Stemwall Foundations and Spread Footings

[medeek]

I've been doing a lot of residential work lately and mostly my concrete foundations are prescriptive in nature which means I don't usually have to dig into the ACI 318-11 which I find very confusing and not nearly as easy to use as my well worn NDS 2012 and ASCE 7-10.

Typically I either have a slab on grade or a stemwall type foundation. Usually the size of the stemwall and footing is determined by the number of stories (local bldg. dept. rules), however I will normally find the wall with the heaviest gravity loads and do a quick check of the soil pressure. I have never had the building dept. ask for more. The designer usually has already called out all of the prescriptive detailing with regards to rebar, depth etc...

Occasioally when I have an extreme point load on a exterior wall I then add a spread footing (ie. 30"x30"x12"). However, with moderate point loads I generally assume the depth of the stem wall is such that the load is distributed to a larger area of the footing (ie. stemwall is 24" deep). What I would like to know is there a general rule of thumb with this sort of thing or a method of rationally analyzing this? or maybe nobody does?

The ACI 318 section on footings doesn't seem to give much guidance in this regard. Also if anyone knows of any good books, software or other resources that deal specifically with typical residential concrete engineering I would be very interested in those.

 

[medeek]

Hopefully this diagram clarifies my question a little:

 

[KootK]

I've heard of a few rules of thumb:

1) Assume that your load spreads through your stem-wall and footing at 45 degrees on each side.
2) Assume that your load spreads through your stem-wall and footing at 30 degrees on each side.
3) Spread the load as far as you like so long as the stem-wall is designed as a beam for the loads.
4) Same as #3 but limit the extent of the stem-wall to 5X the depth of the stem-wall on either side.

Chances are, your judgement will be as good as anyone else's. Give 318 time to warm your heard. It's actually my favourite material code. Especially so after dealing with the Canadian equivalent where the code and commentary are separated by a few hundred pages.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

Nice isometrics. You chose the right line of work.

In the first sketch, note that care should be taken at stem wall end conditions. You need to satisfy rebar development and anchorage detailing requirements there just like you would in a normal concrete beam.

In the second sketch, it's really 2X one way shear rather than punching shear. Probably just a semantic thing. You'll also want to avoid using the overhanging flanges of the footing in your shear calculations. For that matter, don't use the footing at all unless you're happy that horizontal shear friction works along the joint between wall and footing. Usually shear doesn't govern anyhow.

If the soil works under the 45 degree load spread assumption, designers don't typically check for shear capacity at the sides of the wedge.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

In my other life I'm also an artist:

http://www.wilkersonart.com

With the second sketch I think that is really what I am trying to indicate, the 45 degree angle allows for a greater area thereby decreasing the required soil pressure.

An example calc. would be:

P = 5000 lbs

Stemwall Depth = 24"

Assume load concentrated at a point for simplicity and alpha is 45 degrees. Then point load would be distributed to 48" of footing below.

Assume one story building with minimal 12" wide footing (I see a lot of these).

Area_footing = 1' x 4' = 4 ft²

Req. Soil Pressure = P/A = 5000 lbs/4 ft² = 1250 psf < 1500 psf => OK

 

[KootK]

Mechanical engineer, programmer, web developer, structural engineer, artist... You're a modern renaissance man! I'm going to go out on a limb and guess small engine repair as well. Your calc looks spot on. KootK aproved.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

Nope, never been very good at small engine repair.

 

[KootK]

You've started several foundation related threads as of late. And I know that you're a fellow who enjoys a good technical read. Accordingly, I have a recommendation for you: Link. It's the best book that I know of, by far, for elucidating the assumptions and methods that go into routine foundation design. You know: strip footings, strap beams, basement walls. Most foundation text books head straight for the glamorous stuff like pile group testing and earth retention under dynamic loads.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[medeek]

Just purchased me a copy. I think this is exactly what I was looking for, really appreciate it.

 

[ChadV]

I would add, ACI 314 is "Guide to Simplified Design for Reinforced Concrete Buildings" which might be more manageable for residential projects than diving into ACI 318.

I haven't used it much personally so I can't say if it would address your specific question about concentrated loads, but it's worth having a look at in general.