Square Footings combined with Stemwall Foundations: A rational approach 1

[medeek]

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.

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

http://www.medeek.com

 

[KootK]

I don't think that this is often done but I don't see any problem with it. Perhaps a locally wider strip footing could replace the pad. That might save some rebar and excavation.

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.

 

[CANPRO]

If I have a point load on a foundation wall that is large enough that it warrants its own pad footing, I usually just size the square footing to take the entire load.

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.

 

[msquared48]

If the stem wall sees a uniform load in addition to the point load, I would add that to the footing load too to size it properly.

Mike McCann, PE, SE (WA)

 

[SteelPE]

FWIW I was taught to use 1.5x the wall thickness for my load distribution. I have no idea where that came from.... if challenged wouldn't be able to reproduce the literature proving that is OK.

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.

 

[KootK]

ACI chapter 14 seems to suggest 2V:1H. With nominal bottom steel, I'd be pretty comfortable with 2H:1V.

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.

 

[emmgjld]

For the residential foundations I have designed for the last 35 years, I have usually been able to widen just 1 side of the footing,
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.

 

[CBSE]

I was always taught to distribute at 45deg. You are just using arching action, which occurs at 45 deg in both masonry and concrete. Why wouldn't you use that?

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.

 

[KootK]

Emmgld's strategy is very attractive from a constructibility perspective. I'm not clear on how the eccentricity in the footing is resolved however. Stem walls aren't usually detailed to transfer moment to strip footings.

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.

 

[medeek]

I've used both 1.5 x h and 2 x h for the distribution. The 2 x h felt like I was giving away too much at times and so recently I've usually been a bit more conservative.

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

http://www.medeek.com

 

[medeek]

On a somewhat related note: If I have a square footing that requires a bearing area of 10 ft² and a 36x36 footing is coming in shy at 9ft² what is the typical approach to this problem. For some reason I've seem to have fallen into the habit of 6" increments (ie. 18, 24, 30, 36, 42, 48 etc...) but is their any good reason behind this? Would calling out a 38x38 square footing be too non-standard for a contractor? Making the jump up to a 42x42 square footing in this case seems a bit overkill so now I'm questioning the methodology.

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

http://www.medeek.com

 

[jayrod12]

If it's a one off pad I'll generally just go the 6". If there's lots of them with the same load I'll spec what's required, if he wants to provide larger pads then that's his call.

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.

 

[KootK]

I'll let most anything be 5% over stressed. After all, we're making buildings, not pianos. And I don't, for a minute, believe that there's any real economy to be gained by shaving a footing down by less than six inch increments, particularly if that creates new footing types that would otherwise have been repeats. Besides, you have to look out for your own sanity and profitability too. When I start a job, I plug my allowable bearing pressure into a spreadsheet that calculates every conceivable footing design I might need. I then hand that off to be drafted and instruct everyone else on the team to either use those footings or risk a public flogging.

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.

 

[CBSE]

I'm not sure that taking the time to shave a few inches off a footing dimension is economically worth it. You probably take 30-60 minutes to reduce the footing size for calcs, then have to add new stuff to your plans, then potentially have a whole lot of different footing sizes on the plans, which increases the risk of getting the "wrong" footing placed somewhere. I'm economical in my designs, but I'm also consciences that the contractor will be better off with keeping it simple.

 

[wannabeSE]

For distributing concentrated loads on walls, I use 2V:1H for the top half of the wall only (no distribution below the mid point between load and foundation). This is consistent with the masonry code requirements for concentrated loads (e.g. ACI 530-11 section 1.9.7).

 

[medeek]

Explain to me the 2V:1H terminology, this is the first time I've encountered it.

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

http://www.medeek.com

 

[KootK]

V = vertical; H = horizontal.

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.

 

[medeek]

So basically it is saying that one would take twice the stemwall height to get the horizontal tributary length whereas I am using 1.5 the stemwall height?

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

http://www.medeek.com

 

[KootK]

Your method expressed in that nomenclature would be 2V:1.5H.

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.

 

[medeek]

Actually wouldn't it be 2V:1.33H

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

http://www.medeek.com