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HSS Shear Lugs

HSS Shear Lugs

HSS Shear Lugs

(OP)
Hi

I have a HSS shear lug. Inside of the tube is filled with grout and 2'' outside tube as well. Regarding the failure modes of the shear lug tube, local bending of the flange is not a failure mode as the tube is infilled with grout. Any comments?

Thanks

RE: HSS Shear Lugs

Is that supported by code? Then OK.

If not, then I'd check the flange for the same criteria as if it wasn't grouted.

As an academic consideration: If the HSS wall was really thin and a shear tab was highly loaded and wanted to rotate the HSS wall outwards, then 2" of unreinforced grout wouldnt stop it.

RE: HSS Shear Lugs

(OP)
Its not just 2'' of grout. The entire inside of the tube is filled with grout. More like 10''. 2'' grout is only on the outside.

RE: HSS Shear Lugs

Edit: I understand the question is regarding a shear tab.

Bottom of shear tab can't "push in" the tube given the grout inside.
But, top of shear tab will "pull away" from tube, and you'll be relying on the grout in tension, right?

Like below image?



RE: HSS Shear Lugs

Are we talking about a "shear tab" beam connection, or a shear lug welded to the underside of a base plate?

RE: HSS Shear Lugs

I assume OP is talking about a shear lug/key beneath a baseplate. I agree that there is no way for the wall to buckle if surrounded on both sides by the grout pocket.

But for what it is worth I think I've used compact sections the handful of times I've designed HSS shear lugs.

RE: HSS Shear Lugs

I personally would not count on the grout being fully effective inside the tube....at a minimum, I would close off the end of the tube with a cap pl to avoid the open end affect and all it's consequences and then treat the tube as empty....

RE: HSS Shear Lugs

If there is grout inside the HSS and outside of the HSS, the wall cannot buckle or deform. It will fail in the weld connecting the HSS to the baseplate, HSS shear, or the HSS breaking out the side face of the concrete if there is relatively little cover. Just make sure you have adequate means to actually fill the HSS w/ grout. Typically its done w/ a grout hole in the baseplate over the HSS.

RE: HSS Shear Lugs

Quote (OP)

Regarding the failure modes of the shear lug tube, local bending of the flange is not a failure mode as the tube is infilled with grout. Any comments?

I believe this to be a misunderstanding of the statics of the situation. The front and/or back walls of the HSS need to be checked for flexure. If you're confident that you can get the grout in there you can, at best, split the load demand between the forward and rear HSS walls.

The grout fill would prevent local buckling of the sidewalls but I wouldn't dream of putting an HSS in there that needed that for it's survival. This is a mission critical element and and cost difference between thin wall and thick wall is surely zero in the grand scheme of things.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

(OP)
I agree with MotorCity and bootlegend, that the wall cannot buckle. I am going to provide grout hole over the HSS tubes.

@KootK, I dont think the check for the wall flexure is necessary as MotorCity has pointed out. Failure modes will be weld between baseplate and HSS, HSS shear failure and HSS bending failure.

RE: HSS Shear Lugs

Quote (BAGW)

@KootK, I dont think the check for the wall flexure is necessary as MotorCity has pointed out.

Well, there's an easy way to settle this. Why don't you guys put your heads together and post a free body diagram of the situation that doesn't involve front or back HSS wall bending? I'll strip it naked and drag it through the street like Cersei Lannister.

The only model that I can think of that would ameliorate wall bending is shown below. Two takeaways:

1) Technically there's still bending the back wall and;

2) For this not to produce nodal -- albeit confined -- bearing failure at point A, the bearing stresses applied to the lug would need to be so ridiculously low that there'd really be no point having the lug in the first place. Just glue a styrofoam coffee cup under there instead.

I suspect the mechanism shown below is where things would start off at low levels of load. As load increases, local crushing the back corners will spread out and impose an unbalanced transverse load on the back wall. As the back wall deflects, so too will the front wall and then you'll have flexure in both. It's obviously not going to be wL^/8 but just how much less it would be is anybody's guess. At that point it's thesis fodder, not practical engineering.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

I feel like the magnitude of strain in the surrounding grout required to mobilize a significant flexural response in the hss wall would spell more trouble for the brittle grout than the tube itself.

RE: HSS Shear Lugs

I agree with AggieYank that the grout will help resist the compressive load so that the wall can't push in, and that it will do less for the tension load (although in reality it will help).

If you're at all concerned I would be slotting it.

RE: HSS Shear Lugs

kootk - in the free body diagrams you have shown, the back wall of the HSS is free to deform. In reality, I think we are talking about a situation where there is a resisting force there, namely the concrete surrounding the lug. If there were only 1" of concrete behind the lug, I would agree w/ your FBD. But consider if there was 3' of concrete behind the lug. Do you still believe the back HSS wall would deform? Of course, the true situation falls somewhere between those two extremes.

RE: HSS Shear Lugs

@MC: thanks for circling back to further the discussion.

Quote (MC)

in the free body diagrams you have shown, the back wall of the HSS is free to deform. In reality, I think we are talking about a situation where there is a resisting force there, namely the concrete surrounding the lug.

The static picture that I have in my head, reproduced below, is one where the windward wall of the HSS lug is pulling away from the surrounding concrete and therefore derives little benefit from its presence. The motion picture of this that I have in my head is associated with seismic and what form this takes under cyclic loading. Frankly, it's got me questioning whether or not HSS are really appropriate for this application.

In the interest of logical sport, try this on for size. The argument I disagree with goes:

1) Concrete all around thing embedded.

2) Concrete resists deformation in thing embedded.

3) 1 + 2 = no forces developed in thing embedded.

Postulation: if this were true, could the same logic not be applied to the HSS lug macroscopically? If so, then there's no need to design it or its welds for primary bending. Just size your welds for shear and you're off to the races.






I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

The key here, I think, is to properly appreciate the differences in stiffnesses and crushing strengths of the two materials involved. If this were a steel lug embedded into molten steel and filled with molten steel, I'd get in line.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

Do you think the grout could strain enough to allow the tube wall to yield in bending before the grout fails by crushing or some other brittle mechanism? I'm having a hard time visualizing how local bending could become the governing limit state.

RE: HSS Shear Lugs

Quote (bones206)

Do you think the grout could strain enough to allow the tube wall to yield in bending before the grout fails by crushing or some other brittle mechanism?

No. In fact, even with an HSS lug designed not to plastify, I guarantee that you'll get some concrete crushing at the hot spots that I've identified. It's part of what has me wondering if HSS's are appropriate choices for these things. Really, the whole point of designing the walls not to yield is to preclude/limit excessive grout crushing. Faux deflection control and a good application for Sx rather that Zx (North american nomenclature).

It's also dependent on what you decide to call failure here. If you take my last sketch and extrapolate it through a bunch of earthquake cycles, it would probably shake down to a circular blob of beat up concrete in the middle with the HSS walls draped around it like hoop steel under load. Would that have some residual capacity? Sure. Is it a good situation? Not in my estimation. You'd have troublesome movement and be inviting lateral bursting issues.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

the face of the HSS has no significant stiffeness compared to the sides...as koot is pointing out this develops hot spots @ the corners and with , say, seismic cyclic loading may begin to crush the grout locally at first and spread...leading eventually to a loose shear lug in the pocket...

RE: HSS Shear Lugs

I agree that it depends on what you call failure. It may be hard to quantify a local crushing/fracture failure mode, but intuitively, that "failure" would have to occur before the HSS wall bending failure could have a chance to occur. So my argument is not to worry about the HSS face bending because by then your shear lug is already gonna be having performance issues.

If you use KootK's free body diagram and assume a narrow bearing width (maybe 2x wall thickness) at each the HSS walls parallel to the load, you could probably make a reasonable estimate of the bearing pressure on the surrounding grout and compare to an allowable. That is probably what I would use as my failure criterion. This is probably one of those gray areas up to judgement and not covered explicitly in a code.

RE: HSS Shear Lugs

Quote (sail3)

the face of the HSS has no significant stiffness compared to the sides

Yeah, that. You dun speak KootK goodish.

It occurs to me that there is a very good analogy for this situation: the mundane column base plate.

1) You get serious hot spots.

2) No way in hell they work without some local concrete crushing and redistribution.

3) We design them by preventing flexural plastification and this seems to work somehow.

Quote (Bones)

So my argument is not to worry about the HSS face bending because by then your shear lug is already gonna be having performance issues.

Okay. But this is very much like saying that we ought not worry about column base plate bending because by then we'd already be having grout/concrete performance issues. It's true but it's certainly not the conventional approach.

Quote (Bones)

If you use KootK's free body diagram and assume a narrow bearing width (maybe 2x wall thickness) at each the HSS walls parallel to the load, you could probably make a reasonable estimate of the bearing pressure on the surrounding grout and compare to an allowable.

I agree that this would be a valid approach. Interestingly, there's a column base plate design method for lightly loaded base plates that works pretty much the same way: Link.

My beef with this method for this application is that it's highly unlikely to work out in most practical scenarios. In my experience, folks in the wild are sizing these things for monster loads and need the entire HSS width to get it done. OP can correct me if I'm wrong but I believe that we're talking about an HSS 10x10 here and about 500 kip. No 2x thickness is going to move that kind of load unless the key is 4' long.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

In the interest of being constructive, I think that something like this would be a much better choice for serious, woman sized loads. Again, not dissimilar from what Blodgett has us do for monster column base plates. Just sideways.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

(OP)
@KootK Yeah the loads are pretty high. They are in range of 500-700kips. The tubes are close to 12''x12'' and are 2.5' long. So bearing capacity is 0.55 x 5ksi x 12 x 30 = 1000kips. Minimum width of tube I need to resist the load in bearing is 8'' assuming the bearing only on the front face. No way I can get 2 x thickness to get the shear lug to work in bearing.

Quote (KootK)

If you're confident that you can get the grout in there you can, at best, split the load demand between the forward and rear HSS walls.
Maybe I will design the lug for atleast this flexural behavior. I will use wl^2/12 and the HSS wall spanning 'D-2xt'. This should not be difficult to achieve.





RE: HSS Shear Lugs

I'm curious now, will you put your work point at mid height of the lug?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

Yea, I was thinking even while I typed that 2t was too conservative. Maybe there’s some testing out there that could shed some light on the performance of HSS shear lugs at ultimate strength levels. Maybe the stresses at the corners get happily redistributed or maybe the grout spalls away and the lug moves.

Back in the day I did some beta testing for a software company who was developing a FEM base plate design program. They used elastic springs for the concrete to calculate average stress under each element node. If I remember correctly, they would compare the peak bearing stress to the allowable bearing stress without any averaging or consideration of concrete crushing.

RE: HSS Shear Lugs

(OP)

Quote (KootK)

will you put your work point at mid height of the lug?
. Center of column/Center of lug at top of baseplate. That is top of shear lug.Why do you ask?

Quote (bones206)

Yea, I was thinking even while I typed that 2t was too conservative. Maybe there’s some testing out there that could shed some light on the performance of HSS shear lugs at ultimate strength levels. Maybe the stresses at the corners get happily redistributed or maybe the grout spalls away and the lug moves.
. I couldnt find any. Tried looking through all resources.

RE: HSS Shear Lugs

Quote (bones)

If I remember correctly, they would compare the peak bearing stress to the allowable bearing stress without any averaging or consideration of concrete crushing.

I had a copy of RISA base back when it was standalone. I remember trying to model a base plate for a modestly loaded wide flange. For something that calc'd out at less than 3/4", I had to go to 2.5" in the FEM model to rid myself of the concrete bearing stress issues. Obviously, I retired the FEM for my common base plate designs.

Quote (BAGW)

Why do you ask?

This is a monster load and a long lug. With your work point at the base plate and your center of shear resistance 15" below that, you're dumping as much as 875 kip foot into the system at the joint. The question becomes where to put all that moment. If it goes into the base plate and anchor bolts then that obviously complicates the anchorage design. If it goes into the column, then you've got to deal with it there. You'd need something to the tune of a W14x132 to deal with that moment on an Mp basis alone. You'd also want to consider the impact of that moment on the column base rotation if that's where the moment goes. You don't want your lug rotating along with the column in such a way that your lug bearing stress distribution strays substantially from a uniform distribution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

(OP)

Quote (KootK)

Dont we design the shear lug and the lug weld connection for that eccentricity? Assuming uniform bearing on the face of the lug and considering the moment arm from the center of the lug to attachment?

RE: HSS Shear Lugs

how wide is the base pl...a sketch would help....monster load

RE: HSS Shear Lugs

Quote (BAGW)

Dont we design the shear lug and the lug weld connection for that eccentricity? Assuming uniform bearing on the face of the lug and considering the moment arm from the center of the lug to attachment?

We do design for those things but they don't eliminate the moment imposed by the key on the joint that I mentioned in my previous post. They're just part of the load path that delivers the moment to the joint.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: HSS Shear Lugs

I believe the allowable bearing stress formula for shear lugs is somewhat liberal based on the vertical confinement pressure of the base plate above the grout. However, picture a base plate rotating and lifting up on one side, removing that confinement pressure on the tension side. Then consider that the shear lug is trying to do an Appendix D pryout maneuver on that same chunk of now unconfined concrete/grout. Add to that the high local bearing pressures at the "V/2" HSS walls trying to initiate a breakout cone...

I know we are really getting into the weeds answering a pretty innocuous question, but to me there are signs are pointing to brittle failure mode here. I think it's worth getting into the weeds over and it would definitely concern me more than the potential local yielding of the HSS wall.

I did take a few minutes to look over an experimental study this morning while my coffee was brewing. Inn this study they were using plates for shear lugs, but the results at large drifts seem to back up the general failure mode I'm describing:

Quote (pg 244)

Examination of the specimens after testing revealed that none of the shear lugs experienced any
noticeable deformations, and that the welds attaching the shear lug to the base plate all remained
intact. At the smaller deflections (less than 2% story drift), data from the string pots on the
column bases reveals no significant lateral movement of the column base (less than 0.04 in),
indicating that the shear lugs did indeed provide adequate lateral resistance to movement.
It wasn’t until the larger deformations (greater than 4% drift) that the shear lug became
ineffective. As noted previously, it was discovered in post-test analyses that there was a
significant amount of damage to the grout underneath the base plate and surrounding the shear
lug. At these higher deflections, the grout was crushed, removing any bearing surface for the
shear lug, and the shear lug became ineffective. Large lateral movements were observed at the
base of the column at the later end of the test cycles as the lateral resistance of the base shifted
from the shear lug to the anchor bolts.

RE: HSS Shear Lugs

(OP)

Quote (Kootk)

can the gusset plate and weld between the gusset plate column and base plate be designed to take the moment due to eccentricity from shear instead of designing the column or the base plate for the eccentric shear moment?

RE: HSS Shear Lugs

I'd probably be looking at it as:



and

RE: HSS Shear Lugs

Quote (BAGW)

can the gusset plate and weld between the gusset plate column and base plate be designed to take the moment due to eccentricity from shear instead of designing the column or the base plate for the eccentric shear moment?

I don't think so. If I understand your proposal correctly, the welds would transmit the moment to the column and/or the base plate an you're back to the same situation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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