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Stepped footings / Stemwall Design

Stepped footings / Stemwall Design

(OP)
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
www.medeek.com

RE: Stepped footings / Stemwall Design

(OP)
Typical residential stepped footing:



I've noticed I don't usually see a step more than 24" in height, that seems to be a good rule of thumb...

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

RE: Stepped footings / Stemwall Design

At the haunch, I always tie the strip footings together with the bottom bars and thickened concrete section, tapering the bottom of the lower footing up to meet bottom of the upper lower strip footing.

I will allow steps up to 4 feet, although 2 feet is common.

Mike McCann, PE, SE (WA)


RE: Stepped footings / Stemwall Design

Agreed with Mike on tying the 2 levels together. I do limit them to 2 ft steps to keep the tapered portion a manageable size (2 ft long).

RE: Stepped footings / Stemwall Design

For your own sanity, it can be helpful to provide some options. Draw it all once and never get queried on it again. Well, at least not too often.

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: Stepped footings / Stemwall Design

KootK,
Alternative 2 probably does not comply with the IBC. It prohibits slopes on the bottom of footings exceeding 1V:10H.

RE: Stepped footings / Stemwall Design

(OP)
Thank-you KootK for the details, from these I will develop a standard detail(s) to include in my library.

I do have quite a few designers on their plans that will draw something like this (the dimensions and notes are for clarity and questions below):



The step is 24", they usually don't even show the footing at the haunch, I'm am wondering how this actually gets constructed in the field and what the appropriate detailing should be.

Per KootK's detail above, overlapping the footing would result in an large quantity of concrete at the haunch, or at least to my untrained eye.

I'm also wondering about the overlap of the stemwall above the footing.

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

RE: Stepped footings / Stemwall Design

(OP)
It's strange that this topic has never really come up before for me but I guess since most of my projects have been built on a a sand bar (Ocean Shores, WA) sloping terrain has never been an issue. I have quite a few examples now of other residential engineering plans and details that were done in my vicinity but I don't see any details with regards to stepped footings even though some of the plans do have those features. Its seems that this is ignored by some engineers but I would rather not leave anything to chance.

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

RE: Stepped footings / Stemwall Design

Quote (wannabeSE)

Alternative 2 probably does not comply with the IBC. It prohibits slopes on the bottom of footings exceeding 1V:10H.

Would you mind sharing the clause so that I can check it out for myself? I'm not beholden to the IBC these days. The practical distinction strikes me as a bit wishful. It's not like you're getting the hard 90 degree angles shown in detail one unless you're dealing with a very stiff clay and an unusually talented excavator. Most stepped footing end up being sloped footings to some degree.

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: Stepped footings / Stemwall Design

Quote (medeek)

Per KootK's detail above, overlapping the footing would result in an large quantity of concrete at the haunch, or at least to my untrained eye.

Don't sweat this. Remember, most of the cost is in the formwork. A localized blob like we're talking about here is economically irrelevant.

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: Stepped footings / Stemwall Design

(OP)
I guess I would tend to agree with KootK on this one. Detail 1 is going to approximate detail 2 in most soils.

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

RE: Stepped footings / Stemwall Design

Alt 2 per Kootk is my company's standard detail, with the exception that we go ahead and show the diagonal crack control reinforcing inside the wall, (which Kootk probably puts in his wall details). hey wannabese... Please followup on where the IBC prohibits this. 1V:10H seems extremely conservative to me... i could see argument pushing the code to 1V:3H but not 1:10. Is it possible that the 1:10 reference is not discussing stepdowns but the laying of a continuous footing unlevel?

RE: Stepped footings / Stemwall Design

(OP)
@darthsoilsguy2

Explain the diagonal crack control reinforcing in the wall. Typically what I see for most residential stemwalls is 24" in height with one #4 bar in the top and some vertical #4 bars at 24" or 48" o/c. I only specify something above and beyond that when there is significant shear loads on the wall (ie. high aspect shear walls or portal frames that induce large bending moments into the stemwall/footing below).

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

RE: Stepped footings / Stemwall Design

The IBC section on steeped footing is §1809.3

Quote (IBC)

1809.3 Stepped footings. The top surface of footings shall be level. The bottom surface of footings shall be permitted to have a slope not exceeding one unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation of the top surface of the footing or where the surface of the ground slopes more than one unit vertical in 10 units horizontal (10-percent slope).

RE: Stepped footings / Stemwall Design

Step Details my firm has used since'97 and previous by my Father since the early/mid 60's. We have worked with a lot of foundations on slopes, including walk-out basements.

RE: Stepped footings / Stemwall Design

(OP)
I can't see the attached file.

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

RE: Stepped footings / Stemwall Design

You can't upload things to engineering.com that have spaces in the title. Found that out once upon a time.

RE: Stepped footings / Stemwall Design

Quote (BigH)

How do you embed the photo in the post??

1) Bring the thing up on screen.
2) Use the windows snipping tool to capture the image and save it someplace.
3) Use the button circled below to select the file for inclusion in your post.

#2 can be skipped but can lead to weird results with respect to display size etc.

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: Stepped footings / Stemwall Design

Quote (wannabeSE)

The IBC section on steeped footing is §1809.3

Thanks for that wannabeSE. I can empathize with your interpretation but, alas, I've been interpreting it differently.

Quote (IBC Statement)

The top surface of footings shall be level. The bottom surface of footings shall be permitted to have a slope not exceeding one unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation of the top surface of the footing or where the surface of the ground slopes more than one unit vertical in 10 units horizontal (10-percent slope).

Quote (KootK Interpretation)

A footing, in the macroscopic sense (pad, long run of wall strip), shall not have it's bottom surface sloped greater than 10H:1V. Where the grade of the underside of the footing must drop at a rate steeper than 10H:1V, that shall be accomplished by way of localized footing steps which may, themselves, have their lower surfaces sloped at an angle greater than 10H:1V as approved by the geotechnical engineer.

Is my interpretation valid? Dunno. I've been able to get it past code reviewers for good long time. I cling fervently to my previously expressed opinion that it's not realistic to expect those hard 90's in detail one. Neither is it realistic to accomplish a footing "step" by way of incremental slopes not exceeding 10H:1V

I found the blurb below at the journal of light construction website which takes yet another tack. They don't count the portion of the soil in the "wedge" as bearing. I've not accounted for that but I can certainly see the logic in it.

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: Stepped footings / Stemwall Design

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/www.nibs.org/resource/re...

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.

RE: Stepped footings / Stemwall Design



Thanks KootK

RE: Stepped footings / Stemwall Design

(OP)
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
www.medeek.com

RE: Stepped footings / Stemwall Design

(OP)
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
www.medeek.com

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