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Shear Friction Reinforcement

Shear Friction Reinforcement

Shear Friction Reinforcement

(OP)
According to ACI 318, shear friction reinforcement needs to be anchored to develop yield strength on both sides of the shear plane. I assume this means development length per Chapter 12. Question is, can you multiply development length by As required/As provided?

RE: Shear Friction Reinforcement

No - per the shear friction requirements you have to fully develop fy in the bars on either side of the plane. See 11.7.8 for the specific requirement.

This means that As(req'd)/As(prov) cannot be used (see reference to "except where anchorage or development for fy is specifically required..." in section 12.2.5).

RE: Shear Friction Reinforcement

(OP)
Thanks JAE.

RE: Shear Friction Reinforcement

I have been meaning to ask this very question.

I have always assumed that the shear friction development length could be prorated by As_req / As. I`m aware of what the code says but I`ve always interpreted that as an under-nuanced statement.

It`s written in the code in the form of Vr = As x u x fy. As such, one would need to develop fy.

If it can also be interpreted as Vr = As x u x fs, then it seems to me that partial development should be acceptable.

Can anyone shed some light on the reason(s) why shear friction bars need to be fully developed? Making stuff up that I don`t know to be true, here are some possibilities:

1) Maybe the shear friction attributable to each bar varies considerably and some bars need to be able to yield in order to redistribute load to others? Sort of a shear friction version of ductility?

2) Perhaps shear friction is a binary phenomenon rather than one which varies gradually? Maybe in the process of shearing across one set of saw-toothed-ish projections, full fy gets engaged?

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.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

Without full development, you cannot rely upon full tension capacity of the reinforcement. The mechanism of a shear friction failure due to failure of the bar development would be catastrophic and sudden. You are relying upon the aggregate interlock with a very small roughness/amplitude, so a very small slip of the bars would create full failure.

While this could be overkill in some instances, the risk is too high to allow it as a rule. Also, since you don't really know what the slip angle would be between the rocks, it would be impossible to compute a more precise answer.

RE: Shear Friction Reinforcement

I've always thought that one could assume a bar of a lesser yield strength than Fy and thus develop for that lower Fy. Say for example you don't need the full shear friction strength and you run a minimum number of grade 60 bars (420 MPa for the metric crowd) if you don't have the room to develop Fy then I see nothing wrong with assuming you have grade 40 bars and developing for a the assumed lower grade. Obviously your bar is oversized for this but you will get a shorter development length and a lower shear friction strength.

Maine EIT, Civil/Structural.

RE: Shear Friction Reinforcement

TME: I think that your method would be unacceptable for the same reason that mine may be: lack of a ductile clamping mechanism.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

@TX: I was hoping that this would catch your eye. I know that you do a lot of code committee stuff and I've seen you express strong opinions on shear friction numerous times in the past. If you say that ACI's intention is to preclude prorating based on partial development length, then I will accept that as correct. I'd still like to debate the reasoning though.

Parsing your statements:

1) "Without full development, you cannot rely upon full tension capacity of the reinforcement." Right. But then, if our intention is to prorate, we don't require a clamping force commensurate with fy.

2) "The mechanism of a shear friction failure due to failure of the bar development would be catastrophic and sudden." Agreed. However, we accept brittle, catastrophic failure elsewhere in concrete design. The solution is usually just a higher factor of safety.

3) "You are relying upon the aggregate interlock with a very small roughness/amplitude, so a very small slip of the bars would create full failure." I think that your overall magnitude of slip would be less for bars that are partially utilized. I can think of two factors that would come into play. Firstly, most of the "action" in bar development occurs near where the bar enters the concrete. All other things being equal, a partially utilized bar with partial development length should slip less than a fully yielded bar with full development length. Secondly, based on some euro stuff that I've read, a little bit of concrete (~2 bar dia. deep) effectively spalls away where the rebar enters the concrete. Obviously, this would increase slip.

4) "Also, since you don't really know what the slip angle would be between the rocks, it would be impossible to compute a more precise answer." The first page of the attached sketch is a simplified version of my interpretation of this statement. Do I have your intent right? If so, I would have thought that a probabilistic averaging of the slip angles tributary to each bar would iron this out.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

Over the weekend, I have convinced myself that shear reinforcement does need to be developed for fy in many instances. The reason for this that I can see, however, is that often it is difficult to guarantee that bars won't get yanked on with fy level forces for other reasons having nothing to do with shear friction. See the attached sketch for two examples:

1) The first detail is how I've been attempting to use shear friction to keep basement walls from caving in (I mentioned this detail above). In this case, I can't guarantee that intended / unintended flexure in slab won't stress the dowels to fy and thus potentially rip them out through the underside of the slab.

2) The second detail relates to the horizontal cold joints in shear walls. Any dowels placed near the wall tension chord reinforcement are likely to get strained pretty near to fy. If they're not developed for fy, they may get yanked out of the wall before the wall reaches it's intended capacity. In this case, I think that one needs to do more than develop the dowels. Rather they should be pretty much lapped to the vertical reinforcing in the wall.

I'd love to know others' thought on this.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

What friction is being relied upon for the case with an unroughened surface? Obviously you can't stick a bar in 2", but if you are up to some fair % of the development length, it seems unreasonable to use a reduced strength accounting for the less than full development length. I didn't see a limitation on shear friction for only 60 ksi steel. The development length of 40ksi is less than 60 ksi. So you should be able to do calcs based on 40ksi steel with full development length, even though the steel put in the field is 60ksi. I find it hard to believe that shear friction falls apart if the rod buster in the field uses 60 instead of 40ksi bar. That alone gives a 1/3 reduction for development length that I feel the code allows.

RE: Shear Friction Reinforcement

I have had to rely on shear friction more times that I wish due to contractor error (i.e. post installing dowels for anchor rod development). In my experience, it is usually impractical to post install reinforcement to a depth that would develop Fy. I typically have provided additional reinforcement until my ratio of:
As,reqd/As,prov ≤ embedment provided/development length. I rationalize this is my head by comparing it to expansion/adhesive anchors in which ductile steel failure hardly ever controls. I use a Φ factor of 0.75. Maybe I should reduce this since I am designing the failure mode to be brittle.

RE: Shear Friction Reinforcement

@ Dudley: I've taken 0.75 as my phi factor for brittle failure loads. I think that there are precedents for that elsewhere in the code (diagonal tension, over-reinforced beams etc.). I'm curious, is the the situation that you describe using threaded rebar and utilizing shear friction between base plates and concrete piers?

@ Dcarr: for the non-dowel action forms of shear friction, I think that the point would be that pull-out of a 60 ksi bar developed as a 40 ksi bar would still be non-ductile. I agree, however, that for pure dowel action (smooth surfaces), development ought to be less important / unimportant. I find it thoroughly confusing that dowel action is covered under the umbrella of shear friction. For true dowel action, the form of the equation is all wrong and much more attention should be given to edge distances etc. I found some European precast stuff that provides a method for checking dowel action independently of shear friction. I like that much better.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

Maybe I missed something but shear friction shouldn't rely on dowel action at all. The bar is just there to provide a clamping force (As * Fy) and that clamping force is the normal force multiplied by the friction coefficient to get a shear capacity.

Maine EIT, Civil/Structural.

RE: Shear Friction Reinforcement

I agree with your logic TME. However, the code commentary claims that shear friction for smooth surfaces is primarily based on dowel action.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

For smooth surfaces there isn't anything to clamp, thus dowel action.

RE: Shear Friction Reinforcement

Another interesting wrinkle is the issue of cohesion. I just learned about this recently.

Shear friction has two components: friction and cohesion. In the Canadian code the two parts are separated and I generally ignore the cohesion because I don't yet trust it. In the US, the cohesion values are built into mu which is why the US mu values are larger than the Canadian ones.

So, in Canada, we can have shear friction without any reinforcement at all! I'm also surprised that:

1) Cohesion and friction can be used together, in concert.
2) Cohesion can be used at all in sections that one would expect to be cracked (e.g. shear walls).

For something that looks like sixth grade physics on the surface, shear friction sure does seem to spark a lot of debate.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

I want to say I saw something in one of the recent ACI Structural Journals on shear friction research... Maybe I can dig through them before the end of the day.

Maine EIT, Civil/Structural.

RE: Shear Friction Reinforcement

I am looking at this as to how it pertains to retaining walls and slabs built into retaining walls.

@KootK It seems like this is a building heavy crowd, relying heavily on ACI for all concrete matters. However, I am primarily a bridge engineer so my first resource is the AASHTO LRFD Bridge Design Specifications (2013). If you look at Art. 5.8.4.3 AASHTO uses a Mu and cohesion much like the Canadian code, but it has the same Mu values as ACI 318-08. I don't have the 318-14 to know if the Mu values have changed.

I read through other threads on this topic that asked for information on the research of shear friction. ACI doesn't offer many references for the research, but AASHTO list several in the commentary: Loov and Patnaik, 1994; Patnaik 1999, Mattock, 2001; Slapkus and Kahn, 2004. Mattock, Li, and Wang, 1976; Mitchell and Kahn, 2001.

RE: Shear Friction Reinforcement

Current draft of ACI 318-14 still uses the phrase "Reinforcement crossing the shear plane to satisfy ... shall be anchored to develop fy on both sides of the shear plane."

RE: Shear Friction Reinforcement

@ Snow: thanks for the references. I'll check them out. The Loov work was done down the street from my office and is fascinating. I've always found it odd that his general shear resistance model didn't gain more traction.

@ TX: has there been any committee level talk of ditching that provision? Despite numerous threads on the subject, I still haven't heard a decent explanation for why partial development cannot be used.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

It may be because there are old guys on the committee, who like me, still don't believe in shear friction as a reliable means of transferring force across a joint. Shear friction depends both on clamping and dowel action, and the contributions of each are not well understood or quantified.

RE: Shear Friction Reinforcement

One other thought to consider. If you ever get mill certs on your reinforcement, a 60 ksi bar almost always has an actual yield stress of 66 ksi - 70 ksi+. 60 ksi is the MINIMUM required yield stress of the bar. For this same reason, I see nothing to preclude using a lower yield stress in the calculations for purposes of calculating a shear friction capacity.

RE: Shear Friction Reinforcement

@ Lion: that's an excellent point that I had not considered. However, development length testing was done with real rebar, presumably having realistic over strength variation. Might that phenomena already be baked into that particular cake?

@ Hokie: Trust me, I have plenty of my own issue with shear friction. However, I earned my stripes in a post-shear friction world. I don't know anything else. Without using shear friction, how would you resist sliding in, say, a horizontal shear wall cold joint? Regular friction? Dowel action? Shear keys? Part of the problem for me is that I've never seen an established, North American, capacity check procedure for the last two mechanisms. I have seen a dowel strength method in some European precast literature.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

Yes, dowels or shear keys. Primarily shear keys, like slabs supported on the cores of high rise buildings. I don't think anyone uses just shear friction for that.

RE: Shear Friction Reinforcement

Quote (hokie66)

slabs supported on the cores of high rise buildings

FYI - for those that don't peak Australian, "cores" are primarily an Aussie-term for elevator and stair shafts. Not to be confused with the voids in hollow-core slabs.

RE: Shear Friction Reinforcement

So how do you check the capacity of the shear keys hokie? Is there guidance in your codes? Rationally, I feel as though it would be (considering your highrise slab example now):

1) bearing on the underside of the key.
2) Diagonal tension in the slab based on a depth from the underside of the key to the top of the slab.
3) Vertical shearing through the width of the key. I don't know how you'd accomplish this as it's some form of shear that is not diagonal tension but is also not to be addressed by shear friction.
4) Modulus of rupture bending in the key since the load and the bearing area are not concentric.

I would very much like to see a recognized design procedure for this. Does your code provide a method for figuring dowel capacity as well?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

Maybe now it does...I don't know. I have been retired for a few years, and am sure I have missed some recent additions to the code. Possibly someone younger will chime in. But it is essentially just a bearing check, like on unreinforced masonry. There are nominal tie bars used through the joint, bendout bars or coupled bars, Rebox or similar.

http://www.danley.com.au/Product_Connections_Rebox...

RE: Shear Friction Reinforcement

@ kootk - I can't say if that's baked into the development length equations, but I would assume it isn't. I haven't seen testing data, but I would imagine they based fy on the load imposed at yield of the bar. Even if it does account for some increase in actual fy over design fy, there is no limit placed on this ratio, therefore there is nothing, in my mind, to preclude using a design fy of 40 ksi, 42, ksi, or 51 ksi, with an actual fy of 63 ksi.

RE: Shear Friction Reinforcement

What it boils down to, as with every other Code provision, is that an engineer can violate it IF they can provide evidence to the building official that their solution provide equivalent or safer design. The fact that this provision stays through each cycle makes me think there is good reason. I will not rehash my previous posts explaining my understanding of the provision and why it is required, but I still think the provision is valid and should be observed.

I doubt there are very many cases where there would be significant cost savings for cutting this corner without creating risk. I will do a little research and if there is room for changes, I will talk to 318 members about reviewing the provision next cycle.

RE: Shear Friction Reinforcement

I, for one, am not attempting to cut corners. It's not as though I'm concerned about situations where I've got all the space in the world and I want to shave a few inches off of my shear friction dowels. I'm concerned about situations -- and there are many of them -- where you simply can't get the job done if you don't use partial development in your shear friction bars. Here are two examples:

1) Field bent dowels in basement wall to main floor slab conditions. Seem my post and sketch on June 2nd.

2) The vertical joints between the panels of shear walls that we design as composite sections. My code (Canada) specifically states that interconnected wall assemblies be checked to ensure that they've got the mettle to truly behave compositely. Makse sense. And that means shear friction where "flange" panels meet "web panels". By the book, all I've got going for me there is shear friction. And if I stick with developing the shear friction bars for fy, then I'll usually need at least a 250 wall for 10M horizontal bars and at least a 300 wall for 15M horizontal bars. Obviously, the exact value depends on several parameters. The point is that satisfying this provision in this instance is a tough row to hoe and I don't really have any other options to turn to.

Developing for fy has been a thorn in the side of designers ever since I started designing a decade and a half ago. If we're going to continue to live with the inconvenience, it would be nice to have a little commentary elucidating the necessity of it. Surely someone had a rationale for the fy development business when they introduced it in the code? Can't that same party now just pull back the curtain and share that with the design community? And as we all know, the "test out" option for convincing building code officials of things is a non-starter for all but the largest projects designed at the largest firms. I won't be performing shear friction tests in my garage any time soon.

@TX: please don't interpret my frustration with this code provision as frustration with you. We've shaken hands before -- I know that you're a good guy. And I know that you're just relaying what you know about this particular issue, which is alot. I hope that you will table this issue before the ACI committee folks and either get the clause changed or have some explanation added to the commentary. If you can pull it off, there's a grain fed steak dinner in it for you the next time that CRSI sends you up north.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

I'll second KootK's frustration though I usually get around it by headed anchor rods or some other mechanical means of developing fy. Either that or I ignore shear friction and design them as dowels per ACI appendix D.

Maine EIT, Civil/Structural.

RE: Shear Friction Reinforcement

It never occurred to me to use APP.D for rebar dowels. Thanks for that TME. What I generally do is hijack the provision that allows you to assume that 15M or smaller beam stirrups can be considered developed at the bend so long as they're hooked around a bar going the other way. Of course, this isn't explicitly allowed in the code. I'll be kicking myself pretty hard the day that I find out that my cheat is invalid. No... I'll be kicking the commentary writers for forcing me to shame myself in such a fashion.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Shear Friction Reinforcement

Without diving into the code I can't recall anything that makes your "cheat" invalid. My understanding is stirrup sized bars can be considered developed if given hooks around anchoring bars.

Maine EIT, Civil/Structural.

RE: Shear Friction Reinforcement

Jut to add to the discussion about As,reqd/As,prov, ACI 318-11 12.5.3(d) uses the phrase:
(context is reduction factors for Ldh)

"(d) Where anchorage or development for fy is not
specifically required, reinforcement in excess of that
required by analysis.... (As,reqd/As,prov)"

The commentary restates this:
"The factor for excess reinforcement in 12.5.3(d) applies
only where anchorage or development for full fy is not
specifically required."

Which clearly do not allow reduction for hook length so the very least would be 8 db.

As far as using development around a bar, this is not explicitly permitted. 12.13.2.1 applies only to web reinforcement. It does seem reasonable, but we need to fix 318 if it is to be permitted.

RE: Shear Friction Reinforcement

Good info, thanks.

Maine EIT, Civil/Structural.

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