HSS Shear Lugs

[BAGW]

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

 

[AggieYank]

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.

 

[BAGW]

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.

 

[AggieYank]

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?

 

[sbisteel]

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

 

[bootlegend]

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.

 

[SAIL3]

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....

 

[MotorCity]

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.

 

[KootK]

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.

 

[BAGW]

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.

 

[KootK]

@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.

 

[bones206]

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.

 

[Tomfh]

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.

 

[MotorCity]

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.

 

[KootK]

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

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.

 

[KootK]

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.

 

[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? I'm having a hard time visualizing how local bending could become the governing limit state.

 

[KootK]

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.

 

[SAIL3]

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...

 

[bones206]

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.