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
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
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
RE: HSS Shear Lugs
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
RE: HSS Shear Lugs
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
RE: HSS Shear Lugs
RE: HSS Shear Lugs
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
@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
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
RE: HSS Shear Lugs
If you're at all concerned I would be slotting it.
RE: HSS Shear Lugs
RE: HSS Shear Lugs
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
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
RE: HSS Shear Lugs
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
RE: HSS Shear Lugs
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
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.
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.
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
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
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 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
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
. I couldnt find any. Tried looking through all resources.
RE: HSS Shear Lugs
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.
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
RE: HSS Shear Lugs
RE: HSS Shear Lugs
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 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:
RE: HSS Shear Lugs
RE: HSS Shear Lugs
and
RE: HSS Shear Lugs
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.