question about CMU foundation knee walls 1

[gte447f]

Can anyone provide any guidance on the permissibility of CMU knee walls in residential foundation construction. I am referring to the situation where a short stud wall bears on top of a CMU foundation wall below the level of the first floor. If the CMU foundation walls are not retaining any soil, how big of a deal is it that the top pf the CMU is not braced out of plane, as it would be if it extended full height to the first floor level and were braced by the floor joists?

 

[concretemasonry]

Basic engineering - You have a hinge in the middle of a wall that is very possibly load bearing and is subject to lateral loads. A wall with a hinge in in the vertical span can only provide a wall that has problems that cannot be economically and accurately analyzed.

Unfortunately, current engineering practice is not really equiped to provide a realistic quantification since that type of construction is generally pemitted because of most mddel codes that rely on prescriptive requirements.

Since there are many assumptions to simplify the situation, you have to hitch up your belt and suspenders and change what has been accepted.

It is a good thing there is no soil retained because the wood framing is no help and "pinned" base is usually assumed.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JAE]

I believe that the IBC and IRC refer to those as cripple walls.
See IBC Definitions in Section 2302.

Also 2308.9.4. (note that 2308 is an emprical "light framed construction" section more like the IRC.)

So the code acknowledges their use.

To engineer these I'd be concerned with concretemasonry's issue with lateral stability - so sheathing would be required. Also consider issues related to moisture (use treated wood) and venting of the crawlspace. I would also look at the masonry wall stability relative to differences in earth elevation on each side of the wall - some of that may occur even if you detail it otherwise.

 

[msquared48]

I would be very concerned with wind/seismic forces normal to the wall which could not be transmitted properly thorough the hinge to the roof diaphragm and foundation, creating an unstable wall situation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

I have framed many houses that had walk out basements that had this situation in some areas of the framing. Sometimes we did just what was talked about already and simply framed on top.

In a few situations where the vertical height of the framed portion of the wall was more than a four five feet, I framed the studwall from the CMU all the way up to the roof (or second floor) height then "hung" the first floor framing off the inside.
This was never in the plans, but I was just a carpenter/home builder at the time.
I was worried about the "hinge" that has been mentioned. My thinking was that in "balloon" framing the wall up to the second floor elevation (or roof) that the stud wall was essentially cantilevered from the diaphragm elevation at the first floor down to the CMU wall for out-of-plane loads and would provide more stability.
Not sure now that I'd even know how to design for what I "extruded" back then.

 

[SkiisAndBikes]

Within the residential section of the Canadian Building Code, the prescriptive requirements for concrete block foundation walls is tabulated. The values are based upon calculations and/or past proven performance. For partial height concrete block foundation walls, laterally unsupported at the top and retaining soil, typical allowable values of retained soil are in the 2'-11" range for 8" block and 3'-11" for 10" block. The height of the block wall can exceed these values (and should by at least 6"), i.e. overall basement height could be 8'-2", concrete block wall could be 5'-0" high with the exterior grade no more than 3'-11". The remainder of the wall height would be stick framed.

As with most situations, engineering judgement is required and high wind, seismic, unbalanced loading or unusual geometries need special consideration.

 

[BAretired]

Knee wall, cripple wall, I thought they were called pony walls.

Three hinges in a wall makes it an unstable structure. Toad removed the upper hinge. You could also remove the lower hinge by reinforcing the CMU wall as a cantilever retaining wall. Removing the middle hinge would work too, but it is not quite so easy to accomplish.

BA

 

[ACSEngineeringFL]

A sketch of your situation would be helpful, but if I understand your situation, you have a crawlspace foundation wall that is discontinuous. It is CMU from the footer up to a certain height, then it changes to wood from there on up to the floor system. There is no lateral support at this transition point.

As everyone has said, this is an unstable structure. Your job is to make it stable. This is not necessarily difficult, but it IS important!

I see this a lot with porch additions here in Florida: for one reason or another, the slab is forced to be at or sometimes slightly below grade level. The client doesn't want to use CMU for the whole thing, but it's not such a good idea to run wood down to or below grade. Therefore, a short kneewall (3 courses seems most popular) is put down first to keep the wood away from the dirt, and then the remainder of the wall is constructed of frame.
You have to provide a large enough footer on that to make sure the kneewall can be considered cantilevered. It's not hard to calculate the lateral force it has to resist. Then you just have to calculate the width and depth of footer that will keep that kneewall vertical when subjected to both LATERAL and UPLIFT loads simultaneously. Check your soil bearing capacity with your gravity loads, but I doubt that will be the limiting factor here. Next calculate the filled cell spacing you need to resist bending in the CMU kneewall.
You'll end up with a footer than any contractor is going to scream about, but so be it.

Now in your situation, if I have understood it correctly, would it not be simpler to run 2" x diagonals down from the floor system to the transition point to provide the missing lateral restraint? The connections could be something of a mess where studs don't line up with joists or where the wall is parallel to joists, but it might still be the way to go?

 

[gte447f]

Thanks for all the contributions.

I fully understand that this type of construction theoretically results in an out-of-plane hinge in the wall. However, I also know that this type of foundation wall is fairly common in residential construction... even very common in the case of a stepped concrete stem wall with varying height cripple walls on top, as in a partial "daylight" basement.

Presumably, these concrete stem walls are designed as cantilevered walls with fixed bases, although I doubt they are detailed properly to develop a fixed condition between footing and wall, but at least there is some fixity.

In the case of CMU stem walls, I think most residential builders and do-it-yourselfers definitely don't attach the wall to the footing. Subsequently, when a wood cripple wall is built between the top of the stem wall and the first floor framing, the result is the three hinge condition we are discussing now.

In the case of my current project, it is a renovation/remodel project, and the contractor was already planning on replacing the CMU and cripple walls with new construction, because what is there is in poor condition. He wanted to replace with a new CMU stem wall and wood cripple wall, but I designed a full height CMU foundation wall, hinged, but also laterally braced, at the footing and the first floor framing.

For what it is worth, I dug through the 2006 IBC and IRC and found several prescriptive requirements for these types of cripple walls, but no mention of the foundation stem wall required to support them. Seems like a hole in the code to me. The prescriptive requirements for CMU foundation walls are predicated on lateral bracing at bottom and top. There are no prescriptive requirements specifically for partial height walls like what SkiisAndBikes mentions for the Canadian Code.

 

[BAretired]

Attached is Table 9.15.4.2.B of the Alberta Building Code 2006 which I believe is identical to the Canadian Code.

Note 6) below indicates the need to design the footing for overturning and sliding if the height of grade above basement floor exceeds 1.5m (5'-0"). Presumably, if it is less than 5'-0" that is not necessary.

This requirement or one very similar to it has been in our residential code for many years. I have never used it because I can't justify it from an engineering point of view. I have seen a few buildings where it was used and the walls are leaning in as a result of soil pressure. In some cases, interior stud walls normal to the foundation wall are providing lateral support, but this cannot be assumed at design time.

I think Note 6) would be okay if the last part of the sentence, i.e. after the word "sliding" was removed. I have attempted to talk our code officials into changing this but without success.

BA