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Shear Rupture of Uncoped Beam

Shear Rupture of Uncoped Beam

Shear Rupture of Uncoped Beam

(OP)
I am trying to find an answer while reviewing calculations of shear connections. In my connections with bolts at webs of supported beams, the flanges in some cases are not coped. Do we need to check shear rupture for the net section of the web even though there are flanges present? I have always said yes, but am being told otherwise by the connection designer. Some other input, or a documented reference would help, I do not see anywhere that AISC specifically says where/where no to perform this check, so I interpret that as check anywhere you shear section is reduced.

Thanks.

RE: Shear Rupture of Uncoped Beam

Unless I'm grossly mistaken; no. Just think how could you shear the web off without shearing off the flange(s)?

Obviously check block shear though.

Maine EIT, Civil/Structural.

RE: Shear Rupture of Uncoped Beam

I vote yes. Imagine if the shear / tension planes in your shear rupture model were replaced by thin air gaps. Would you feel good about hanging the beam from the top flange then? I wouldn't. You would have to shear off the flange however:

1) There would be considerable deformation before you fully mobilized the flange in shear and;
2) The deformation mentioned in #1 might even be enough to initiate and unzipping kind of failure.

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

RE: Shear Rupture of Uncoped Beam

Shear has to be checked whether or not the flanges are coped, but it will govern only on short, heavily loaded beams. The shear capacity Vr, listed in the steel handbook applies to an uncoped beam.

BA

RE: Shear Rupture of Uncoped Beam

Absolutely check it. Flanges only are considered as bracing elements for webs - not load-resisting elements in combination. The elastic stress distribution on the I-section is parabolic through the web, right? So the flanges aren't resisting much shear. The only "resistance" the flanges could provide is that at real web rupture the flanges *might" keep the member from collapsing entirely, and the beam may hang on to the connection like a wet noodle. Interesting to think about, but not practical.

RE: Shear Rupture of Uncoped Beam

(OP)
I agree with you all about checking it. I relayed this to the connection designer and he said he will get more info as to why it need not be checked, something regarding test info by AISC and prior communications with them. Fundamentally though, I believe it should be checked.

RE: Shear Rupture of Uncoped Beam

If the connection designer manages to send you something convincing, please share.

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

RE: Shear Rupture of Uncoped Beam

No, shear rupture of a section with at least one flange is not a valid limit state to be concerned with for connection design. So if the beam is uncoped, or even if it has a top cope but no bottom cope, shear rupture on the web need not be checked. (Of course, you still have to check shear yielding on the remaining portion, and block shear on the web, and all other regular checks.)

As for documentation, the AISC design examples that accompany the 14th edition Steel Construction Manual don’t check it. Neither do the 13th edition examples. Neither do the examples in the silver Volume II companion to the 2nd edition LRFD manual. The one exception is the Volume II companion to the 9th edition ASD and 1st edition LRFD manuals (the green and blue book). There is one example there where they check shear rupture on the web of an uncoped beam. But I’ve spoken with people involved with the creation of these manuals, and I believe this check is there because it was easy to calculate and include, not because it should have been checked. Regardless, we have several years’ worth of documents that have come out since the green and blue book that indicate it is not a valid limit state.

If you insist on checking it, you would have to include the flange area in your area calculation. If you were going to tear out the web, you’d have to take the flanges with you.

RE: Shear Rupture of Uncoped Beam

I don't think you need to check this limit state.
If you did try and check it what would you use for the Anv? Do you assume the whole flange, none of the flange, or just the k portion of the flange? In reading the commentary they suggest other examples of where block shear strength limit states apply, other than the coped beam end case (CJ4.3 paragraph 2) but none of those examples ever cross an out of plane element.
For further support Table 10-1 does not consider block shear of uncoped beams. The beam web available strength only considers bolt bearing for uncoped beams. If AISC intended this to be checked I would think they would have gone through the calculation like they did for coped beams. It is not explicit but the weight of evidence is toward not checking in my mind.

RE: Shear Rupture of Uncoped Beam

Nutte / Rob,

You guys have convinced me. I change my answer and no longer believe that shear rupture needs to be checked in this situation. Thanks for the education. Still, I would very much like to understand why shear rupture doesn't need to be checked. The arguments presented above to the contrary are pretty attractive.

I'm not at all convinced that the flange participates in providing shear resistance. Before the flange tips would be fully mobilized in shear, I feel that one of two things -- maybe both -- would occur:

1) You'd have to develop a ton of shear deformation in the web, perhaps enough to cause rupture and;

2) You'd have to yield the flanges locally in bending.

Check out the attached sketch for an illustration. At the very least, I would think that this would be a lousy serviceability condition. Maybe serviceability isn't pertinent here.

I did think of one significant benefit that would be provided by having the flange present above the tear out block. Without the flange, the tear out block requires bending stress on one or both of the failure planes in order for equilibrium to be satisfied. With the flange, the tear out block doesn't require these bending stresses. Rather, equilibrium can be satisfied purely through shear stresses. This is analogous to a building with bracing on three sides. Again, check out the attached sketch.

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

RE: Shear Rupture of Uncoped Beam

Theoretically, maybe you could experience some kind of shear rupture failure in an uncoped beam. But practically speaking, some other connection element would fail first. The bolts would fail, or you’d shear through the shear tab, or you’d exceed the shear yielding capacity of the beam.

Another item to consider: Have you ever heard of a beam failing in this way? If not, that might be a reason not to worry about this proposed limit state.

RE: Shear Rupture of Uncoped Beam

I agree on both counts. However, if you didn't already know AISC's feelings on the matter, you could apply the same arguments to a coped beam.

If a logical hierarchy of failure modes precludes shear rupture for an un-coped beam, that should able to be demonstrated by calculation somehow.

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

RE: Shear Rupture of Uncoped Beam

I don't know if this makes sense, but I will try. After reading the AISC commentary (and 1978 reference) and reviewing the block shear equation, it seems that: The only way there is a chance to initiate block shear rupture, is when there is the possibility of tensile rupture (they are in tandem). If there is no chance of tensile rupture, then the shear rupture cannot mobilize. It seems that if there is a flange, it will prevent tension rupture from occurring, and then a precipitating block shear rupture.

RE: Shear Rupture of Uncoped Beam

Quote (KootK)

If a logical hierarchy of failure modes precludes shear rupture for an un-coped beam, that should able to be demonstrated by calculation somehow.

So try it. Figure your net shear area, including the flanges, and see what you get.

RE: Shear Rupture of Uncoped Beam

If I'm understanding MacGruber correctly I agree that there can't be tension and shear block failure on a simple beam connection as I don't see the tension component of the loading.

RE: Shear Rupture of Uncoped Beam

Live and learn.

BA

RE: Shear Rupture of Uncoped Beam

Live and learn indeed.

@Nutte: You'll have to hold on to your gauntlet. Since I don't buy the flange participation bit yet, my numbers would be the same as for the coped case and the exercise would bear no fruit. That, and I'm way too lazy for a homework assignment

@Jayrod: if I understand correctly, the tension is an internal stress rather than an external loading.

@MacGrubber: your point ties in with what I was trying to get at with regard to the moment -- or lack off -- on the rupture block. The Ubs factor in J4.3 penalizes the capacity due to non-uniform stress on the tension rupture plane, at least for multiple row connections. I speculate that the presence of the flange minimizes that non-uniformity and might contribute to a higher capacity. I'll read the paper over the weekend. Thanks for digging it up.

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

RE: Shear Rupture of Uncoped Beam

I still fail to see how a pure shear load (simply supported beam) can create tension and shear block failure. The moment at the end of the beam is zero so there is no internal tensile stress?

RE: Shear Rupture of Uncoped Beam

See figure 1.5 of MacGrubber's paper. It's page six of the document, page 18 of the PDF. I can see how it might cause confusion. Without those flexural stresses that I mentioned above, equilibrium isn't satisfied.

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

RE: Shear Rupture of Uncoped Beam

jayrod12: Are you talking about a non-top flange coped beam end? Or, categorically you don't understand the whole block shear failure mode?

Quote (jayrod12)

The moment at the end of the beam is zero

No, it is not exactly zero. It is "zero" when considering transfer of moment to adjacent members. In coped connections, there is a moment that needs to be considered that is the reaction times the distance from the cope length. That moment can induce non-uniform stresses on the web in the locale of the bolts. But, regardless of a moment forming, there the group of bolts pull upward, creating a tension force in the plan(s) between the holes. The assertion is that when the top flange of the beam is intact, those tension forces cannot develop to a failure limit state before some other limit state occurs (bolt bearing, faying surface slip for SC bolts, bolt shear, etc).

RE: Shear Rupture of Uncoped Beam

Ugh...my grammar. *the plane between the holes*

RE: Shear Rupture of Uncoped Beam

I mean in this specific example where the beams are uncoped

RE: Shear Rupture of Uncoped Beam

Well, there will be tension there, no doubt. The question is of what magnitude? It seems like not enough to make a difference before another limit state controls, hence no block shear for non-coped beams.

RE: Shear Rupture of Uncoped Beam

(OP)
The questions is specifically about shear rupture. Not a block shear failure. If we can, let's stay on topic. In calculating the gross shear capacity of the beam we strictly use the area of the web. So I'd like to pin-point where AISC may say that we specifically do not need to check rupture of the beam web across the shear plane through the bolt holes if the beam is uncoped. To this point, I do not see where it states we do not check it.

RE: Shear Rupture of Uncoped Beam

Like I mentioned above, AISC provides several documents with example connection problems. This is not a limit state they check (in anything that has come out in the last 20 years). That ought to be good enough.

For yielding of the beam web, we all agree the flanges don't contribute to the capacity. For rupture, you have to actually tear through the section. The flanges would have to go as well.

RE: Shear Rupture of Uncoped Beam

@Chris: I thought that shear rupture was block shear rupture? As Nutte has pointed out, we've pretty solidly established that AISC doesn't think it should be checked.

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

RE: Shear Rupture of Uncoped Beam

@KootK,

No, shear rupture is not the same as block shear (though block shear does have a shear rupture component). If you have a copy of the AISC 13th or 14th edition, you can check out the difference in J4.2 (Strength of elements in shear - yielding and rupture) and J4.3 (block shear strength).

RE: Shear Rupture of Uncoped Beam

You are speaking about row tear-out, I believe.

RE: Shear Rupture of Uncoped Beam

Point taken Winelandv. However, the two provisions are mathematically identical. It's just a matter of the what geometry one assume's for the failure plain. Certainly, in the context of a basic shear connection, I think that it makes sense to be considering shear rupture and block shear rupture under one umbrella.

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

RE: Shear Rupture of Uncoped Beam

It makes no difference. If the tension rupture is not a possibility, how are the rows going to rupture? If there is no block shear tension+shear rupture, there is no row tear-out (shear rupture), right?

RE: Shear Rupture of Uncoped Beam

(OP)
AISC Steel Solutions has answered that this is not a limit state they consider for coped beams because:

"It is felt that the flange will provide enough strength to preclude these failure modes..... This issue has been discussed by the Manual Committee. and at this point we have not chosen to include these checks for uncoped beams. If we ever do choose to include the checks, they will take into account the strength of the flange in some manner."

A bit elusive in response but, in writing, they state the checks are not required...

RE: Shear Rupture of Uncoped Beam

If the flange prevents block (shear+tension) rupture, it prevents row tear-out (shear rupture). You would have to have some unconventional mixture of over-sized and standard holes, with some in bearing and others in slip-critical condition to allow a local row shear rupture, right?

RE: Shear Rupture of Uncoped Beam

What do you know, we do rely on the flange strength. I shall now scour the internet for test data...

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

RE: Shear Rupture of Uncoped Beam

Ha! Good luck, KootK. By the sounds of the Steel Solutions' answer, it might have been a subjective decision without empirical validation.

RE: Shear Rupture of Uncoped Beam

KootK: Did you find anything?

RE: Shear Rupture of Uncoped Beam

Nope. Thirty minutes of intense Googling yielded nothing. Oh well, knowing what's to be done, even if I don't fully understand why, is a step in the right direction.

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

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