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.

 

[TehMightyEngineer]

I don't have the time right now but what happens if you run the more advanced shear strength calculation using equation 11-5 in ACI 318 taking into account flexure?

Maine EIT, Civil/Structural.

 

[TehMightyEngineer]

It's important to recognize that the situation that you've described here is not shear friction as there is no sliding parallel to the adjacent surfaces.

Hmmm, yes I see that now and agree.

Maine EIT, Civil/Structural.

 

[KootK]

I'm not sure what would happen TME. I'm going to leave that check on the table as I'm already satisfied that shear friction on a vertical shear plane is a matter of purely theoretical interest. As I mentioned in my first post of this thread, I believe that there's also a shear-compression field generating clamping force as well. Adding that into the mix would put the capacity way over the top I reckon.

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

 

[KootK]

We're approaching fifty posts with this thread. In my experience here at Eng-Tips, that's about the maximum life span. The end is near. With that in mind, I'd like to wrap up with this:

1) Thank you all for your participation in this largely pedantic discussion.
2) Now for some cake.

Cake you say? Yes... cake. For practical minded folks like yourselves, this thread has probably been a bit like downing a meal of gruel and green beans: frustrating and unsatisfying. Such a meal should be followed by desert. And I have some... sort of.

I believe that there is a very practical application for my "imaginary shear friction plane" concept for those who choose to ascribe to it. Thinking in those terms allows me to be more flexible when considering unplanned cold joints. To understand how that is, please read these two snippets from another thread that is currently active (Link):

...Columns don't have inclined joints, and neither should STM struts.

You raise an interesting point regarding columns Hokie. As you know, you've been helping me out with a related shear friction issue lately (Link). While you're no doubt sick of indulging me on the SF front, I believe that the concepts in the other thread have application here. Try his wacky supposition on for size:

I believe that columns -- all columns -- do in fact have inclined joints. Several actually: 15deg, 30 deg, 60 deg... and at every location along the columns too. Those joints are just monolithic, held together by shear friction (or the atheist equivalent), and perhaps... imaginary.

Why would I bother to say something so fru-fru and bizzare? Imagine me in seated lotus position as I type this. For me, the imaginary shear friction plane idea from the other thread has a very practical application. It allows me to be less conservative when dealing with surprise cold joints. Here's how it goes:

1) Contractor calls me up and proposes / tells me about a unplanned cold joint.
2) I freak out. You put a cold joint WHERE? It's the worst possible location.
3) I remind myself that there was always a slip plane at the proposed cold joint location. The only difference is that it was originally a monolithic shear friction joint and now it will be a roughened shear friction joint.
4) I check the numbers (mu = 1.4 vs 1.0) and, if things work out, I carry on.

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

 

[bookowski]

It seems that the biggest confusion is coming from 'satisfied' vs 'checked' question. There is no arguing that equilibrium must be satisfied across any section that you draw - vertical, 2 degrees, 10 degrees etc. So some shear mechanism has to be at work across your vertical section. Whether or not that is the ACI shear friction is where it goes off track and I don't think you can derive a true answer as that method is something that is acknowledged to not be entirely correct.

Also note that in my quick derivation previously the M/4d approximation for steel area does a unit conversion so there are mixed units in the final result in that L is in ft. and d is in inches. To make it more useful you can convert and say that for a simply suppt'd beam as long as L/d > 2.3 the flexural reinforcing will force the shear friction equation to work. Because of the different phi factors you need to scale that by 0.9/.75 so it's actually L/d > 2.7 - and that is only an approximation because of the estimate for steel area. This checks out if you look at two beams:
beam 1: Wu = 10k/ft, L = 6ft, b = 12", d = 14", (2)-#6 bars flexural, 5ksi. L/d = 5.1 and shear friction is satisfied (mu = 1.4) by the flexural reinforcing
beam 2: Wu = 35k/ft, L = 6ft, b = 12", d = 28", (3)-#6 bars, 5ksi. L/d = 2.6 and shear friction no longer satisfies Vu.
You could derive similar symbolic results for various beams but I am guessing you'd always get a similar result, that it take a very highly shear controlled beam to be worth a look (i.e. tiny span, large depth, high shear/moment ratio).

Conclusion: There clearly has to be some limit at which you could force a direct shear mechanism, the capacity is not limitless. In 99% of typical cases this is never close to controlling so it is 'satisfied' and passes by inspection (or more likely never considered). In some cases, i.e. very very shear controlled it may require checking. (See link to paper I posted earlier, there are several papers on direct shear actually occurring - mostly it seems to come from dynamic, i.e. seismic or blast).

 

[KootK]

The "correctness" of the shear friction provisions is another interesting issue.

My understanding is that part of the reason that the mu values were calibrated upwards was to account for cohesion. In Canada, cohesion is a separate term and, as a result, mu values are lower.

To some extent, I wonder if the shear friction provisions weren't calibrated upwards to ensure that monolithic shear planes would never govern. It all just seems to work out way to nicely. Or maybe it's truly connected to some unifying mechanical principle that just jives.

If you look at detail "B" in the sketch that I provided with my initial post, you'll see that my theory predicts that a compression field will develop to automatically take care of the vertical plane shear friction business so long as the angle of the assumed struts doesn't exceed 55 degrees. What's the most commonly quoted upper limit recommended for struts? Fifty five. Freakin'. Degrees! And I didn't fudge the numbers on that; they just fell out of the analysis that way.

I know, I'm starting to sound like an Area 51 conspiracy theorist.

Back to the more pedestrian issue of where SF needs to be checked, the A23.3-94 concrete code stated:

interfaces between elements such as webs and flanges, between dissimilar materials, and between concretes cast at different times or at existing or potential major crack on which slip can occur

"

That definition is a bit more expansive than others that I've seen.

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

 

[bookowski]

from Structural Concrete article: "As previously mentioned, direct shear failures can occur in uncracked elements at locations near a support when a static load is applied in its proximity. However, tests have shown that concrete elements can also fail in a direct shear mode under the action of an intense dynamic load distributed along the length of the element [2]. In these tests the roofs of reinforced concrete box structures were subjected to loads from detonating explosive charges. The box structures were cast monolithically with two open ends and, for one series of tests, with dimensions and reinforcement as shown in Fig. 6. The roof slab did not have any prior crack planes through its thickness. The test results show that the slabs failed in direct shear in several cases and that the slabs were completely severed from the walls along vertical failure planes. Most of the slab reinforcing bars were pulled out of the wall, with a few broken bars exhibiting minor necking. It was further observed in several tests that the central portion of the roof slab remained relatively flat, as though no flexural deformations had taken place (shown schematically in Fig. 7).

Little is known about the actual fracturing and direct shearing process in dynamic events and therefore it was assumed in [14] that the direct shear failure in the dynamic case behaves in accordance with the shear transfer mechanism under quasi-static loading conditions. It was stated in [14] that the direct shear failure of the roof slabs is characterized by the rapid propagation of a vertical crack through the element depth. Since direct shear is associated with crack planes perpendicular to the longitudinal axis of the element, such failures are also possible in elements designed for flexural shear. Failure curves for reinforced concrete elements were developed in [14] and used in a parametric study of direct shear failures, see Fig. 8. The failure curve is unique for the specific element in question such that a family of curves could be generated for elements with different properties. A failure curve is constructed such that the combined values of pressure and rise time below the curve relate to no failure, and values above the curve relate to direct shear failure. The increase in pressure for an increasing rise time implies that the element is able to resist direct shear at higher pressure levels if the load is applied more slowly. The shape of the failure curve also indicates that for very small rise times, in this case < 0.1 ms, the maximum pressure appears to be approximately constant.

The analyses in [14] indicate that the resistance to direct shear increases as the element span to effective depth ratio L/d for uniformly distributed loads increases. However, the influence of L/d diminishes for small rise times and disappears at rise times close to zero. Another case is the comparison between elements with fixed support conditions and reduced end restraints. Two failure curves were generated and the difference between these curves diminishes for small rise times. Further results in [14] suggest that strength enhancement due to strain rate effects increases the shear resistance such that the entire failure curve is shifted upwards. Thus, even for the case with zero rise time, the failure curves do not coincide. It was also mentioned in [14] that the load duration does not affect the direct shear failure curve significantly. Other investigations have involved theoretical analyses of the direct shear mode of concrete elements [24–26]. However, this work is not the focus of the present paper."