Shear-Friction Design Method for Reinforced Concrete 4

[clemsonC]

I have ACI 318-02 (11.7.4) and a reinforced concrete textbook. Here is my understanding of the shear-friction design method:

This method of determining Vn considers shear capacity of "dowels" as well as the friction between adjacent faces of a plane of concrete, whether the plane is the result of cracking or separate placements of concrete.

1) Is this correct?

2) What qualifies as shear friction reinforcement?

3) What are the limitations of this method, beyond the prerequisite that the bar be tensioned by the shear force?

4) What are some appropriate scenarios for use of this method?

Thanks.

 

[miecz]

In the design of a cantilevered retaining wall, the interface of the stem and the footing has a compressive force (due to the weight of the stem) and a bending moment. Ignoring the compressive force, it seems from the forgoing discussion that it would be acceptable to use the tensile reinforcement at the back of the stem as shear friction reinforcement, and eliminate a shear key. However, I worry that this reinforcement cannot provide the necessary clamping force, as it has only 2 inches of cover to the face of the wall. I would seem that any shear displacement could spall off that 2 inches of concrete and bend the bar without providing the clamping force.

 

[J1D]

Just came across this thread and found it is an interesting discussion. The “double yielded” point sounds right, but not 100% from my view.

1. The reinforcement across the interface contributes to the shear resistance with the tension caused by crack/interface opening under shear.
2. Consider a construction interface at the base of a retaining wall, it subjects to shear and bending. And assume the rebar in tension is At and rebar in compression is Ac. Case 1, consider the bending first to the ultimate state (At yielded, Ac in compression but may not yielded, the concrete at the ultimate fibre reached to the max. strain), then apply the shear. The shear will be resisted by the friction of the concrete in the compression zone. Because of the rough interface, the shear will cause a widening of the crack. The change of the internal force will be: additional tensile strain in the At and compression release in the compression zone. Once the At can accommodate the strain, the shear friction doesn’t reduce or undermine the bending capacity.
3. Case 2, have the shear go first and stress the rebar to ultimate: At*Fy and Ac*Fy. Under the additional bending, the interface will perform somehow like a prestressed section, the interface compression around At will compensate the tension from bending. In another word, the bending doesn’t cause another At*Fy. Like pretensioned anchor bolts (e.g. stress the bolts to 0.7Fy doesn’t mean the interface has only 30% bending capacity left.

The actual loading and internal force should be more complicated. But if ACI has experimental demonstration, maybe it is about right.

 

[miecz]

J1D - In case 1, when you add the shear, I agree that the shear will cause a widening of the crack. But if the tension steel (At) had already been stressed to yield, any widening will cause little further stress in At, assuming a flat stress/strain curve beyond yield. As I see it, the widening will cause an increase in compression in the concrete near the front face. The increase in compression in the concrete will be balanced by a tension in the steel at the front face (Ac). As such, the steel at the front face is the "shear friction steel".

In case 2, when you add the bending moment, the interface compression around At will decrease, and the compression around Ac will increase. The stress in At is constant, while the stress in Ac decreases, but remains in tension. Again, At resists the bending and Ac provides the shear friction steel.