Using slab on grade to brace a basement wall

[RichF2]

I'm trying to understand the the typical detail where a basement slab-on-grade meets a basement wall. Everywhere I see this joint discussed, an expansion joint material is called for to isolate the slab from the wall. This allows shrinkage (horiz. movement) or vertical movement of the slab (settlement, or swell) without affecting the wall.

What I'm confused about is it's typical practice to assume that the slab restrains the bottom of the concrete wall. How does the slab brace a wall if there is an expansion joint material, or the slab may have shrunk away from the wall some amount?

I've searched this forum, as well as general web searches. ACI 302.1 (3.2.5.1) and ACI 224.3 (5.3) both call for these isolation joints, but don't discuss the load path of the slab supporting the wall.

How do people deal with this? It doesn't appear to be a problem. Basement walls everywhere are being braced by slabs that were installed with compressible expansion joint material. I'd just like to better understand the load path and preferred materials.

Thanks!

 

[oldestguy]

Yes it "braces" the wall especially with no expansion joint material. However, in my experience (in USA)seeing occasional walls cave in or bow in, there never was an expansion joint, yet failure took place farther up the wall. With relatively stable temperatures in basements, why would you want sn expansion joint to begin with? If anything happens, it usually is loss of moisture in time and shrinkage of some degree occurs. Maybe ACI calls for them, but seldom if ever used in my experience.

 

[JAE]

If we depend on the slab to brace the wall, we specify a 30 lb. felt bond breaker instead of an expansion joint material.

An Example:

https://www.tarcoroofing.com/produc...cally-attached/asphalt-saturated-organic-felt

ASTM Reference:

https://www.astm.org/Standards/D226.htm

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[RichF2]

I like the idea of the felt bond-breaker. Thanks!

 

[PEinc]

I would not count on a slab on grade to brace a basement wall. Slabs are often minimally thin and often get removed for various reasons. Also, the basement wall shouldn't be allowed to be backfilled until the basement slab is in place and has sufficient strength.

http://www.PeirceEngineering.com

 

[jayrod12]

If you weren't willing to rely on the basement slab to brace the wall, you'd be out of the market around here. Even a thin slab tends to have ample capacity to provide the level of bracing for a standard basement. 3rd parking level is a different animal.

Agreed on the no backfilling until the slab is cured, this is like note 3 or 4 on my notes page.

 

[Ron247]

We use more of a bond breaker (felt) than an expansion material. If we do use expansion material, it is thin (1/4" or less). Basement slabs do not expand that much since they are in contact with unfrozen ground all the time in my area.

The slab does not provide a horizontal brace until the fill material is put in place and it pushes on the slab. Once you pour the slab, it shrinks some anyway, so that gap has to be made up. Basically, a little movement does occur. You still have the passive soil bearing against the edge of the strip footing and the friction under the strip footing that has to be overcome first.

It will brace against a failure because sliding of the wall will cease when it completely compresses the expansion material. At that point, the slab struts the load to the other side that is acting like a brace. I have seen countless failed basement walls that buckled around eye-level but the base was pretty much still in the original location. Most of these failed walls were hollow-block with no French drain. In the end, the base was still intact.

 

[Guest262653]

Half of my country is in a high seismic area so we don't rely on the slab on grade to brace basement walls either. We use a combination of ground beams (parallel and perpendicular to the wall) to dissipate horizontal loads to other fixed points. Additionally, slabs on grade can get a pretty rough treatment, with cuts for gutters, infrastructures, pumping pits, etc, so we can't rely that it will be there throughout the entire life of the stucture.