Stepped footings / Stemwall Design 5

[medeek]

When it comes to footings combined with stemwalls for most of my residential projects I am usually concerned with bearing loads (ie. sizing the footing for 1,500 psf or less) or if there is unbalanced fill I sometimes have to look at retaining wall designs. Up until now I have not given much thought to how the designer/contractor is actually building steps into their footings and stemwalls. Most of my homes, until recent, have been on level ground but as I've seen more jobs with sometimes very complex stepped footing configurations I've begun to give this some more thought.

Are there any specific details or resources which delve into this subject further, both from a construction standpoint and engineering?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[wannabeSE]

KootK,
Our typical stepped footing detail is similar to your alternative 1. I did a quick search of the internet to find the reasoning behind IBC §1809.3 without much luck. The only thing I found is in FEMA 232 Homebuilders' Guide to Earthquake-Resistant Design and Construction (2006) where it states "To aid in providing sliding resistance, the bottom of the footing should be level." See oage 60 (pdf page 12) in

https://c.ymcdn.com/sites/

http://www.nibs.org/resource/resmgr/BSSC/FEMA232_Chapter3final.pdf

I think most engineers wouldn't upsize the footing to compensate for the little wedge of excavation at a steep angle. While I have nothing against JLC, the cranked bars at the wedge don't make much sense to me unless they are more fully developed to prevent cracking at the reentrant corner.

 

[dik]

My standard stepped footing detail:

 

[BigH]

Thanks KootK

 

[medeek]

I appreciate the overwhelming response to my question. I now have a much better idea at what the typical stepped footing should more or less look like. However, a number of points from the various details shown so far a contradictory. My client usually likes to use a 24" step (usually multiple steps on a sloping hillside), based on some of these examples the overlap length of the footing should be equal to the step height which seems excessive to me. So far the detail provided by "dik" and the journal of light construction detail show steps that are most representative of reality, at least for what I see.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Would something like this be wrong?

dik's detail gives the max. step rise to run as 0.70
JLC gives it as 0.75

I would probably opt for a more conservative 0.50 ratio. That way a double step in the stemwall would have a min. 48" overlap as shown above.

It is very interesting to me how the building practices tend to be very geographical. In certain areas (ie. Portland) for example they like to set the floor joists inside the stemwall rather than on top with a rim board, I find this very strange but I guess it works. This tends to make the crawl space rather tight with a standard 24" stemwall height and not easy to work in but I guess it is done for the overall cost savings on the structure.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com