Shear Strength of Dowel from Continuous Footing into Wall
Shear Strength of Dowel from Continuous Footing into Wall
(OP)
I am looking at a 12" concrete basement wall. Its connection to a mat foundation consists of a single row of dowels from the mat into the wall that align with the outer mat of rebar in the wall. In my way of thinking these dowels provide almost no shear resistance at the bottom of wall to soil and groundwater loads. This is because the dowels are located at the loaded face of the wall. What would others use for shear strength of the connection under this condition? No shearkey is present






RE: Shear Strength of Dowel from Continuous Footing into Wall
1) There will be flexure at the joint that will stress the dowels in tension on the exterior side.
2) The tension in #1 will be balanced by a compressive force on the interior side.
3) The compression in #2 will provide the necessary clamping force for non-dowel action shear friction to develop.
I share your concern, however. I always have to fight the urge to place some dowels on the interior face as well. When I have dowel action stuck in my head, it feels as though the exterior dowels will just pop out of the wall.
Another thing that causes me grief is that the shear friction mechanism that I described above DEPENDS on a moment being developed at the joint. If the load on the wall is well distributed, as one would expect, then developing the necessary moment ought not be a problem. If, hypothetically, the resultant lateral load were to be applied one foot above the base of the wall, however, you'd be in trouble.
I worry about a similar thing for ramp slabs cast against CIP walls. I feel good about shear friction when I imagine cars driving down the centre of the ramp. I feel less good about cars driving beside the walls.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear Strength of Dowel from Continuous Footing into Wall
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
It's also important to note that, in this situation, you can use 100% of your flexural reinforcement as shear friction steel and vice versa. This is because, when the bars are stressed in tension due to flexure, you get a corresponding compression force which provides the clamping required for shear friction. You get to double dip so to speak.
It's not As = A_flex + A_sf.
Rather, it's As = max (A_flex, A_sf).
Since you likely designed your wall as pinned at the bottom anyhow, it really just comes down to A_sf.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear Strength of Dowel from Continuous Footing into Wall
I don't agree on the double dip with a smooth surface. With a rough surface maybe, but not with a smooth surface.
RE: Shear Strength of Dowel from Continuous Footing into Wall
Like you, I don't agree with the double dip -- or even the logic of a clamping force -- for a smooth surface. However, ACI does. ACI even explicitly recommends putting your dowels in the tension zone. While this might not alleviate your conscience altogether, it should be a comfort to know that you'd be fine in court.
If you're desperate, you could:
1) Use your dead load as a permanent clamping force in lieu of rebar. Even just the weight of your basement walls will add up to something.
2) You could use a seperate cohesion value of 0.25 MPa like the Canadian code allows. Obviously, using provisions from another jurisdiction's code requires careful consideration.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
I read up on this recently. Apparently shear friction has two components: cohesion and friction. The code mu values are inflated to incorporate the cohesion component, not because of dowel action. If you have MacGregor's book, he gets into this in some detail.
KootK
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Shear Strength of Dowel from Continuous Footing into Wall
RE: Shear Strength of Dowel from Continuous Footing into Wall
I am not a proponent of shear friction, but your statement that the location of reinforcement crossing the joint is obviously incorrect for a situation where significant moment is present. Without reinforcement on the tension side, there is nothing to resist the compressive force required to create friction. There was a recent discussion in this forum of the same subject.