Shear Friction Reinforcement (ACI 318-11) 1

[humanengr]

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.

 

[TLHS]

In the large number of discussions we've had, I'm pretty sure everyone agrees that the code says the bar needs to be developed to the full strength of the bar.

People disagree about whether that's actually physically required for shear friction to work, but it's definitely what the code says.

 

[TehMightyEngineer]

I'll second what TLHS said. Essentially you need to fully develop the bar at the critical section for the shear friction to be used. You get no credit for over-reinforcement that I'm aware of per ACI 318.

[Enter KootK STAGE LEFT to provide a highly technical discourse on the rational behind this.]

Maine Professional and Structural Engineer.

http://www.fepc.us

 

[KootK]

What's that? Free pizza? Shear friction?. I'm there.

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.

 

[MacGruber22]

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.

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."

 

[humanengr]

Thank you all for the feedback and responses. I accept your responses at face value and accept that the intent of ACI 318-11 is that the bars at critical section shall be fully developed.

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.

 

[MacGruber22]

Hate to be-labor the obvious, but what if permanent net compression is adequate to resist all but 1% of the shear friction?

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."

 

[humanengr]

MacGruber22, I understand that (what you have quoted from ACI). My point, however, is that I need full development length to resist just the remaining 1% when far less would be sufficient.

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.

 

[TLHS]

In that case you could likely rationalize it with dowel action of the rebar, or cohesion between the shear planes.

 

[TXStructural]

The answer is "yes" (to whatever the question was.)

When shear acts along a crack, one crack face slips relative to the other. If the crack faces are rough and irregular, this slip is accompanied by separation of the crack faces. At nominal strength, the separation is sufficient to stress, in tension, the reinforcement crossing the crack to its specified yield strength. The reinforcement in tension provides a clamping force Avf fy across the crack faces. The applied shear is then resisted by friction between the crack faces, by resistance to the shearing off of protrusions on the crack faces, and by dowel action of the reinforcement crossing the crack.

 

[KootK]

@humanengr: several, unofficial hypothesis were proposed in that thread that I linked previously. Of them, I find the most convincing argument to be a lack of ductility in the connection when bars are not fully developed on either side of the joint. Try the following example on for size:

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.

 

[MacGruber22]

humanengr, oops. My response was incomplete. I meant to follow the ACI quote with, "If you used this ACI provision properly it seems you are so close you could rationalize that 1% to go away." There is the little quote icon in the text editor (2nd one left of the red/yellow present-looking button).

Also, if you have flexural reinforcement across this shear-friction plane, you can use that A_s 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."

 

[abusementpark]

Most engineers I know prorate the shear friction capacity when the bar is not fully developed. There are MANY, MANY common details where this occurs and has occurred for years. If it were a major problem to do this, I'd think we would have heard about it by now.

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.

 

[KootK]

I expressed a nearly identical sentiment in a past thread abusement (Link).

It has always seemed to me that a number of commonly used details would not work if shear friction development length cannot be prorated. In my neck of the woods, foundation walls do not get poured higher than the underside of ground floor slabs. As such, the only thing keeping the basement walls from caving in is shear friction at the slab / wall interface. I attempt to muster that shear friction by running small diameter dowels up from the exterior of the basement walls and bending them horizontally into the ground floor slab. Generally, there is not enough slab depth available to fully develop the bars. As a belt and suspenders thing, I place longitudinal 15M bars in the bend and hope that I can consider the bars fully developed at the bend like you can with beam stirrups.

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.

 

[MacGruber22]

I suspect a great deal of that apparent "over capacity" is due to redundancy/redistribution of forces when a local overload occurs.

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."