Underpin lightly loaded residential foundation walls

[sbouvia]

We are having a discussion in regards to providing underpinning for lightly loaded residential walls. The existing basement wall is 6'-0" tall (from brick slab on grade to underside of existing floor joists) 8" brick masonry with a two story plus attic wood framed structure above. The existing grade is at the first floor level. The intent is to underpin the foundation so that the basement can have a clear height of 9'-0" to the underside of the floor joists. So a new concrete underpinning wall will be constructed below that is approximately 4'-0" tall to allow a 4" s.o.g., insulation and drainage material to be installed on the basement side of the wall. My issue is because the walls are so lightly loaded above that the interface shear between the two materials is almost impossible to resist without adding some sort of supplementary support such as a horizontal angle.

How does everyone normally look at the interface shear between the new and existing? What about the analysis model utilized?

I have seen essentially a few different types of analysis models utilized for this condition.

One is to assume the new and existing will act together and just be a basement wall. So this could have a pinned top and pinned bottom and then have to resist moment and shear at the joint between the two members.

The second option is to have a fixed base, a hinge at the interface between the existing/new and a pinned top. This essentially means the loading on the existing wall is unchanged.

 

[jayrod12]

Teh second option is easier from a design and construction standpoint, i.e. you don't have to detail a moment connection at the interface. And tehn you don't have to rely on the contractor installing it correctly either which is easier for everyone involved.

 

[sbouvia]

I like the second option, however with such a light loading I am having issues transferring the shear between the existing and new.

One option is dowels into the existing brick masonry from below. Not really what I want to do as it is only 8" thick brick masonry and you are installing dowels overhead.

Second option is providing a continuous angle that I would post install into the concrete underpinning by making the underpinning thicker than the wall which essentially leaves a shelf to install the angle and post installed anchors. Thinking of making the underpinning below 13" so I have a 5" shelf to work with.

Third option is to assume the the joint (dry-pack) between the new and existing is essentially a large mortar joint and then look at using like 10psi times the area of that joint (8"x12") to figure out an allowable shear through the joint in lbf/ft.

Fourth option is to use a continuous horizontal plate in bending to deliver the load to anchors installed in the concrete underpinning after it has reached design strength.