Shear-Friction Design Method for Reinforced Concrete
Shear-Friction Design Method for Reinforced Concrete
(OP)
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.
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.






RE: Shear-Friction Design Method for Reinforced Concrete
1) Yes
2) both the longitudinal rebars and stirrups are shear Friction reinforcement,
3) stirrups crossing the failure plane are assumed to be at yield, shear forces are limited so that will not cause compression failure in the concrete, and the beam is as strong as its weakest shear plane, and
4) it is allowed to use the shear friction method to consider interfaces between elements such as webs and flanges, between dissimilar materials and between concrete cast at different times or at existing or potential majot cracks along which slip can occur.
RE: Shear-Friction Design Method for Reinforced Concrete
DaveAtkins
RE: Shear-Friction Design Method for Reinforced Concrete
RE: Shear-Friction Design Method for Reinforced Concrete
Is this correct? - I would say no - the shear friction method is all about the friction between two surfaces of concrete - dependent on the roughness of the concrete. The shear friction works due to the ability of the reinforcing to hold the surfaces together and this engages the rough edges such that slippage is minimized.
So the concrete works in shear, the reinforcing works in tension. Shear capacity of dowels are not involved.
RE: Shear-Friction Design Method for Reinforced Concrete
RE: Shear-Friction Design Method for Reinforced Concrete
JAE & cap4000: ACI 318-02 defines Vn = Avf*fy*mu. So, you're clarifying that any reinf crossing the shear plane is acting in tension, not shear. Right?
It seems that you've all discussed existing shear planes, due to noncurrent/nonmonolithic placements of concrete. So here's another question:
If a monolithically placed reinforced structure with no shear reinf (say, a 2-way slab with reinf EW) fails/cracks in shear, THEN I can account for the reinf that crosses the shear plane, because now that reinf functions as dowels?
RE: Shear-Friction Design Method for Reinforced Concrete
For concrete cast against hardened concrete not roughened in accordance with 11.7.9, shear resistance is primarily dur to dowel action of teh reinforcement and tests indicated that reduced value of mu = .6 x lambda specified for this case is appropriate.
So for very smooth conditions, they do seem to depend upon shear resistance of the dowels...but not for the other cases where there is a roughness present.
RE: Shear-Friction Design Method for Reinforced Concrete
RE: Shear-Friction Design Method for Reinforced Concrete
DaveAtkins
RE: Shear-Friction Design Method for Reinforced Concrete
When you have a bending moment acting on the section in question, it creates a force couple on the section...compression on one side, tension on the other.
These forces are equal and they are due to the bending moment. In this case you are better off just using the compression force for your shear friction and ignore the tensile force of the reinforcing which will be needed for flexure.
If you had the same section but it was only used to resist shear (any moment was insignificant) then you would utilize the AsFy of the steel crossing the section as your shear friction resistance since you would have no sustained compressive force to use.
If you have a significant bending moment AND you want to use the AsFy of the reinforcing (in lieu of the compresive force) for shear friction you DO need to use a combined stress approach....usually the compressive force alone is enough to get you there in my experience.
RE: Shear-Friction Design Method for Reinforced Concrete
When moment acts on a shear plane, the flexural tension stresses and flexural compression stresses are in equilibrium. There is no change in the resultant compression Avf(fy) acting across the shear plane and the shear-transfer strength is not changed. It is therefore not necessary to provide additional reinforcement to resist the flexural tension stresses, unless the required flexural tension reinforcement exceeds the amount of shear-transfer reinforcement provided in the flexural tension zone. This has been demonstrated experimentally.
RE: Shear-Friction Design Method for Reinforced Concrete
I suspect it does not, and what the commentary is really saying is that you can use the compression component of the force couple without having to add any additional AsFy.
AASHTO does a better job of explaining this I believe.
RE: Shear-Friction Design Method for Reinforced Concrete
So your SF equation would be:
φVn = (C + Avf(fy)) x μ
RE: Shear-Friction Design Method for Reinforced Concrete
...it's a good thread though, shear friction should be better understood by all (including me!)
RE: Shear-Friction Design Method for Reinforced Concrete
RE: Shear-Friction Design Method for Reinforced Concrete
after reviewing ACI and the PCA Notes I agree with your formula but only if the "C" compression force in the formula below is due to a sustained compressive force NOT already utilizing the AsFy.
?Vn = (C + Avf(fy)) x ?
If you have gravity loads that are causing a significant compressive force you can add this to the shear friction capacity....
..but if a bending moment is causing the "compressive force" AND the reinforcing in question is already being used to resist this flexural stress then you cannot use this compressive force in addition to the AsFy.
In summary, unless you have a significant gravity load component then ACI will only allow a shear friction capacity of ?Vn = Avf(fy)) x ?
RE: Shear-Friction Design Method for Reinforced Concrete
DaveAtkins
RE: Shear-Friction Design Method for Reinforced Concrete
The bending causes a compressive force which can be as high as AsFy but that's all you get without an additional gravity compressive force...period.
You can't use the compressive force due to bending as the "additional compressive force" and add this to AsFy.
The AsFy is already using this bending compressive force based on your own definition above.
RE: Shear-Friction Design Method for Reinforced Concrete
RE: Shear-Friction Design Method for Reinforced Concrete
RE: Shear-Friction Design Method for Reinforced Concrete
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.
RE: Shear-Friction Design Method for Reinforced Concrete
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.