Residential Concrete Stem Walls 1

[medeek]

Okay, I readily admit my understanding and use of the ACI 318 is still quite limited. I'm much more comfortable with the NDS and everything having to do with wood engineering.

I've recently been doing some work on some detached garages. One story structures with a 24" stemwall resting on a separately poured concrete footer. As far as sizing the footer width, I've usually just looked at the total load at the highest loaded section of wall and if does not exceed 1500 psf (default allowable soil bearing pressure) then I'm good to go.

However, the designer on two very similar projects called out a 6" stemwall on one and a 8" stemwall on the other. Is there a quick, rational check/calculation that I can do to verify that the stemwall size is adequate without getting to muddled up with the details.

My first thought was to simply take the load on the stemwall at is base (so that it includes the selfweight of the stemwall) and check the pressure and compare that to the strength of the concrete (2500 - 3000 psi). However, with the reinforcing (horizontal and vertical) this is probably to simplistic of an approach. I guess it time to really crack open the ACI 318.

 

[KootK]

Are the stem wall dimensions that you've provided widths? If so, 24" sounds enormous, 8" is what I most commonly see, and 6" seems workable depending on the details (sill plate connection, brick support etc). So long as the walls don't transfer a monstrous amount of shear or retain a significant amount of unbalanced soil, the reinforcing design is usually pretty trivial. I generally see the rebar designed for two things:

1) Temperature and shrinkage.
2) Nominal flexural capacity to allow the stem wall to span over soft spots in the soil.

This usually adds up to t&s wall reinforcing with an extra 2-#5 continuous top and bottom.

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

 

[medeek]

Sorry if that is confusing. Both stemwalls are 24" in height with one at 6" wide and the other at 8" wide. Reading through the ACI last night I noticed 22.6.6.3 which limits ext. basement walls and foundation walls to 7.5 inches. However, does this only apply to full height basement walls (ie. 8' tall with substantial backfill) and not to stem walls?

 

[KootK]

Got it. In that case, my original answer still applies. The ACI limit would only apply to a wall that retains significant unbalanced earth pressures. You're in the pacific northwest, right? Isn't frost depth for heated structures closer to 48"?

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

 

[medeek]

Both of the structures will be in Grays Harbor County with a frost depth of only 12".

 

[medeek]

The ACI seems less than helpful. The only document that seems to address this directly is the IRC (which seems to be generally despised by most structural engineers) but even then there is no specific table or section dealing with stemwalls where lateral soil pressure is really not an issue.

 

[TLHS]

The ACI code is incredibly helpful and could be a tool to solve this problem. ACI isn't going to tell you specifically what to do without additional information because it's intended to give requirements for determining that sort of thing based on loading conditions. To use it, you need to determine the forces on the structure as per your local building code (probably the IBC or some modification thereof which would then reference ASCE 7) and then design your concrete section based on those moments.

You're not going to find a clause that tells you the wall width for some specific stem wall height, because I could design a stem wall that takes only compression, or one that's got shear and moment on top of that along with an uplift case. My stemwall could be holding up a one floor garage or a ten story apartment building.

The IRC is a prescriptive code, so it limits its scope significantly and then it can tell you what details to use for buildings within that scope. If you fall within the scope of the IRC, go ahead and use it.

 

[KootK]

Under ordinary circumstances, stem walls are barely even structural elements. I wouldn't sweat it too much. I find IRC quite useful as a guide to what residential contractor expectations will be. I find that, if I drift to far from IRC (NBCC Part IX in Canada), I get a lot of push back.

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

 

[bookowski]

As others have said the width required for your wall detailing will almost always be sufficient. No code restriction from ACI.

One note from your original post "take the load on the stemwall at is base (so that it includes the selfweight of the stemwall) and check the pressure and compare that to the strength of the concrete (2500 - 3000 psi)" - if you did want to run the calcs you should using factored loads (ACI is based on LRFD) and you can't compare it to the concrete strength directly. You'd need the appropriate code section for whatever you are checking, you were referring to bearing so you'd want that section of ACI which includes phi and reduction factors (you'll end up closer to 50% of f'c).