Shear Friction Reinforcement (ACI 318-11)
Shear Friction Reinforcement (ACI 318-11)
(OP)
ACI 318-11, R11.6.8 states; "shear-friction reinforcement... should have full tensile anchorage..."
I take that to mean the anchorage must develop or resist the tensile force used to calculate shear friction, and not develop the full tensile strength of the reinforcement. Does anyone disagree with that?
If the reinforcement was equal to required Avf, then that is one and the same. However, if it is over-reinforced, I'm thinking I don't need to fully develop the reinforcement. That would be similar to reducing the required re-bar development length based on excess reinforcement.
I take that to mean the anchorage must develop or resist the tensile force used to calculate shear friction, and not develop the full tensile strength of the reinforcement. Does anyone disagree with that?
If the reinforcement was equal to required Avf, then that is one and the same. However, if it is over-reinforced, I'm thinking I don't need to fully develop the reinforcement. That would be similar to reducing the required re-bar development length based on excess reinforcement.






RE: Shear Friction Reinforcement (ACI 318-11)
People disagree about whether that's actually physically required for shear friction to work, but it's definitely what the code says.
RE: Shear Friction Reinforcement (ACI 318-11)
[Enter KootK STAGE LEFT to provide a highly technical discourse on the rational behind this.]
Maine Professional and Structural Engineer. www.fepc.us
RE: Shear Friction Reinforcement (ACI 318-11)
X3 for TLHS's response.
The reason for requiring full development on shear friction has never been explained to my satisfaction I'm afraid. I would refer humanengr to this thread: Link. To my knowledge, that is the most theoretically comprehensive thread on this subject. Certainly, that's the one where I went to the mat trying get things sorted. There's some characteristically great stuff from TXStructural there too.
If KootK ever, ever finds out that the only reason for fy development on shear friction is because that's the only way that it's been tested so far... heads is gonna roll.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Shear Friction Reinforcement (ACI 318-11)
Lol! I wouldn't be surprised if that is why. However, if one day there is an allowance for As provided/ As req., that would be a good day.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Shear Friction Reinforcement (ACI 318-11)
However, logically, I just don't see it. If the shear to be resisted is 30 kips, for example, a certain quantity, area of rebar, Avf(o) is required. If the load is reduced to 5 kips, I can use (5/30)Avf(o) or one sixth the original reinforcement and that would be OK per code.
If, however, I'm checking an existing condition and reducing the reinforcement is not an option, having more reinforcement than needed, somehow reduced the capacity?? I just can't see that logically. By accounting for less than full development of the bars, isn't that effectively the same as having less reinforcement - an amount of reduced reinforcement that is adequate for the applied load?
Hate to be-labor the obvious, but what if permanent net compression is adequate to resist all but 1% of the shear friction?
Do I still need rebar for 100% of load to resist just 1%?
I wonder if ACI has confirmed this was their intent, or acknowledged its a limitation due to lack of testing.
RE: Shear Friction Reinforcement (ACI 318-11)
ACI 318-14 22.9.4.5
"22.9.4.5 Permanent net compression across the shear
plane shall be permitted to be added to Avf fy, the force in the
shear-friction reinforcement, to calculate required Avf."
"R22.9.4.5 This provision is supported by test data
(Mattock and Hawkins 1972) and should be used to reduce
the amount of shear-friction reinforcement required only if
the compressive force across the shear plane is permanent."
"It is imperative Cunth doesn't get his hands on those codes."
RE: Shear Friction Reinforcement (ACI 318-11)
BTW, how did you get that quote in your post? I tried using the "quote" icon, but it did not work. I must not have done it correctly.
RE: Shear Friction Reinforcement (ACI 318-11)
RE: Shear Friction Reinforcement (ACI 318-11)
RE: Shear Friction Reinforcement (ACI 318-11)
Imagine a shear wall joint where the shear wall is interrupted by the slab pour. The shear transfer across the joint is classic shear friction. Additionally, let's assume that:
1) We have 75% more rebar crossing the joint than we need.
2) Because we have more rebar than we need, we reduce the development length by 25%
3) For some reason, the concrete on the left 50% of the wall is rougher than the concrete on the right 50% of the wall. Let's say the amplitude of the roughening is 8 mm vesus 6 mm.
One could envision the road to failure looking something like this:
1) Given the same initial shear displacement, the dowels on the left half of the wall would be strained, and stressed, 33% more than the dowels on the right half of the wall due to the greater roughness on the left side.
2) At 75% x fy, the bars on the left half of the wall would reach their maximum stress based on bond strength. Simultaneously, the bars on the right would be at 1/1.333 x 0.75 fy = 0.5625 fy. At this point, we've only achieved 87.5% of the clamping force required for full capacity.
3) With further strain, the capacity of the shear friction dowels on the left side of the wall drops to zero. This is because bond failure is brittle. The remaining, effective dowels on the right side of the wall now get stressed to 75% Fy prior to brittle bond failure. At this point, we've only got 50% of the clamping force required for full shear capacity.
The moral of the story is this: unless you can guarantee and even distribution of shear friction dowel strain, providing for <fy development is dangerous and may result in a brittle, unzipping style failure.
Of course, this is just my theory. To my knowledge, It appears in print nowhere other than this forum.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Shear Friction Reinforcement (ACI 318-11)
Also, if you have flexural reinforcement across this shear-friction plane, you can use that As to count towards your shear friction.
KootK - that is a very convincing argument to maybe why they have omitted the reduction. You can't count on what you can't count on. Particularly, when you are counting on an intentionally-roughened surface. You have to trust that the contractor will provide the 1/4" amplitude surface consistently throughout, and that inspections catches when it is not provided consistently.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Shear Friction Reinforcement (ACI 318-11)
If you are prorating the capacity, it would be prudent to make sure there is a nice bit of additional reserve capacity provided. I never like to push shear-friction to a stress ratio of one, full development provided or not.
RE: Shear Friction Reinforcement (ACI 318-11)
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Shear Friction Reinforcement (ACI 318-11)
Can't rely on that so much when you are designing a beam/column construction joint, repair, etc.
"It is imperative Cunth doesn't get his hands on those codes."