Torsion or shear cracks and solutions

[holteg]

Have you seen anything like this yet? Is this indication of torsion or shear cracks and what should be the remedial actions? The beam is 12 inches wide and 20 inches deep with closed stirrups distanced 4 inches apart. And the location of it is in the following (shown in red):

This is zoom of the cracks (crack width similar to other cracks).

There is sufficient longitudinal, tranverse, torsion and shear stirrups inside. The concern is the beam has lost the rotational restraint (due to the cracks).. should steel bracket, etc be used to make it fixed again? What do you do in your experiences?

 

[hokie66]

They do appear to be shear cracks. But as far as I can tell from the photos, they are fine cracks, probably described as hairline cracking. I find it significant that the cracks apparently do not reach the top and bottom flexural reinforcement. Do you know if there is any longitudinal reinforcement other than at top and bottom? Not sure why you are worried about "rotational restraint", but I would not be overly concerned. Do you know if there was an overloading condition at this corner?

 

[holteg]

It has only top and bottom longitudinal rebars size 0.8 inch diameter. No middle rebars. But there is tight 3 leg close stirrups inside. No overloading conditions. It's just that in many buildings and malls. I have never seen such hairline cracks. Have you or anyone seen such hairline cracks before? Is it not beam must be designed such that at service loads, the concrete must be sufficient in shear strength Vc such that it should never even form hairline cracks?

Could it be a web shear crack instead of torsion?

 

[jayrod12]

If your framing plan is correct, then I wouldn't be surprised to find those cracks. If you check the beam for shear and torsion at service level loads, do you exceed Vc or Tcr?

 

[hokie66]

holteg,
You said closed stirrups, now you say "3 leg close" stirrups. Which is it?

I think the cracking is caused by a combination of web shear, and no longitudinal bars in the web. But it is most likely this is not a strength issue, but rather one of serviceability.

 

[holteg]

Vc exceeded. Are you not supposed to exceed Vc when its Vc + Vs for total shear capacity?

This is what is meant by 3 leg atirrup hookie.

Serviceability means?

 

[jayrod12]

Serviceability is is unfactored loads.

If you exceed Vc or Tcr you need stirrups. But in order to engage stirrups there n÷as to be some cracking. That's why I said I'm not surprised.

 

[holteg]

I know. If the width is increased by say 2 inches or 4 inches. It may not exceed Vc.

So the question is. When you design beam, do you make it such that at service loads it won't exceed Vc? That is. You make the width wider? And yet still added stirrups. Or do you design your service loads to take both Vc + Vs capacity?
I don't see such cracking in most beams. So I wondered if they were designed not to exceed Vc by forcibly making the beam width wider (yet still has stirrups for ease of placements or other safety).

 

[hokie66]

Serviceability does not refer to unfactored loading. The term "service loads" is misleading in that respect. Serviceability can be many things which are undesirable but not strength limiting. Deflection, vibration, cracking, durability...there are others.

I don't think most of us have a requirement to limit service loading to Vc. But you are likely on the right track about the beam width. A 12" wide beam with 3 stirrup legs is a bit tight IMHO. But I think the lack of side face reinforcement is the main reason the cracks are showing. They are inclined like shear cracks, but they probably started as restrained shrinkage cracks. The bars top and bottom provided the restraint, and there was nothing at mid-depth to control the shrinkage.

 

[jayrod12]

Hokie's description of serviceability is correct. I meant it more in the sense of service level loading (which is unfactored loading).

 

[holteg]

I don't think most of us have a requirement to limit service loading to Vc. But you are likely on the right track about the beam width. A 12" wide beam with 3 stirrup legs is a bit tight IMHO. But I think the lack of side face reinforcement is the main reason the cracks are showing. They are inclined like shear cracks, but they probably started as restrained shrinkage cracks. The bars top and bottom provided the restraint, and there was nothing at mid-depth to control the shrinkage.

Can any pattern or form of restrained shrinkage crack jeopardize aggregate interlock? Imagine the shrinkage crack occurring diagonally. Can this occur such that the stirrups would be further stressed because the diagonal shrinkage crack was able to disengage aggregate interlock?

 

[jayrod12]

With 3 legged stirrups at tight spacing, do you need Vc to make it work? Or can you ignore the contribution of concrete and still be ok?

 

[rapt]

The cracks look very steep to me for shear cracks. How deep is the beam and is there any side face reinforcement? I would suggest shrinkage cracks.

Hotleg,

Sounds to me like you need top read some text books on concrete design and also the ACI code!

 

[holteg]

I have read many of most popular textbooks and the ACI code and familiar with them.

Just want to hear your experiences. Most of my colleagues always oversize beams such that no crack would form (they put stirrups only as code requirements). So I want to hear from those who use Vs (some of my colleagues do but they don't see cracks often).

But if they are shrinkage cracks. Why are they still slant? I understand you mean diagonal cracks are 45 degrees.

 

[jayrod12]

They're slanted because the slab at the top restrains the top half, and the bottom half is free to srink at more regular spacing.

 

[hokie66]

I think they are drying shrinkage cracks, but the flexural behavior of the beam affects their orientation. So they are minimally inclined to reflect the shear stress component.

 

[holteg]

Since the drying shrikage cracks were inclined.. then it's like it imitated shear crack (more steep) so for all intent and purpose it can be considered as semi shear cracks.

Anyway. What would occur if steep cracks occur between two stirrups towards the beam ends (not at midspan). How would it behave compared to bonafide shear cracks where there were stirrups holding them (in the latter.. but not in the former)? It's not in any of the textbooks.

 

[rapt]

Hotleg,

From some of your discussions above, you do NOT understand concrete and shear design! You might have read about it, but that is not understanding!

In more than 40 years as a concrete designer, I have seen few shear cracks. And I do not want to see many, especially if no shear reinforcement has been supplied. Your colleagues are not being conservative by supplying shear reinforcement even if there are no shear cracks.

For what you have said, there is fairly closely spaced shear reinforcement. But I doubt there is any longitudinal side face reinforcement which is required for any member more than about 2' to 2'6" deep and which would control those cracks.

 

[holteg]

I really know about beams and shear. Our company always use D/4 or less so all beams ever designed have closed stirrups. We also put inner stirrups if there are internal longitudinal bars. In fact, we have file photo of it. I didn't design it but our company. We only put side longitudinal bars if the beam is more than 20 inches.

and from the book Design of Concrete Stuctures the following is shown.

I know when the stirrups haven't yielded. the contributions of the cracked concrete is from
1) dowel action 2) aggregate interlock 3) uncracked concrete

But what happens if the crack is between the stirrups on either sides of midspan or between midspan and either support?

We can eliminate uncracked concrete. So there is only dowel action and aggregate interlock? It may not match the pattern in the principle stresses trajectories but still there is crack induced by restrained shrinkage.. so how do you model this or the behavior of such crack?

 

[rapt]

It is not a shear crack.

Longitudinal reinforcement will control the crack. You still have not told us the beam depth, but it would appear to be > 20" from the picture.