Need some details of a "chain wall" and a good definition of it 1

[Ron247]

I have never heard the term "chain wall" used and I have been doing foundations over 30 years. Now I have heard the term used twice in 2 weeks on projects that are unrelated. I asked the 2nd person that used it to describe to me how it was built since he actually built the one he was referring to.

The house is a Slab on Grade and the SOG is only about 4" above exterior grade. He described a strip footing with a stem built up off it that the floor slab ultimately sets on. There is no void under the slab. It is backfilled. The chain wall went around the perimeter and connected the midpoints of the exterior to effectively quarter the slab.

I then looked on the internet and see a description of a concrete system that sounds like it sets on a strip footing that elevates the structure above some "flood elevation". Both projects I was reviewing are only about 4" above the ground and both are SOG. There were not any construction details on the internet to depict a chain wall system so that is the point of this posting.

Does anyone familiar with either of the 2 chain wall descriptions I have mention have a detail that depicts it? I am more interested in details of the Internet described one but also need one for the SOG. The one described by the Client sounds like a strip footing with any type of stem (grade beam or masonry block) that I am familiar with but never heard it called a chain wall.

Strange that you can go 30-40 years without hearing something and then get bombarded with it every time you turn around. Kind of like Roulette, bet the 12 until you are bankrupt and it never hits, then when you have no more money to bet, the 12 hits 3 times in the next 5 spins.

 

[dauwerda]

Per FEMA,

https://www.fema.gov/media-library-data/20130726-1707-25045-4311/chapter6.pdf:

Stem wall foundations are similar to crawlspace foundations. They consist of perimeter foundation
walls (typically masonry or concrete), but the interior space that would otherwise form the crawlspace
is backfilled with soils that support a floor slab. In the Gulf Coast region, these foundations are often
referred to as “chain walls.” Figure 6-7 shows a cross section of a typical stem wall foundation.

 

[Ron247]

Thanks a million dauwerda, exactly what I was looking/hoping for. That is how it was described to me. The only difference is the slabs are on top of the stem and not formed into the side of it where I am looking.

 

[JoelTXCive]

I live in the gulf coast region and have never heard that term.

Sometimes people say "cheek walls" for slabs or foundations that drop and have an exposed face. Maybe that is what they meant?

Learn something new everyday I guess....

 

[Ron247]

Oddly enough I created a post months back about similar construction. I have seen cracked block walls on a Slab on Grade where they made the wall about 5' to 10' above the ground. It was like a reverse basement. I never had found anything in the IRC about building them other than a note that walls taller than 4' required an engineer. These were similar to these new-fangled chain walls but the height was ridiculous. They were unreinforced 8" block in may cases. Cracked and leaning.

 

[r13]

I doubt the wisdom of using block foundation wall without reinforcement, except as permitted by code/standard.

 

[KootK]

Will these walls typically get braced on the exterior side until the SOG restraint is in place?

 

[dauwerda]

Here is the rest of the excerpt from the FEMA document linked above:

Anecdotal evidence suggests that during flood events stem wall foundations have performed better than
many crawlspace foundations. However, it is important to note that the prescriptive designs set forth in
the building codes often limit stem wall height to just a few feet. While higher stem wall foundations can
be designed, the cost of suitable fill and proper fill placement often makes their use impractical. The use
of stem wall foundations is prohibited within V Zones and is not recommended for use within Coastal A
Zones; however, their use is appropriate within A Zones where wave heights are 1.5 feet or less and where
footings are deep enough to resist scour and erosion.

Stem wall foundations do not require vents to equalize the pressures exhibited by floodwaters. They do,
however, need to be strong enough to resist lateral pressure from retained soils. Because the retained soils
can become saturated during a flood event, additional reinforcement is typically needed.

FEMA 550 has prescriptive flood- and wind-resistant designs for stem wall foundations. Designs are
provided for two types of walls: cantilevered and laterally supported. In cantilevered walls, reinforcing
steel provides sufficient strength to resist lateral forces without relying on the floor slab to laterally support
the top of the wall. In laterally supported walls, the top of the wall is tied to the floor slab; the typical
result is that less vertical reinforcement is needed in the foundation. Laterally supported walls must be
braced to prevent movement or collapse when backfilling.

 

[Ron247]

No KootK, where I live, they lay up the block, place the backfill but do not compact the backfill much and then pour the slab. There is no exterior bracing installed that is used until the slab to stem joint is cured. I had one contractor explain how you loosely install the backfill to keep from having it bow outwards before the slab in installed. Yeah, an later it compacts on its own to some degree and still pushes the wall outwards. It would then have a sag in the slab, cracks in the slab and a leaning wall that is really cracked near the corners and center.

Their slab to stem joint a lot of times is only concrete poured into a header block with no steel. Also, the block wall is hollow. That tends to loosen the header block at its horizontal mortar joint.