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Shear Lug Breakout Failure When Force is Parallel to the Edge

Shear Lug Breakout Failure When Force is Parallel to the Edge

Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
Hi all,

I am connecting a steel beam to a concrete column and having a shear lug to take the huge shear forces from gravity loadings.

I am designing the shear lug as per ACI 349, the code does not discuss the checks of the shear lug incase the force is parallel to the edge (not perpendicular). However, going through ACI 318-11 appendix D, it says for anchor bolts '' The case of shear force parallel to an edge is shown in Fig. RD.6.2.1(c). The maximum shear force that can be applied parallel to the edge, V||, as governed by concrete breakout, is twice the maximum shear force that can be applied perpendicular to the edge, V⊥''.

My question is does this apply also to the shear lug case? Also, can i use the same special reinforcement specified in ACI 318 appendix D to resist the breakout around the lugs? How and what is the breakout perimeter?



Thanks in Advance

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Is the beam cantilever out from column? For the shear lug, I suggest to use plate with shear studs instead of the tee.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
Thanks retired13, the beam is simply supported, appologies for the non-clarity on the drawings.

The studs provides a reduced shear strength as per ACI 318 breakout checks, that is why i proposed the shear lug to have greater bearing area.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

The reasons of my suggestion are:

1) The stem may be in the way of vertical rebars.
2) There is a potential for air voids to form under the stem. The stem and flange forms a dead corner, which is difficult to vibrate.

You may think to turn the tee 90° if it works by strength requirements. But I personally don't trust the shear friction developed in such arrangement.

I don't have code on hand, so my suggestions are purely from practical point of view. If you can provide ACI 318-11 App. D Fig. RD.6.2.1(c), I think more people will join the discussion and offer their opinions.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

I'm also for studs. Two reasons to add to retired13's list:

1) I think that it's generally safer to stick with what is considered typical practice in a given situation. You know, things that have worked well for others in the past and and for which accepted evaluation procedures exist. For this situation, I feel that is the nelson stud configuration.

2) In my opinion, your lug situation lacks an important, mechanical component of the connection: you don't have anything to resist the shear pryout mechanism of failure. That would be your anchor bolts in a base plate connection or the studs themselves in a nelson stud application. Not considering this failure mechanism may, in fact, be why the numbers are appearing to work out favorably for you on this. You might be able to get around this if you're willing to consider your HSS to be moment connected to the embed plate. Even that would be a little dubious in my opinion, however, given thermal restraint effects, fitup etc.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

With really high bearing near an edge like that, does one need to consider some sort of blowout? I understand it wouldn't be blowout from a pure tensile type load on the head of a nut, but there are some really concentrated forces on a shear lug that can produce high pressures where blowout is a concern. Could be a corollary to KootK's shear pryout concern depending on relative lug/plate/beam rigidity.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

I'll add my advocacy for studs, as well. The shear lug creates a significant discontinuity in both the column concrete and reinforcing. If there's a failure mode for the studs to break out, the same mechanism would apply to the shear lug. The shear lug would likely be worse for breakout, since it isn't locked in to the concrete the way the heads of the studs do and there's nowhere to develop reinforcing between the lug anchorage and the face of the column along the failure plane. If you need more capacity for breakout, add horizontal ties in the column crossing the failure plane to increase the capacity for breakout and/or add more studs.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
Thanks all for your comments, as i mentioned the studs provides much less shear strength, there is a solved example (4.8) in the AISIC Design Guide 01 - Column Base Plates and Anchor Design_2nd Ed, where shear lugs are used on its own to resist the whole shear force with a defined breakout failure plan which is perpendicular to the nearest edge, the check is based on ACI 349, i thought i can use the same principle to optimize my design, the only difference would be that the force is parallel to the edge not perpendicular.
See below extract from the design guide:

I understand all your concerns and i personally would prefer the studs since the calculation is more defined in the code, however in this case the design is very tight and no matter number of studs we add into, the breakout checks wont work with a limited space.


RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
@Kootk, I dont see how pryout failure would form at the shear lug face, the only breakout that would make sense is the shear edge breakout and the concrete capacity for the bearing stress, any thoughts??

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

In the application of shear lug in base plate, the embedment is usually set in the preformed shear pocket, then grouted in place to ensure the positive contact between the tee and the grout. The filling of the grout space is verified through the return of grout from pre-drilled hole(s). Concrete bearing is positive in this case, but not in your application/design, as the potential to have air voids that compromise the quality of concrete bearing. Also, potentially a weak plane is created in the column.

As you go further down the road of engineering, many times you will find construction concerns can be very critical, and influence the way of design. We don't want to see a structure, while satisfies all strength requirements, but failed in service due to lack of consideration on construction practice that may result in increased uncertainty, and lower quality of work. Be mindful in your design.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

For large load beyond the capacity can provide by the more positive anchorage (studs), I may consider to provide beam seat, if possible, to take away some of the load. It also provides redundancy into the design.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Another note besides your design. When shear force is very large with relatively small vertical force, you shall consider embed the base plate level with the ground, as the raised grout tends to be pushed out of position and crumble, resulting in loss of bearing of the column above. Just something to think if you encounter such conditions.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
Many thanks retired for your valuable comments, i will put that into my considerations.

With regards the beam seat, we will still need to transfer the load of the seat to the concrete so same issue will occur.

I agree with your comments on potential air voids to form in my application, maybe we can introduce a hole in the lug to allow better concrete flow and confinement while pouring and deduct that area from the resisting??

What is really frustrating is the lack of enough research on these tricky connections while they have been extensively used by many contractors and designers by having very different approaches and assumptions. I am just trying to do my best to understand the behavior based on the available research we have.



RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

In my suggestion on beam seat, you will have two connections to share the load. One more thing for you to consider, as you stated before, the beam is simply supported, but if you have large shear force, you might have to put more weldment to the beam connection. For relatively long beam, that weld configuration might produce a moment that is negligibly small, but for shorter span, it could be substantial. The beam seat should help for the latter case.

I agree with you on the lack of guidance and unified approaches for connection design. . But if design work as simple as 1+1=2 we will be all unemployed. The lucky thing is the engineering never been one dimensional, we just have to put our knowledge and training to work on various situations. Good luck.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

You could pour a corbel to provide the beam seat. This would revert back to standard concrete design that would be more reliable.

The applicability of this depends a lot on the situation, but it is common enough.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Quote (OP)

@Kootk, I dont see how pryout failure would form at the shear lug face, the only breakout that would make sense is the shear edge breakout and the concrete capacity for the bearing stress, any thoughts??

1) I stand by my pryout concern per the sketch below.

2) Why is it that you cannot use a long-ish plate with a zillion studs? Whether you use the lug or nelson studs, your ultimate resistance mechanism is the concrete. And, in my experience, it doesn't take all that many studs before you've fully mobilized the concrete with that solution to just as great an extent as you would with the lug.

3) What are the loads that you are delivering to this connection?

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

I still do not favor this connection typology over studs but, if I were forced to do something like this, here's how I'd adjust it up to address the pryout issues.

Note that this is to effectively construct jayrod's corbel but to put it inside the formwork. While I very much like the concept of using a true, external corbel, if you're already enthralled to your contractor, I'm sure that they'll veto such a thing because of the formwork costs.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

KootK's sketch bring out another monster - shear friction.

The reaction developed under the stem tends to chip the concrete away in a manner similar to beam seat with concentrate load on notched concrete wall. Shear friction capacity of the concrete is to be evaluated, and more often than not, special reinforcement is required to resist the force that causing shear, and to prevent the failure of concrete in un-predicable and brittle manners. Note that the critical shear plane is usually steeper than 45°, measured from column face.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Quote:

(OP)

My question is does this apply also to the shear lug case? Also, can i use the same special reinforcement specified in ACI 318 appendix D to resist the breakout around the lugs? How and what is the breakout perimeter?

You are comparing apples and oranges. Appendix D is not applicable here. And really neither is the AISC Design Guide #1. DG #1 has been found (in testing) to overestimate capacity in certain circumstances. The reference to use is this:

https://www.aisc.org/globalassets/aisc/research-li...

And it takes into account edges (as far as the failure planes go).

As far as reinforcing the connection goes (where capacity cannot be met).....not sure I'd try that. I'd just make the lug bigger.

I vote for the studs too (if possible).

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Even if you address the prying action, as KootK suggests or otherwise, you still have the issues of how to adequately reinforce the local shear failure zone and how to get adequate concrete consolidation under the shear lug plate. The column base plate example is different in a couple of significant ways 1) The shear lug is vertical, so consolidation of the concrete around the lug isn't an issue. 2) A column would have substantial permanent axial load, providing sustained force from the steel into the concrete, which is not present in the beam connection.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
Kootk, I totally agree with you, I do need to add more welded rebars or studs to take out the tension from the eccentricity of the load.
I think we need to differentiate between concrete edge breakout from shear and pryout failure.
The actual breakout plane in your image would be a result of shear pryout which i can understand your concern.However, i have never came across pryout failure mechanism in shear lugs, i only can see it in anchors and studs.
My main question was related to the normal shear edge breakout plane in my case where the force is parallel to the edge and not perpendicular.

My thoughts was to apply same concept of the anchors by considering the following:

1. If the column was with infinite dimensions, I wouldnt check any except for bearing.
2. The column in my case is with limited dimensions, hence concrete edge break out shall be checked.
3. The force is parallel to the near edge and not perpendicular to it, so i would use the same as ACI 318 App D (The maximum shear force that can be applied parallel to the edge, V||, as governed by concrete breakout, is twice the maximum shear force that can be applied perpendicular to the edge, V⊥'')
4. Do my checks on the shear lug by calculating the shear capacity in the perpendicular direction and then multiply by 2 to get the capacity of shear applied parallel to the edge.
5. I wouldnt check pryout failure since AISIC and ACI349 does not mention to do so.



I just need to back it up with any reference.
My principle engineer thinks that the reinforcement of the column will take care of these failures and i dont need to worry about. I am not convinced so i need to do some checks to let it go.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

(OP)
Thank you all for your valuable comments, the corbel option is not feasible due to architectural reasons.

@WAROSE, what makes you think that this research is more reliable than the AISIC guidelines, they also have thier own testing as well and also considers the edges same as the research you linked.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Quote (Mahmoud Belal)

The actual breakout plane in your image would be a result of shear pryout which i can understand your concern.However, i have never came across pryout failure mechanism in shear lugs, i only can see it in anchors and studs.

The shear lug could have negligible pryout or tension resistance, since it relies completely on bond and friction for that. If pryout is a concern, you need something that has a anchorage in the concrete. If not, then pryout for studs isn't a concern either.

I'm not trying to be belligerent, but I'll put in one more plug for the approach with studs. If you're primarily concerned with shear capacity, providing studs with a total equivalent area to the cross section of shear lug should provide approximately the same shear capacity, without interrupting the vertical reinforcing, and without the concerns about voids in the concrete at a critical location that will not be visible for inspection. Also, the vertical stiffener in your most recent diagram would interrupt or eliminate the horizontal ties through the same critical location below the shear lug.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Quote (retired13)


KootK's sketch bring out another monster - shear friction.

The reaction developed under the stem tends to chip the concrete away in a manner similar to beam seat with concentrate load on notched concrete wall. Shear friction capacity of the concrete is to be evaluated, and more often than not, special reinforcement is required to resist the force that causing shear, and to prevent the failure of concrete in un-predicable and brittle manners. Note that the critical shear plane is usually steeper than 45°, measured from column face.

There is a methodology laid out in ACI-349 to evaluate shear friction capacity of an embed plate assembly with shear lugs, but tension anchors must be present in addition to the shear lug. Further guidance can be found in the code commentary.

RE: Shear Lug Breakout Failure When Force is Parallel to the Edge

Quote:

@WAROSE, what makes you think that this research is more reliable than the AISIC guidelines, they also have thier own testing as well and also considers the edges same as the research you linked.

I don't know what "ASIC guidelines" you are referring to.....but if it's Design Guide 1, the paper I referenced provides testing results that show the 45 degree method given in DG 1 is light in certain circumstances. AISC agrees because the first time I heard about this, it was in a seminar given by AISC, where they themselves pointed this out and warned us about the 45 degree method.

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