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# Diagonal compressive failure of concrete

## Diagonal compressive failure of concrete

(OP)
Hello,

this is my first post, I usually watch other threads and find most of the answers so thank you for that.

My question is does anyone here know where the expressions for diagonal compressive failure of concrete come from?
To clarify, I'm talking about the maximum shear resistance that an element can have before the diagonal strut fails under compression, in eurocode it's denoted as VRd,max.
Now, I do know how to get the expression from some basic statics and geometry, but I want to check how the actual experiments to confirm it were done. I want to do this because I think that the expression is valid only for elements with low ductility. Here is my reasoning behind it. In the standard procedure you basically check the strut for failure and it should be the same at any point on the strut, but at the point where the strut connects with the compression area (caused by bending) a 2D stress field occurs. In other words, if bending causes large stresses in compression area, at that point less diagonal stresses can be transferred (basically I check main stresses and not just normal and shear independently).

Why do I care for all this? Well, because of walls in earthquake. Eurocode 8 states that you should reduce this strength to 40% (that's 2,5 times reduction!) of it's basic value for high ductility class DCH (EN 1998-1 section 5.5.3.4.2 if you want to check), but no reduction for medium ductility. Further they say that this is "due to dynamic nature of the loading", but the dynamic nature is present in both ductility classes and usually tests are made by static pushing of the wall so it makes more sense that it's due to ductility.
I'm trying to figure out what really happens because this reduction is HUGE, this strength can not be increased by adding reinforcement and shear failure is non-ductile and should be avoided at all cost. It just feels very important.

I'm sorry if this is not coherent, too long or if I posted it in the wrong place. I can provide way more explanations if needed but I didn't want to put too much in the first post.
If you can give any insight into anything related to this I'd be very grateful, thank you!

### RE: Diagonal compressive failure of concrete

Stay with it. In time some structural guys will show up. You are somewhat over my head, but I'm not a guy that into structures.

### RE: Diagonal compressive failure of concrete

The reduction in codes to shear strength in beams, walls, columns etc is due to the fact that under cyclic loading there will be cracking, member elongation, spalling of the cover concrete potentially, crushing of the confined core of concrete, and the reliance on aggregate interlock as part of the concrete shear mechanism is reduced or becomes less certain. Here in NZ we take concrete shear = 0 for higher levels of curvature (or local ductility demand at potential plastic hinges). Keep in mind ductility = controlled damage to dissipate seismic energy in simple terms.

Note this local ductility demand is quite separate to the global ductility you might use to determine the overall seismic loading on the structure. A short deep beam will undergo much more plastic rotation in the hinges than say a longer shallower beam for the same overall lateral drift.

### RE: Diagonal compressive failure of concrete

Not familiar with that code but presumably higher ductility class relies more heavily on major deformations to absorb earthquake, in which case shear rupturing is a much bigger concern (since the shear capacity is not ductile, and heavily deformed concrete has reduced shear capacity)

### RE: Diagonal compressive failure of concrete

(OP)
@agent666
You say that aggregate interlock can't be relied on and I agree, but aggregate interlock is a mechanism relevant to resist tension force. As you mentioned, it's usually completely neglected and that's fine because you can simply put more reinforcement. Compressive failure on the other hand can only be avoided by making a wall thicker and that's usually a problem.
You mentioned some other problems that affect shear strength like cracking. That is actually considered in eurocode for concrete already so this is a reduction in addition to that because of cracking. Spalling of the concrete cover is also considered in a way that you consider only the part of wall inside confinement reinforcement. Regarding member elongation, do you mean because of axial force? Because that's also taken into account with the basic formula.
Now regarding concrete crushing it really depends will it happen and how much will it crush because that depends on the way you detail your wall, axial force, the amount of reinforcement and as you mentioned slenderness.
However, none of those problems are specific to the cyclic nature, but rather to the fact that large plastic deformations occur, right?

Note that this reduction is specifically given ONLY for walls of high ductility (not for beams or columns, nor for any element of medium ductility). It also completely disregards the way you detailed it or anything.
Yeah I know about ductility, both local and global, but thanks for the info.
My main problem is that I feel like this could be the case for medium ductility (most often used in practice) and that it's not that dependent on "cyclic effects" but more on ductility.

Just to paint a picture what "a medium ductility" is in eurocode, it usually (now this can obviously vary) implies required curvature ductility of a wall is 5 or larger. High ductility is 7 or larger.
Also the fact that it's given only for walls and not columns doesn't make any sense to me.

@tomfh
I agree and that is certainly the case. The problem is the following, I saw an article from which that provision is taken supposedly and there they had a lot of experimental data and their conclusion is that for high ductility shear compressive strength should be 40% of basic value (that's what the code also says). BUT HERE'S THE THING, according to them for medium ductility (standard structures, like 90% in practice) this reduction is roughly 51,5% (that's their exact number and the code requires no reduction). That's HUGE! Code requires no reduction at all for those structures.
In other words, I'm not really concerned with why is there a reduction of 2,5 times for high ductility, it's why there ISN'T a reduction of 2 times for medium ductility.

Question for all of you not from europe: Is there such reduction in your code? I'm talking specifically about compressive shear strength. I know that there is a "force amplification" factor but it's independent of this phenomena.

### RE: Diagonal compressive failure of concrete

Ok, I understand your point. It does seem a double standard.

What year version of Eurocode? I looked online and the versions of 1998 available on the web are 2004 edition. The 5.5.3.4.2 clause doesn't appear to differentiate between DCH and medium?

### RE: Diagonal compressive failure of concrete

(OP)

#### Quote (Tomfh)

What year version of Eurocode? I looked online and the versions of 1998 available on the web are 2004 edition. The 5.5.3.4.2 clause doesn't appear to differentiate between DCH and medium?
That's the last version yes. Clause doesn't say it's about DCH because the whole document is poorly written. The whole chapter 5.5 is about DCH, it's called "Design for DCH".

### RE: Diagonal compressive failure of concrete

Aggregate interlock has little to do with tension resistance. Even in tension, the mode of failure resisted by aggregate interlock is shear.

A Great Place For Engineers to Help Engineers

### RE: Diagonal compressive failure of concrete

(OP)

#### Quote (Ron)

Aggregate interlock has little to do with tension resistance. Even in tension, the mode of failure resisted by aggregate interlock is shear.

Imagine a member under shear force (only shear).
It will fail on a diagonal, right? It won't be cut vertically.
That's because either principal tension stress was too large or principal compression stress was too large.
I'm saying that shear failure caused by exceedance of principal TENSION stresses depends on interlock, but the one caused by exceedance of principal COMPRESSION stresses doesn't.

That's what I meant by "tension failure" or "compression failure".
We're not talking about axial tension.

### RE: Diagonal compressive failure of concrete

Tension capacity doesn't rely on interlock of the aggregate though, it relies on the tensile capacity of the concrete matrix (cement paste + aggregate).

Aggregate interlock is the mechanism by which shear is resisted/carried in the concrete after it cracks.

#### Quote (hardbutmild)

Regarding member elongation, do you mean because of axial force? Because that's also taken into account with the basic formula.
For member elongation I'm not talking about the elastic elongation or shortening due to axial load. I am referring to in a ductile system, if you are yielding the tensile reinforcement in one cycle, but in the next opposing cycle the reinforcement is not yielding in compression. The elongates the reinforcement due to the strain not being recovered in the opposing cycle. With each successive cycle this elongates the member as a result. This effect for example in terms of beams can end up pushing columns out from the floor plate and causing large cracks in the floor especially in the corners. In the worst cases a loss of support for any flooring units can occur in these regions. In walls it can mean the distribution of gravity loads changes if it also supporting continuous beams because you are in effect trying to push these beams upwards when the wall is longer. Walls often have a lesser confinement requirement than say columns.

This is a real effect and I'm not sure how its treated in any code but my own (NZS3101). Our code was amended a few years ago now to address all these lessons learned from our 2011 Christchurch and 2016 Kaikoura earthquakes. These earthquakes resulted in some regions of the response spectrum at least 2 times the design basis level earthquake. We has increased awareness of this as some of our recent earthquakes have had quite a few high profile failures in this regard, one building in particular in the 2016 events had a number of double tee units lose their seating and come down on the floor below. Thankfully this earthquake occurred near midnight and the buildings were not occupied, but people would have died for sure based on the photos (if you are interested in reading up on it google statistics house floor collapse.

It's sometimes called 'frame dilation' as well when its discussed for moment frames if you are looking for literature on the effect.

#### Quote (hardbutmild)

I'm talking specifically about compressive shear strength.
I've never heard of the term compressive shear strength? In our code there is some enhancement due to axial load, but its far less pronounced in ductile members.

We have for reference 3 separate levels of detailing, nominally ductile plastic regions (NDPR), limited ductile plastic regions (LDPR) and ductile plastic regions (DPR), these are based on limiting curvatures which sounds the same as what Eurocode may have. I'm sure Eurocode must have references on which there minimum recommendations are based. Perhaps delving into those will help answer why the code is the way it is.

Keep in mind that codes only prescribes the minimum requirement, if you want to go further based on what you know, research or otherwise there is no one stopping you. In this part of the world we've dealt with these rules for neglecting shear in potential hinge regions since the conception of our modern standards in the 1970's so it's not a shock to us, it's just the way things get done. We look at other codes with the more relaxed requirements around things like member confinement and being able to take concrete shear capacity when ours don't, and wish them the best for the future, we've already been there and learned our lesson so to speak.

### RE: Diagonal compressive failure of concrete

#### Quote (Agent666)

I've never heard of the term compressive shear strength?

He's referring to VuMAX, where the compression struts crush.

### RE: Diagonal compressive failure of concrete

Ah right, it's simply termed maximum shear strength limit or something similar here.

### RE: Diagonal compressive failure of concrete

(OP)

#### Quote (agent666)

Tension capacity doesn't rely on interlock of the aggregate though, it relies on the tensile capacity of the concrete matrix (cement paste + aggregate).

I'm talking about shear induced tension. Tensile stresses because of shear forces. The thing that causes diagonal cracks. I used the term "tension" to distinguish it from "compression" or crushing. Shear can always be substituted by principal stresses. Why do you think cracks due to shear are at a diagonal? Because tension stresses caused by that force are diagonal. Just draw trajectories in a beam and you'll see failure is caused by tension. If the failure was truly due to shear crack in a beam would be vertical (just like bolts may fail)

@agent666
I never thought of that! It completely makes sense actually and should be considered, but it's not so much related to capacity, rather to forces and structural response.

I think eurocode was based off of new zealand codes (or at least that's something I heard long time ago) so they're probably similar in a way.

I checked the background documents for the provisions given by the code and their explanation is "we didn't want to put any reductions for medium ductility so that wall systems could be economical"

Now just to clarify again, we DO neglect shear strength completely, but that's not my concern at all.

The problem is maximum shear limit (the force at which crushing occurs). I have checked research and I only found some experiments, no theoretical explanation as to why does this limit drop or does it drop with rise in ductility indefinitely or is there a limit.

To further clear it up, I'll try an example. Imagine a wall. You get with standard analysis a force V acting on that wall (let's say it's 2000 kN). You increase it due to "dynamic magnification" (let's say V = 3000 kN after amplification). After that you say strength = 0 and put enough horizontal reinforcement to take the whole increased force. After that you have to check that struts don't crush. You get by your code expression (usually found in a part of code about concrete, not seismic part) that they crush at a force of 4000 kN. You think you're safe, right?
Our code says "if you have high ductility multiply it by 0,4". That would mean that crushing occurs at the force of 1600 kN. Now since 1600 < 3000 it's really bad and it'll fail way before the required ductility is achieved.
But our code also says "if you have medium ductility it remains the same". That would mean that crushing occurs at the force of 4000 kN and that you're safe. In reality it makes no sense at all to me. There should at least be some reduction to it.

Problem here is that if some reduction of that shear strength limit occurs for medium ductility structure, it'll fail in a very non ductile way! And difference between 1600 kN and 4000 kN is huge. The only way to avoid that failure is to require a thicker wall. That changes it's stiffness and so on.

No mechanisms you mentioned before actually influence the maximum shear limit (I think) so I wonder why does it reduce. Because the only documents I could find (most of them research) all say it's because of a dynamic nature of loading.
But dynamic nature makes no sense to me especially since dynamic effects occur even if you design a building to remain elastic.

As I said, I haven't been able to find any research papers explaining this and I've been looking for it on and off for a few years now.

### RE: Diagonal compressive failure of concrete

Before I start - Just a question hardbutmild..
You have 2 identical walls - You calculate one using DCH provisions and one using DCM - Do you think VRd is the same for both cases?

### RE: Diagonal compressive failure of concrete

(OP)

#### Quote (Klitor)

You have 2 identical walls - You calculate one using DCH provisions and one using DCM - Do you think VRd is the same for both cases?

I'm talking only about VRd,max (crushing) so if the geometry and concrete class are the same, yes. So the basic value is the same!

BUT, because of different ductilities, this strength will reduce more for DCH. The question is why does it reduce?

I don't think that final strength should be the same for DCM and DCH, but if the reduction for DCH is 0,4 for DCM it should be 0,5.
Although I think it shouldn't be based only on the ductility class, but I don't know what should be considered.

### RE: Diagonal compressive failure of concrete

They are not the same :)
For DCM, VRd,max is not a fixed number
If you take a look at the DCH item you referenced, EN 1998-1 section 5.5.3.4.2(a), it says that VRd is calculated using theta angle of 45 degrees... But for DCM it is not so (you can freely select angle between 28 and 45 degrees) ... so for instance your same wall from example that has VRd,max=4000kN will have VRd,max=3350kN if you select 28 degrees.

Also, fallacy in your example is that force V is 3000kN for both DCM and DCH...
In fact q factor (or R for our north american friends) is 3.00 for DCM, and 4.50 for DCH .... So for the same building the DCM force V in the wall will be multiplied by 1.5 (4.5/3) and will be 4500kN (Also doesnt pass) ... So in essence you are reducing capacity, just in another way.

### RE: Diagonal compressive failure of concrete

(OP)
@klitor
nothing you actually said has anything to do with the fact that for some reason shear strength reduces, but I'll address your post.

#### Quote (Klitor)

so for instance your same wall from example that has VRd,max=4000kN will have VRd,max=3350kN if you select 28 degrees.

It's usually considered either 45° or 39,8° for standard design. Using 22° (the lowest allowable value) seems unreasonable and you usually have to prove that you can actually provide that angle. It assumes a large redistribution of forces which is not a good idea in a critical section where dissipation is supposed to happen. It seems to me that choosing an angle different than 45° already assumes some plastic behavior and to have a same mechanism addressed by two factors seems confusing at best. Also, in earthquake walls cracks tend to be at a 45° angle.
Also, by assuming the lowest possible angle you get around 80% of the original strength. Experiments suggest that this value is closer to 50%, so it would be 2000 kN. Do you understand my point? It's not something that should be ignored since it's still just 60% of the smallest value that you suggested.

This variable inclination thing only adds to some differences in the design, but it doesn't address the problem of shear strength reduction.

And "they're not the same" is a maybe. You can chose 45° regardless and I always tend to do that when doing seismic design so yes, they are the same

#### Quote (Klitor)

So in essence you are reducing capacity, just in another way.

You're fundamentally wrong. This increase has nothing to do with the reduction of strength, it's a phenomenon on it's own. Also, the basic value is 2000, it's multiplied by 1,5 for DCM and by a larger value for DCH. So if it's 2000*1,5 for DCM for DCH it might be 2000*(3/4,5)*2,25 which gives the same value. This increase for DCH can kind of vary, but it's usually between 2 and q. For most structures (period smaller than corner period) it tends to be on a higher side. So usually DCH force is larger than that for DCM AND the strength reduction is larger.

This force has nothing to do with my question though because I'm concerned ONLY with why does strength reduce. This force part is quite easy and straightforward. My point in that simple example was only to show what the problem is since people probably aren't familiar with eurocode procedure.

### RE: Diagonal compressive failure of concrete

Indeed if you get multiplicator 2,25 you are just choosing the inappropriate method for calculation (in that case you should calculate this specific building using DCM).
I stand by my comment.. If you look into eg. Fardis, the point of DCM/DCH is only choosing the right method for specific structure (not some much higher level of safety, or material savings) and 0,4 is just a safeguard that you dont "accidentally" get too small shear force (and you CAN manipulate alpha0/alpha1,ε and bunch of DCH related items)

I did add strut inclination thing just as an indicator of one of EC8 many many small items that you CAN "use" to tweak you designs ... unfortunately, as you know, EC is sometimes very vague, and it is not exactly pure science :)
Just my 2 cents

### RE: Diagonal compressive failure of concrete

(OP)

#### Quote (Klitor)

Indeed if you get multiplicator 2,25 you are just choosing the inappropriate method for calculation (in that case you should calculate this specific building using DCM).

Nonsense, the minimum value for that factor is around 2, so 2,25 or larger is the majority of cases. DCH tends to behave better in earthquakes that are unexpected since it has larger ductility and although it should be as safe or as economical it really isn't. Just look at the thing I'm talking about. Experiments show that there should be reduction of 0,4 for DCH and 0,5 for DCM. They take them for DCH, but not DCM. Same exact phenomenon in the same exact research paper! How can you claim it has the same level of safety? Also, do bear in mind that DCH doesn't require more strict regulations on the actual placement of reinforcement and it HEAVILY depends on the high level of crafstmanship, more so than DCM. In practice, they differ significantly.

Note that this increase is only for shear checks, when you determine moments they don't increase. So it makes PERFECT sense to get larger design shear for DCH. Because your structure needs to go deeper into the ductile area you need to ensure that shear doesn't govern for a longer time. Shear failure is non ductile so if you want ductility 1 you just need to ensure that it doesn't fail in shear up to idk 1,1. If you have ductility 2 you need to ensure that ductility 2 is provided and then it's allowed to fail in shear at 2,1 and so on. Larger the ductility, larger the increase of the shear force. It can be as large as elastic force, while for DCM it's half the value of elastic force.

#### Quote (Klitor)

and 0,4 is just a safeguard that you dont "accidentally" get too small shear force (and you CAN manipulate alpha0/alpha1,ε and bunch of DCH related items)

0,4 has nothing AT ALL to do with shear force. It is only related to CAPACITY. you need to consider BOTH the increase of the shear force and the decrease in the capacity. Latter doesn't exist in american codes.
0,4 isn't a safeguard it's actual result from an experiment, not a safety factor.

The point is not "how can i manipulate and trick the code". I know how to do that. The point is "WHY DOES SHEAR CAPACITY VRd,max REDUCE?". Does it have to do with the dynamic nature of earthquake? Does it have to do with ductility? If so, does it mean that if I design a system to behave in a plastic way for standard loads (no earthquake whatsoever) what is the VRd,max?

It's not a safety thing, fardis who you mentioned SPECIFICALLY says that in the designers' guide to eurocode. He says that "similar reduction should exist for DCM, but it wasn't introduced into the code because it would limit the usability of wall systems in practice". That's a very poor thing to do, you can't say "oh it's too expensive so I'll just ignore that this critical thing is in fact half as strong"

### RE: Diagonal compressive failure of concrete

I didnt see that one before - I was talking about the Fardis book..Why didnt you put excerpt in the beginning, it would be quicker..everything is written in it :)

Im putting it here if someone wants to continue, but I doubt something better can be found, excluding a email to Fardis or comitee members :)

### RE: Diagonal compressive failure of concrete

(OP)
@klitor
I thought it'd be a lot quicker to just ask if someone knows how the original VRd,max was acquired because the expression given in the picture you provided has a lot of "problems". First of all, ductility stops influencing the strength at a low value (between DCM and DCH lowest DCH value). It makes no sense. Also, reinforcement shouldn't influence the VRd,max at all so what's that all about? Also, reinforcement has no upper value! So if you put a huge amount of reinforcement you could avoid crushing?

I know it's a fitted expression, but it's not even dimensionally correct and mechanisms are kind of unclear.

I've looked at that article they say is the source but it is very confusing, it seems like they used only a few walls of ductility above 5 and a lot of tests were done by an equivalent static method.

I was just curious if someone saw an article explaining mechanisms that cause this reduction or if someone knows how tests are done for the original VRd,max. Because according to the picture you attached this is also true for ductility 1 (or 0 as they say). They also don't mention if this is dependent on the rate of loading (I'd expect that if it's because of dynamic nature). Because if original VRd,max was done only for plastic ductility 0 then maybe the same thing would happen for non-seismic structures when they behave non-linear.

It's also very weird because I'd expect that it depends not only on the current ductility, but also the total ductility capacity.

### RE: Diagonal compressive failure of concrete

“we didn't want to put any reductions for medium ductility so that wall systems could be economical"

### RE: Diagonal compressive failure of concrete

Not economical if you have to replace or repair after each shake though.

Slightly more economical to build in the first place, but if the first decent shake damages it beyond repair, isn't that false economy?

Then you have to ask is it really economical if you consider you might be paying for it twice when you roll the dice on whether you'll see the big one within the life of the building.

Most of the ductile concrete structures in the NZ 2011 earthquakes functioned well in terms of life safety (except the two that collapsed with a good proportion of the 165 odd dead inside them), however most have been knocked down now because of the economics of repair (not entirely structure based, often the fitout/facade damage meant they are not recoverable from an economic standpoint as I understand it). A lot of the new structures going up favour structural steel out of interest. I'd put some money on it that if we give it some time and some corporate amnesia that we'll be building the same concrete structures in a few decades ready for the next big one!

### RE: Diagonal compressive failure of concrete

(OP)
@agent666
I totally agree about the economy part and that's what makes me kind of mad. What's even worse, people don't follow the code even where it's clear what's correct (capacity design is often ignored completely). As you said, everyone seems to forget about earthquakes in a decade or so.

I got a glimpse of a new draft to the code and they seem to be going in a direction of not taking this reduction even for high ductility structures.

@Tomfh
Well, I was hoping for something that explains the mechanisms that cause the reduction of the strength to better understand it.

For example, if you ask why does shear force increase, there's a logical explanation that it's because energy is usually dissipated by bending in the first mode, but not in higher modes so influence of the higher modes on the shear force should not be reduced.
This example has nothing to do with resistance, but I just wanted to show an example how there's a certain logic to it, you can understand why is it happening.
I don't like provisions that just say "oh, reduce this 2,5 times because I said so", especially if it's because of a certain phenomena and not a safety thing. Because there is a logic behind it, it can be explained.

### RE: Diagonal compressive failure of concrete

There is a known degradation in shear capacity with increasing curvature, it's not something someone made up for no reason, i.e. something like this:-

This reduction is the outcome of many of the contributing factors noted in discussions above with respect to shear capacity at higher ductility/curvatures in plastic regions.

EDIT- added this as it explains it in words:-

The text they refer to is this one:-
Displacement Based Seismic Design of Structures by M.J.N. Priestley

### RE: Diagonal compressive failure of concrete

(OP)
@agent666
Yeah, i know it's not made up for no reason. Thank you for a suggestion on reading material. I've already seen this book many times, but was never able to get it.

The graph kind of makes me sceptical, this expression is used for determination of shear strength if failure is due to tension. I think I read one of the articles mentioned in the text and it was tensioned based failure. I attached a picture to show what I mean by tension or compression failure.

I seem to mention this a lot, but tension controlled failure means nothing because in design you consider Vc = 0. You don't however consider Vc,max = 0 (or VRd,max or Vu,max). You can't influence Vmax by adding any type of reinforcement, just by changing the thickness of a wall.

Since those are different mechanisms, their degradation shouldn't be the same. For example, due to dynamic nature the cracks smooth so mechanical interlock becomes weaker. While that may be important for tension controlled failure it doesn't make any difference when talking about compression controlled failure.

I'm sorry if I don't make much sense, I don't know, maybe I think there is something that doesn't exist.

### RE: Diagonal compressive failure of concrete

I think you are possibly confusing concrete cracks (principle tension issue at crack location) with zero or limiting shear strength occurring due to crack forming. To be honest I'm having a few issues following your argument, however I'll point out again that its been explained a few times that this isn't the primary mechanism related to shear strength in concrete, its aggregate interlock in relatively simple terms, the bits between the cracks can still carry load more dependably. You keep coming back to the tension argument on a macro level being a driver to explain the behaviour, but it seems flawed to me (at least) as its one part of a number of complex concrete mechanisms that are no doubt going on.

You posted a number of failure mechanisms related to shear, most standards design rules probably cover these mechanisms. They just aren't exposed to the user. Our code has specific rules for preventing sliding shear (which only governs at very high shear demand). All this does is make sure you have more shear reinforcing, the contribution from the concrete is still the same. But one function of the addition of the reinforcement ensures also that you can develop this concrete component in a dependable manner.

A member could be cracked to hell and still have shear strength. That curve I posted is a result of many mechanisms, I don't think you can boil it down to a single factor which you might be searching for?

### RE: Diagonal compressive failure of concrete

(OP)
I think you are in fact wrong. I'm sorry, I just don't know how to better explain it. It's different mechanism altogether. Imagine a flexural failure. A beam can fail in flexure either by tension reinforcement yielding or by concrete crushing. Those are the two possibilities and two different things completely separate from each other. It's the same thing here.

#### Quote (agent666)

You keep coming back to the tension argument on a macro level being a driver to explain the behaviour
i don't know what you're talking about. just look at the end of my post, I don't know how to explain it to you. You obviously don't understand that there are compression and tension controlled failures and I posted a pic. I simply can't understand, how does someone not see it, it's right on the pic for god's sake. I can't even argue anymore, it's like we're speaking in different languages.

#### Quote (agent666)

All this does is make sure you have more shear reinforcing, the contribution from the concrete is still the same.
HOW? How can you even claim that? HOW? Do you not see the picture? Sliding occurs along a HORIZONTAL crack and tension controlled along a diagonal line. If "concrete contribution" was the same that would mean that if you had no reinforcement whatsoever that tension controlled failure and sliding would occur at the same exact moment. Because their strength is the same. For god's sake, "more shear reinforcing"????? Sliding reinforcement is either VERTICAL or diagonal, shear reinforcement is HORIZONTAL. I don't know... i just give up.

It's simple. Imagine you have a concrete wall. You also have a shear force. You increase this force from 0 to infinity.

At one point due to tension you will get a crack. If you don't have reinforcement it will fail (that's the strength defined by tension).
So you put enough reinforcement to avoid that, but the force keeps getting larger. At one point concrete crushes!!!!! No amount of reinforcement is going to save you from that! No sir, not possible to avoid by adding steel.

Now what does this tell us? That since tension and compression failure aren't simultaneous they must be caused by different mechanisms.

As a matter of fact, here is an article (I found it on the site of a uni so I guess it's openly available) http://www.strulab.civil.upatras.gr/sites/default/...

Look at the chapter 4. You can see that there are different expressions for shear in tension and compression and their strength reduces differently.

In fact, look at this formulas taken from eurocode, this one is for tension controlled shear failure

k takes into account size effect, ro is the influence of longitudinal reinforcement, sigma is the influence of axial load

Now look at the expression given for compression controlled failure

alpha is the influence of axial force (okay, same or similar mechanism as in tension)
ni is the influence of cracking
theta is the influence of strut inclination (doesn't exist in tension failure)
where is the influence of longitudinal steel???
This expression shown in the second picture can easily be derived from simple statics and geometry, just see when you get compressive strength on a diagonal because of shear force.
The first expression on the other hand usually can be described by 3 different mechanisms
1) part of the section in compression due to bending resists some shear due to friction basically and since it's compressed that helps
2) mechanical interlock along the crack
3) dowel action of tension reinforcement

Influence of each one of them depends on things such as size of the element (remember, it seems to not be important for compression controlled failure) and also on the amount of reinforcement (find me where's the reinforcement in the second formula please)

I can't believe we even have to discuss the fact that compression controlled and tension controlled failure rely on different mechanisms and that their reduction is MOST CERTAINLY different.

#### Quote (agent666)

That curve I posted is a result of many mechanisms, I don't think you can boil it down to a single factor which you might be searching for?
I'm not looking for one factor, but for the love of god man... it's NOT THE SAME MECHANISMS! you not seeing that is simply unbelievable to me. It's absurd. We're discussing something that is clearly different. Why would different failures even exist if all of them depended on the same mechanisms? They can have some mechanisms in common, but OBVIOUSLY not all. You want proof? well look at the reduction suggested by priestley and the one I said from a paper by biskinis and fardis and a few others. They are different. I just can't anymore... I simply can't. It seems trivial to me that different modes of failure are in fact different.

### RE: Diagonal compressive failure of concrete

Dude calm down alright, I'm trying to help contribute to your random question on the internet. I'm not getting anything out of it at all. It seems that whatever reasonable explanation people have had in this thread isn't for you, you don't believe or want to rely on well recognised texts, phenomena, etc. There is zero point coming here and asking a question and then berating everyone that tries to help because you don't believe part of what they have to say.

Yes there are all different types of shear failure. I recognise that, never disputed it. For practical design, who really cares, codes evaluate a lower bound solution that you hope precludes all different types of failures. Can you say for certain for a given configuration what type of failure will occur knowing how variable concrete and reinforcement factors can be.

As to which limit state ultimately governs depends on the load compared with the capacity for that limit state, inclusive of concrete and/or shear reinforcement capacities, etc.

Sliding shear for example occurs at higher levels of shear in reversing plastic regions, to prevent sliding shear you need more shear reinforcement as I noted (at least this is how the NZ standard treats it, sometimes diagonal reinforcement or inclined bars are required above certain limits of shear stress because as per the graphic you posted traditional orthogonal reinforcement doesn't cross the shear failure plane).

This additional reinforcement does not create more 'concrete capacity'. The amount derived from the concrete component is constant irrespective of the mechanism of shear failure (the first equation in your post above for Vrd,c). Your standard may have different means of treating/checking these types of failure mechanism. NZ standard covers 'sliding shear' and 'standard shear' and your 'compression shear limit' as an upper bound on shear strength.

In the NZ standard the maximum shear limit is a function of concrete strength or a constant stress (I think 0.2f'c or 8MPa from memory, though the later changes depending on the type of component, beams/walls/beam-column joints all have varying max shear limits), it is simply an upper limit cutoff to be compared to concrete + reinforcement shear capacity. If your traditional shear capacity calc is higher than this limit, then that limit governs the capacity instead, if your load is less than this you are good to go. YThere is no means of enhancing the strength beyond this limit except increasing the concrete strength or wall geometry, and even then you run into the 8MPa limit at some point. In practical design the compression limit Vrd,max would very rarely be the limiting factor, obviously if this is an upper limit, and in ductile design the Vrd,c was reduced to zero then there is no way the other Vrd,max limit even comes into it even if there is a different reduction applicable. I agree there may well be a different rate of reduction, but my guess is the reduction is a lower bound, applied equally irrespective of the final limit state at which the member might fail (see last few paras below).

In Eurocode it works exactly the same way as far as I can tell for conventionally reinforced members, I'm only stating this so I know we are all on the same page here...
Vrd is the lesser of
Vrd = Vrd,max
or
Vrd = Vrd,s + Vrd,c

So the point I want to make is the V,rd,max limit is compared to the strength of the sum of the reinforcement + concrete strengths, it's not a limit on just the concrete shear strength in case there was any misunderstanding on this, to achieve that capacity it is reliant on there being some minimum level of reinforcement present. This is identical to how the NZ standard is laid out. I don't think I said it had anything to do with increasing if we added more reinforcement, as noted above I was referring to the sliding shear limit state when I stated this (sorry if this was unclear).

The curve I posted isn't a function of the many types of individual failure limit states being reached, its a function of what mechanisms are going on inside the concrete at a given yield curvature ratio such as degree of cracking, degree of concrete crushing, degree of aggregate interlock, degree of reinforcement elongation, other factors you've noted, etc as it affects the shear capacity derived from the concrete contribution. These mechanism applies equally to the concrete shear capacity (your first equation for Vrd,c above). The 'material' mechanisms I referred to are these, not the individual ultimate mechanisms of failure to clear up your point about not being the SAME MECHANISMS.

They are different modes of failure, and they may well have different rates of decay, but the same 'stuff' is going on inside the concrete up until any one of these failure mechanisms is reached if you like and I guess that the codes take the most conservative value in developing the degradation curve, a lower bound value that may not recognise the actual capacity if a given configuration happened to fail in another manner. Does it always reflect reality, maybe not, but does it make the economy of design easier for the poor designer not having to evaluate 6 different failure states by knowing that you only need to evaluate the most critical code approach, hell yes. In a cyclic hinge, concrete is getting hammered in reversing compression/tension/shear, so arguably at that point a universal reduction is palatable because all those mechanisms are occuring in the same bit of concrete in a full cycle.

Have you plotted out those degradation curves from the paper so you can share the differences? I guess in terms of what you design for only if they crossed and one was more critical at certain levels of load than the other would it be of any practical use.

### RE: Diagonal compressive failure of concrete

The code rule being refered to only reduces VR,max. So is that saying if you are well below VR,max that shear degradation need not be considered?

### RE: Diagonal compressive failure of concrete

(OP)
Well, depends on how "well" bellow the VRd,max you actually are. If you're bellow 0,4 VRd,max then technically yes, but ONLY because you already say Vc = 0. Since the code says that no contribution of concrete should be considered when determining tension controlled failure they didn't introduce any reduction for that, but it obviously should exist and should be different than the reduction of VRd,max.

So according to the code, if you have no reinforcement, tension controlled failure occurs at V = 0 and compression controlled occurs at V = 0,4 VRd,max.

If you add some reinforcement, tension controlled failure changes, but compression controlled doesn't.

### RE: Diagonal compressive failure of concrete

(OP)

#### Quote (agent666)

you don't believe or want to rely on well recognised texts, phenomena, etc.

I do recognise them and accept them. They just address the question of tension controlled failure and not the one I'm talking about. It's as if I ask about a chicken raising manual and you give me a duck raising manual. Close enough.

#### Quote (agent666)

The amount derived from the concrete component is constant irrespective of the mechanism of shear failure (the first equation in your post above for Vrd,c).

It isn't, but I don't have the energy to discuss this anymore.

#### Quote (agent666)

If your traditional shear capacity calc is higher than this limit, then that limit governs the capacity instead, if your load is less than this you are good to go.

exactly, and I'm talking about the reduction of this limit, not the reduction of Vc. and it's not "just an upper limit".

#### Quote (agent666)

In practical design the compression limit Vrd,max would very rarely be the limiting factor, obviously if this is an upper limit, and in ductile design the Vrd,c was reduced to zero then there is no way the other Vrd,max limit even comes into it even if there is a different reduction applicable.
It does, I've seen it happen a fair amount of time in practice since it's reduced to just 40% of the standard value. Especially for walls with smaller thickness when you consider that concrete cover spalls.

#### Quote (agent666)

I agree there may well be a different rate of reduction, but my guess is the reduction is a lower bound, applied equally irrespective of the final limit state at which the member might fail (see last few paras below).
As I pointed out a number of times it's a lower limit for DCH, butnot for DCM (DCM is mostly used in practice).

#### Quote (agent666)

Have you plotted out those degradation curves from the paper so you can share the differences? I guess in terms of what you design for only if they crossed and one was more critical at certain levels of load than the other would it be of any practical use.
They most certainly cross. Because as you said V = Vc + Vs and Vrd,max are in question.

So when you have a large enough amount of steel V will be larger than Vrd,max and I saw examples in practice and had a few people call me about it.
It's so obvious. We even talked about it in this thread. There's a picture of a guy who wrote the code saying "we didn't put it in because it would limit the usage of this system". That's the proof that it would have influence.

All I tried to say by "different mechanisms" is the following. Consider the picture

both of these can fail either by one triangle sliding on the other along a line or curve.
Another failure can occur by crushing of the diagonal.
Now, both of these will crush at the same force, but won't slide at the same force. That's all I meant, that some things that are important for one type of failure aren't important for other type. So if you say that reduction of sliding capacity occurs because the bottom pic became top pic, crushing won't care because it's independent of the roughness of the crack. So one of them can decrease without the other one decreasing.

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