Foundations: A rational approach
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Square Footings combined with Stemwall Foundations: A rational approach
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.
RE: Square Footings combined with Stemwall Foundations: A rational approach
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Square Footings combined with Stemwall Foundations: A rational approach
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.