Shear Friction: Where and When? 2

[KootK]

Over the years, I've made a rather unexciting hobby out of asking other structural engineers a seemingly simple question: "when do shear friction provisions apply?" I ask because, frankly, I don't know myself -- not with any certainty at least. I get a range of responses, often in combination:

1) Shear friction applies at cold joints.
2) Shear friction applies as an alternate when Vc + Vs can't be made to work. This is dangerous in my mind.
3) Shear friction applies at abrupt changes in cross section, like the interface between the flange and web of a tee beam.
4) Shear friction applies at any assumed future crack. This seems pretty vague to me.

I have come to believe that shear friction must be satisfied at all locations within a member where shear is present. This includes cold joints, abrupt changes in cross section, assumed future cracks, and anywhere that diagonal tension would be checked. Basically, anywhere that a shear diagram is not zero, shear friction needs to be satisfied. Please refer to detail "A" of the attached PDF for an illustration of my thinking on this. I believe that if one imagines a vertical cut through a monolithic concrete beam between stirrups, equilibrium of the resulting free body diagram will demand that a shear resisting mechanism falling under the shear friction umbrella be developed.

Now that I've expressed my heretical view that shear friction needs to be satisfied at all locations in monolithic members, the next logical question becomes: "do I need to check shear friction at all locations?" Every time that I've designed a beam in the past, should I have divided it up into ten segments and checked shear friction at each section? I hope not. In fact, I've come to the conclusion that shear friction need only be checked at cold joints in properly detailed concrete members. Please refer to detail "B" of the attached sketch. I speculate that the compression fields present in most concrete members simulate longitudinal prestress and result in the automatic satisfaction of shear friction demands for monolithic members. One hole in this theory is the very fact that the code provides mu values for monolithic concrete. If shear friction need only be checked at cold joints, why bother with a monolithic value?

So my questions for the forum are:

1) In what situations do you think that shear friction needs to be satisfied?
2) In what situations do you think that shear friction needs to be checked?

Thanks for your help.

KootK

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

 

[hokie66]

While I am not a proponent of the shear-friction theory used in North America, I thought it was primarily intended for perpendicular construction joints. The diagonal tension provisions of various codes have served us well, so there is no need for an additional check for that type shear.

 

[mtu1972]

I have to admit that I only used it at cold joints or at column brackets (infrequently used). I only used vertical stirrups, but always wondered if diagonal stirrups would be preferable so as to have more than one stirrup cross any vertical plane. You have obviously thought about this in far more detail and your question regarding the monolithic value is very pertinent. I hope other comments can help clear this up for all of us.

gjc

 

[dcarr82775]

I have only looked at it at construction joints.

 

[KootK]

Thanks for your contributions gentlemen. So far, everyone's been answering question #2: when should we check shear friction? And it sounds as though we're quickly coming to a consensus. We've been checking at cold joints.

If you'll indulge me, I desperately want your answers to question #1. I want to know what you think of my conclusion that shear friction needs to be satisfied, although not necessarily checked, everywhere.

To steer the conversation to the heart of the matter, please review the attached sketch. In that sketch, I suggest that shear friction must be at work on a vertical cut through the beam in order for equilibrium to be satisfied. Do you agree or disagree? I love explanations but will also happily accept a yes/no vote.

@Hokie: I realize that you're a not a fan of shear friction, perhaps not even a believer. With that in mind, what shear resisting mechanism do you see at work across the vertical section cut in my sketch? I'm betting that it's shear friction by another name: aggregate interlock, compression block friction, dowel action. I've waited a long time for my chance to try and lure you to the dark side...


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

 

[dcarr82775]

I pretty much only use it at #1 when trying to connect something new to something existing. I have never used it as described in #2. I may have used it at #3 (I can't recall senator), and never as #4.

I don't think your sketch applies. The shear cracks happen at the 45, not vertically. You do not get vertical shear cracks (principal stresses and all that). I tend to look at shear friction at the construction joints since there is no(?) other codified (ACI anyway) manner to transfer the load across that joint. I actually think shear friction makes perfect sense for the unroughened and roughened cases per ACI.

 

[DaveAtkins]

I agree with dcarr82775. Shear cracks are not vertical--they are diagonal. You have sketched a failure mechanism which is not consistent with experience.

DaveAtkins

 

[KootK]

Yes. The applicability of my sketch is precisely the issue Dcarr. I'm going to sell it a little harder to see if I can't make a convert out of you.

For any given location along the length of the beam in my sketch, I argue that shear capacity must be satisfied for all possible orientations of the shear crack (15 deg, 30 deg, 45 deg...90 deg). The reason that we check a 45-ish degree crack in practice is simply because that's the one that generally governs as a result of concrete's inherent weakness in tension. Just because a 90 degree shear crack doesn't govern, doesn't mean that a shear mechanism isn't required along a 90 degree plane. And I argue that the only shear mechanism available for the 90 degree plane is shear friction.

Looked at another way, it's just a matter of statics/equilibrium. If you take the FBD that I've drawn, with the section cut at 90 degrees, diagonal tension isn't available as a shear resisting mechanism. Something else must be getting the job done. Again, I think that something is shear friction.

As an aside, note that for a prestressed member, it is entirely appropriate to think of your shear crack at an angle other than 45 degrees.

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

 

[KootK]

I should reiterate that I am not suggesting that shear friction needs to be checked gratuitously in monolithically cast members. Rather, I'm asking:

1) Is shear friction, as a necessary mechanism, not present everywhere that shear is present? It seems to me that it is.

2) We're all confident that shear friction doesn't need to be checked in monolithic concrete. How do we know that other than testing and experience? I pitched my theory at the beginning of this thread.

These two points are just rephrased versions of the two questions that I posed originally.

I've attached a good article by Loov that gets into this a bit. I'm sure that no one will have the time to read it but I thought that I'd throw it out there anyhow for sport.

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

 

[hokie66]

KootK,
Sorry, but I'm not going to be drawn into debating the METHOD of shear design. Research is good, but it sometimes produces METHODS which are not required. Based on the various threads here indicating confusion with even the intent of the shear friction theory, I will just choose not to use it.

 

[KootK]

I understand Hokie. Despite the title of this thread, shear friction buy in isn't really necessary for participation. I'll rephrase the question in a way that doesn't point to shear friction:

For the vertical shear plane that I've been harping on in my sketches, what shear resisting mechanism do you think is at work?

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

 

[hokie66]

As you listed above. Aggregate interlock, dowel action, etc., but I just think of a truss analogy.

 

[PicoStruc]

My little contribution to kootk and Hokie discussion.

Canada Concrete Code required minimum shear reinforcement only where it is required : Vf > Vc (Clause 11.2.8.1)

So in my opinion, you cannot used the 'Interface Shear transfert (11.5)' equation with the reinforcement dowel action effective. Just use Vc (11.3) plain simple !

 

[KootK]

@Hokie: thanks for clarifying your view. It may interest you to know that it was the truss analogy that got me thinking about this. I've started a separate thread about that: Link. It's related but deserving of it's own discussion I think.

@PicoStruc: thanks for joining in the conversation. Vc is, of course, diagonal tension failure. As such, it will not contribute to shear resistance over the vertical section that I've directed attention to in this thread. I'm not sure how 11.2.8.1 impacts a designer's ability to utilize shear friction. Can you elaborate on that?

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

 

[dcarr82775]

I think it doesn't control so the limit state doesn't apply. Your sketch still gives you the diagonal tension afterall. I suspect the vertical case you are curious about is so far from controlling that it doesn't matter, you can't practically get there. This is all speculation on my part of course.

 

[KootK]

@Dcarr: I mostly agree. In the majority practical applications, shear friction on a vertical plane won't govern. I wouldn't say that it's miles away from mattering however. The monolithic value is only 40% stronger than the roughened value after all. And, in the general case for monolithic construction, the designer is not paying any explicit attention to shear friction reinforcement in the form of +ve/-ve steel.

I'm not suggesting for a second that I think that vertical plane shear friction will govern in monolithic construction. In fact, in my initial post, I proposed a theory attempting to "prove" why it won't ever govern. What I'm trying to accomplish here is to build myself a world view of sorts that will allow me to better understand the mechanics of shear friction and concrete in general. Your speculation is exactly what I'm seeking here.

Your sketch still gives you the diagonal tension afterall

Can you expand on that Dcarr? I am of the opinion that, at any given location, shear needs to be addressed on both a diagonal plane and a vertical plane (every conceivable plane really). Having satisfied diagonal tension doesn't automatically mean that shear friction on the vertical plane is satisified. That is, unless you're buying into my "proof" in my original post. And I'm not even sure that I believe my proof.

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

 

[PicoStruc]

Kootk,

Yes the Vc+Vs is based a strut/Tie model, but Vc can be used alone in a design point of view (see one way shear) as described in clause 11.2.8.1 to resist shear AND torsion !!!

11.2.8.1 impacts a designer's ability to utilize shear friction because your 'Interface Shear Transfert' resistance that include dewel action cannot be greather than Vc. If it is the case, then minimal shear reinforcement is required.

 

[TehMightyEngineer]

To me ACI 318 provides very clear guidance on when to apply shear-friction. Per 11.6.1: Provisions of 11.6 are to be applied where it is appropriate to consider shear transfer across a given plane, such as: an existing or potential crack, an interface between dissimilar materials, or an interface between two concretes cast at different times.

The commentary for 11.6.1 states: "With the exception of 11.6, virtually all provisions regarding shear are intended to prevent diagonal tension failures rather than direct shear transfer failures. The purpose of 11.6 is to provide design methods for conditions where shear transfer should be considered: an interface between concretes cast at different times, and interface between concrete and steel, reinforcement details for precast concrete structures, and other situations where it is considered appropriate to investigate shear transfer across a plane in structural concrete."

So, to me shear friction is to be applied only when the normal diagonal shear crack design assumption is not valid.

Maine EIT, Civil/Structural.

 

[KootK]

@TME: Thanks for joining the discussion.

"...or potential crack...". That leaves rather a lot of room for interpretation wouldn't you say? Will any old thermal restraint or flexural crack do?

"...other situations where it is considered appropriate to investigate shear transfer across a plane in structural concrete...". Equally vague.

So, to me shear friction is to be applied only when the normal diagonal shear crack design assumption is not valid.

I disagree with this statement. For any shear friction plane that you investigate, there's usually going to be a diagonal tension check that needs to be performed either at that same location (cold joint) or right next door (dowelling into existing). Diagonal tension and shear friction are generally checks that need to be performed concurrently, not independently.

Notwithstanding the above nitpicking, I agree with you. I only check shear friction at cold joints. However, this begs the question: if shear friction doesn't need to be checked on monolithic concrete-to-concrete interfaces, then why doe codes provide mu values for that?

Where to check shear friction is really of much less interest to me that the question of where does shear friction need to be satisfied. Care to tackle the question that I posed in my 11:26am post? In your opinion, what's the shear transfer mechanism at work on the vertical plane that I sketched?

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

 

[KootK]

Yes the Vc+Vs is based a strut/Tie model, but Vc can be used alone in a design point of view (see one way shear) as described in clause 11.2.8.1 to resist shear AND torsion !!!

Vc is diagonal tension and would not apply to a vertical shear plane as I have proposed Pico. The diagonal tension shear plane is at roughly 45 degrees to the vertical shear plane of interest. Satisfying Vc isn't always sufficient to guarantee that you've satisfied shear friction, as evidenced by design issues at cold joints.

11.2.8.1 impacts a designer's ability to utilize shear friction because your 'Interface Shear Transfert' resistance that include dewel action cannot be greather than Vc. If it is the case, then minimal shear reinforcement is required.

I'm afraid that I don't follow your logic here Pico. I've got 11.2.8.1 in front of me as I type this post. It doesn't mention shear friction anywhere within that clause so I don't see how it would place limits on its use. Perhaps that limitation is implied in some way that isn't apparent to me.

11.2.8 Minimum shear reinforcement
11.2.8.1
A minimum area of shear reinforcement shall be provided in the following regions:
a) in regions of flexural members where the factored shear force, Vf, exceeds Vc + Vp;
b) in regions of beams with an overall thickness greater than 750 mm; and
c) in regions of flexural members where the factored torsion, Tf, exceeds 0.25Tcr.
Note: Footings and pile caps designed using strut-and-tie models in accordance with Clause 11.4 need not satisfy
the minimum shear reinforcement requirements of Clause 11.2.8.

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