Knee wall - foundation wall 'hinge' 3

[RFreund]

I'm wondering how other engineers feel about this situation and how they typically handle it?

This is a detail that I see often in homes with a crawl space. Usually the foundation wall is about 4'-0" tall 8" wide with an 18" wide footing. Instead of the floor framing bearing on the foundation wall there is a knee wall built usually about 2' or so tall then the floor framing bears on the knee wall.

A 'hinge' is formed at the bottom of the knee wall / top of foundation wall. I have seen quite a few houses with this condition and there is nothing visibly failing. However it seems hard to justify. The foundation wall would either have to span horizontally or the weight of the wall needs to resit overturning. If this detail comes up in design how do you handle it? Cantilever retaining wall, horizontal beam, kickers ?

If this has been discussed elsewhere let me know as I tried to search but couldn't find too much. Is there another name for the 'knee walls' maybe?

EIT

 

[concretemasonry]

RFreund -

This is a common question that most engineers "sweep under the carpet". Usually it is not critical enough to bother with because of the lack of familiarity and technical ability to thoroughly analyze for such a minor condition that is controlled by minimal prescriptive code requirements and not the real world.

Obviously, the wall would be much stronger if the lower wall was full height and there was not a hinge. It is just a question of economics and traditions. The overturning is minor and too difficult to calculate accurately from an engineering standpoint.

Dick




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

 

[SteelPE]

Is this detail not covered by the IRC? I thought there was a chart that listed the unbalanced soil height vs the foundation construction.

I have a wall in the basement of my house that is 8' high with 4' of unbalanced load with a 4' high wall. Of course there are jogs in the wall every 14' or so.

If you are not comfortable with the detail you can run the numbers yourself to see if they work. There is nothing wrong with using the weight of the wall to resist overturning. You can even add the weight of any floors/roof above the help.

 

[concretemasonry]

SteelPe =

You have an 8' high wall that is continuous and that is much stronger than a wall with a hinge. An 8' high masonry wall is routine irregardless of the amount of unbalanced load.

With a hinge, you have a weak point and have to justify the strength of two cantilevers (wood and masonry) and the connections. If you have enough vertical load, the problem is minimized.

The IRC Code is just a minimal prescriptive "crutch" to use to avoid facing the real situation.

the codes to not take into account the effects of horizontal continuity and the restrain provided by corners or "jogs"/pilasters.

Dick

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

 

[msquared48]

I have to strongly disagree with concretemasonry here. An 8 to 9 foot high concrete foundation wall with the top two feet or so framed as a knee, or pony wall, is not an issue to take lighly and ignore. I have seen rotational failures of such walls, but I have also seen many showing no distress. It depends on the specific structural support conditions.

If the concrete portion of the wall is a free standing retaining wall - a yielding wall - there is no problem with the pony wall above. However, if it is a non-yielding wall, pinned at the base and the top, there is a problem - a lateral kick at the top of the concrete wall that must go somewhere. If there is no soil load against the wall, you still must consider wind and seismic forces where appropriate. You are looking at a minimum of 180#/ft lateral force to resist at the top of the concrete wall with 6 feet of soil retention.

Usually, either kickers as you suggest, or a balloon frame condition that most framers would object to, are good solutions. Either will transfer the force properly if adequately designed. The kickers to not need to be large to work.

It is true that this condition is ignored much of the time, but here, any concrete wall over four feet must be designed by an engineer, and with good reason. If ignored, the wall will experience a force at the top of the wall, that could with time, cause it to rotate inward, putting the wall out of plumb. Owners do not like this.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SteelPE]

Maybe I am not getting the detail

If the concrete/masonry foundation wall is designed/check as a cantilever then the wood knee wall can span from top of the foundation wall to the floor as a simply supported member. Most of the foundation walls constructed in my area are done out of concrete. I can't remember seeing one done out of masonry unless it is pretty old.

I guess I just don't get it.

 

[MiketheEngineer]

If you are in a NO-WIND or NON-SEISMIC area - no problem. Other than that I would treat it as a "short wall" and design it as shear wall with all the factors thrown in!! ETC, ETC, ETC

 

[concretemasonry]

Sorry, but I unfortunately erred when I missed the "If" in my first sentence when I referred to the continuous wall (masonry or concrete). Obviously, a hinge or break in the wall is weaker and takes some consideration.

Normal construction and codes require a basement wall to have anchor bolts connecting a sill plate to the floor system. the normal procedure is to block the first several bays of joists (depending on local codes) to provide a lateral resisting floor diaphragm. at the non-bearing walls. - The codes do have some engineering features which is fortunate in common cases.

If you have a hinge, there is always a lateral resistance problem that must be addressed. A concrete or masonry wall can easily be constructed as a cantilever, but the frame wall portion of the wall must be able to resist the lateral loads by using "kickers" or sophisticated connections. I have seen the effects of the lack of lateral strength while investigating tornado and hurricane disasters. Unfortunately the lack of inspection or controls, the omission of nuts on the anchor bolts cause most of the problems.

Unfortunately, there was no mention of the wall thickness of the lower wall, but they are commonly 8", 10" or 12" supporting a thin 4" frame wall and the non engineered simplistic codes do have reinforcement table (usually 4' o.c.) that provide the desire cantilever strength effect, but do not address the lateral strength of the frame wall above unless the wall below has enough resistance.

Just avoid getting into the "propped cantilever" concept of a foundation because that is a "can of worms" that I learned 30 years ago.

Dick

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

 

[JAE]

RFreund - can you provide a sketch of your theoretical knee wall condition? I think the responses above might be based on something different than what you are describing.
Thanks,

 

[msquared48]

That would be good.

Also consider that the condition you describe is ripe for the p-delta effect sinde it is a bearing wall. When any lateral deflection in the wall occurs, it is magnified further by the vertical load.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[wannabeSE]

RFreund,
Are the short wood framed walls (knee, pony, or cripple) detailed with sheathing so they act as shear walls and the floor designed as a diaphragm?

 

[RFreund]

Sorry for some reason I wasn't getting or didn't see the notifications in my email.

I believe you are answering my question. I will try to post a sketch later. These foundation walls/footings are not designed as cantilevers. The foundaiton walls are pined top and bottom. The knee wall is sheathed but not ballooned framed. The floor is a diaphragm but that is not helping.

Is it the IRC or the IBC that address this situation in a prescriptive method?

Also I just found out that my parents house is framed this way. It is a walk about basement so the 2 sides have concrete walls that step down from retaining 8' of soil to 0'. So at one of these steps there is 4' of soil being retained and a 4' knee wall on top of that. I asked what rebar is in the wall - (2)#5 top, mid, bot.

EIT

 

[jhoulette]

HA! I design repairs for these exact problems all the time. The engineers that keep ignoring this are nuts.

For residences with a hinge or better yet, lack of proper support by using the main floor to top of foundation will most likely fail. Or I should say will fail, just a matter of time.

This happens for garden level, walkout basements and just random low grade situations where the builder wanted to save some concrete.
I also see it happen with long ranch homes with a basement and full height concrete walls. The sill plates split and the wall moves in.

Yes, the foundation wall can be designed as a cantilevered retaining wall that does not require bracing at the top, but that requires a larger footing typically. And what do you do when the foundation has drilled piers?
Yes, you can count on jogs in the foundation wall that brace the foundation, but if that wall length gets much over 18ft the center of the wall can start to lean in. As engineers we don't get to decide on the house geometry. Also, corners add costs to the foundation.

I suggest using counterforts or buttresses to brace the walls.
They help brace the wall during backfill as well since they can't build the main floor until after the walls are backfilled. Darn OSHA ;o)




Jim Houlette PE
Web:

http://www.evstudio.com

Online Magazine:

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[IceNine]

I looked at a wall of this type that failed years ago, during construction ( the house was nearly finished). It was an 8" concrete wall approx. 6' tall with a 3' wood knee wall on top. There were large cracks in the wall, and it was bowed about 4", but luckily didn't collapse. I asked the contractor how it was reinforced and he said '(2) #4s at the top and (2) #4s at the bottom'...no vertical steel. The house was about 8' from the adjacent house, soil was wet, and the mason drove a bobcat with a load of bricks between the houses, causing the failure. His plan for repair was to remove the wall and rebuild it the same way. Luckily, the homeowner wanted a second opinion.

 

[RFreund]

Very insightful and thanks for the replies. I'm still trying to find the 'prescriptive' code. I believe it's in the IRC, which we do not have a copy of but I may be wrong. just so I can read through it. I'm not a fan of the detail either unless it can be justified through the methods mentioned above. I just know that I've seen a number of these conditions on old houses and have not seen any problems. Having said that, most are crawl spaces not retaining much soil and the horizontal span is not too long either. Given the posts above I am now reassured that this is not always the case and this still is not a good detail.

I asked my ol'man about it (family business is residential G.C.) and he said "go ahead and tear it down then. You'll see why it's still standing." I laughed but I'm sure there is some truth there. Kinda like when saying "brick veneer can not support anything." I've had to tear that down, its not as weak as you might think.



EIT

 

[jhoulette]

RFreund - what is the length of the wall?
Remember concrete can span along the length of the wall, not just the height.
For a crawlspace wall, no brace at the top from the main floor, no brace from a basement slab then the wall is supported at the corners. If this distance from corner to corner is short enough then it may be fine.
For what you described I would place that distance at roughly 16ft, maybe a bit more depending on the soil type.

I don't think you will find any prescriptive methods for this.
You're an engineer right? Design it.

Jim Houlette PE
Web:

http://www.evstudio.com

Online Magazine:

http://www.evstudio.com/blog

 

[woodman88]

RFreund - In looking at the details (Figure R403.1(1), ...(2), ...(3) in my 2003 IRC, all of the details show the retaining wall (wood, concrete and masonry) to extend from the footing to the floor framing. The hinge you are asking about is not shown as a detail.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[RFreund]

@jim - I don't have a particular situation in mind. I was just thinking about the topic and wanted to get some feedback.

@woodman - thanks. So they do not refer to any "cripple walls"?

EIT

 

[woodman88]

Okay I found Figure R602.3(2) FRAMING DETAILS that shows a cripple wall from the floor framing to the top of the foundation wall. And in Part II the definition "CRIPPLE WALL. A framed wall extending from the top of the foundation to the underside of the floor framing of the first story above grade plane." Now from the details (R602.3(2), R403.1(1), ...(2), ...(3)) and the definition (IHMO) I would assume that the top of the foundation is to be considered a fixed bearing for the connection of the cripple wall.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[jhoulette]

A quick search in my IRC 2009 and I found this. It has a revision line for this section, so it may not be in previous versions of the IRC. I starred the important part. Over 48" and not braced at the top puts you to sectionR404.1.3, Design required.

IRC 2009
R404.1.2.2.2 Concrete foundation stem walls supporting light-frame above-grade walls. Concrete foundation stem walls that support light-frame above-grade walls shall be designed and constructed in accordance with this section.

1. Stem walls not laterally supported at top. Concrete stem walls that are not monolithic with slabs-on-ground or are not otherwise laterally supported by slabs-on-ground and retain 48inches (1219 mm) or less of unbalanced fill,measured from the top of the wall, shall be constructed in accordance with Section R404.1.2. ***Foundation stem walls that retain more than 48inches (1219 mm) of unbalanced fill, measured from the top of the wall, shall be designed in accordance with Sections R404.1.3 and R404.4.***


R404.1.3 Design required. Concrete or masonry foundation walls shall be designed in accordance with acceptedengineering practice when either of the following conditions exists:1. Walls are subject to hydrostatic pressure fromgroundwater.2. Walls supporting more than 48 inches (1219 mm)ofunbalanced backfill that do not have permanentlateral support at the top or bottom.

Jim Houlette PE
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http://www.evstudio.com

Online Magazine:

http://www.evstudio.com/blog