RBS (Dogbone) on an HSS 6

[Althalus]

We have an ordinary moment frame using HSS. As you can imagine, welding is a big deal. We have some seismic activity to worry about. So, Omega on the connections are creating welds that are much larger than the wall thickness of the HSS.

So, I was thinking of introducing a reduced beam section by cutting out an oval on the top and bottom of the HSS. Calculating the reduction in Zxx and Ixx would be fairly simple.

Is there any pitfall here that the community can warn me about for this methodology for HSS?

 

[JoshPlumSE]

I wouldn't do this, if I were you. Some other options:
1) ConXtech CONXL connection: which allows for HSS columns with RBS WF beams.
2) Split and splice the HSS column at the connection. See the through plate connection (example 4.2 in the AISC Design Guide on HSS connections)
3) Use some form of a haunched connection to reduce the weld demand.
4) Some form of an end plate connection or side plate connection.

 

[Althalus]

That sounds like a great alternative (if we can manage it in our shop).

But could you enlighten me on the reason why you wouldn't do it with HSS beams?

 

[KootK]

I think that the concept is sound so long as you could:

1) Brace for LTB near the RBS.

2) Find a fabricator willing to cut that oval without creating stress concentrations. This sounds a lot more difficult than the WF case.

3) Ensure that you get no local buckling in the two, short lived channel sections that you create by doing this.

I also would not do this for the simple reason that I feel that this is a little too "off reservation" for my liking without there being some testing to back it up. Heck, I wouldn't even be willing to dogbone WF if there were not testing and precedence available.

 

[Althalus]

Thanks, Kootk.

1) How do we take care of LTB for WF RBS?
2) The fabricator has a robotic plasma cutter. They have no problem getting the exact size of ellipse with a smooth curve all around.
3) I can look at the local buckling.

 

[KootK]

1) How do we take care of LTB for WF RBS?

1) On the column side of the RBS, we consider the column itself to LTB brace the WF. This is probably even better for HSS.

2) On the span side of the RBS, we toss in some fly bracing, a roll beam, etc...

 

[Althalus]

Fly beam, roll beam...

How is that allowed? We can't attach anything to the "protected zone" of the RBS.

If you're talking about bracing the overall span, that goes without saying. And I can look at an equivalent weak axis and see how it holds up for beam spans.

But we need to do the same thing for WF. So, why is it different for HSS?

 

[KootK]

How is that allowed? We can't attach anything to the "protected zone" of the RBS.

It's located just to the side of the protected zone.

But we need to do the same thing for WF. So, why is it different for HSS?

I never said that it was different, only that it needed to be done. HSS beams are often that cross section precisely to obviate the need for LTB bracing. As such, I thought it worth mentioning.

 

[JoshPlumSE]

The first problem I have with this is that this type of connection hasn't been tested. So, right then I say there is no way to use it in an IMF or SMF.... short of doing the testing.

Second is that I worry about local affects around the cut out. Whether it be fracture (which is a big deal when you start cutting members at locations of inelastic demand), LTB, local buckling or something unknown.

 

[Althalus]

Weren't you saying that if I designed it as an OMF, then many requirements would go away?

Would that include the need for the particular member shapes to be tested?

 

[JoshPlumSE]

Weren't you saying that if I designed it as an OMF, then many requirements would go away?

True. I think your type of connection would be excluded (without testing) if you were doing an IMF or SMF. But, you might get away with it for an OMF. But, you've still got to get the building official / plan check department to buy off on it.

I still worry about local effects and buckling and such. This is a large enough deviation from common connections, that I'm probably not going to be comfortable with it. Elastic behavior doesn't worry me so much, it's the reliance on post elastic ductility (even limited ductility) that scares me.

Note: It's kinda silly, but using it for an R=3 system as opposed to an OMF (R = 3.5) would make a big difference in my mind. Just because when I say OMF, I'm saying that this system is specifically detailed for seismic resistance.

 

[Althalus]

I still worry about local effects and buckling and such.

I have taken a while to figure out why something didn't seem to be making sense. You've mentioned the things that I'd need to take into account. And all of it seemed reasonable. Yet I didn't feel like my questions was being answered. Here's what I've determined:

My question was essentially: What would be DIFFERENT about using an RBS on an HSS as opposed to a WF. But all the comments basically said:

1) Make sure you take into account all the same stuff as you would for a WF (Yes, of course I would).
2) But remember that an HSS is different too.

So, I'm still wondering "what is different?"

Of course the shapes are different. But with regard to their behavior using an RBS for seismic design, what is different? If anything, it seems like it would be better than a WF for lateral buckling.

What is different that would make an HSS worse?

 

[JoshPlumSE]

Althalus -

I like the way you're thinking about it. Your last post helps (me, at least) to better understand my own discomfort with the connection you describe.

So, what is different between an HSS and a WF type of RBS connection? My thoughts would be:
1) RBS for a wide flange is codified.
2) RBS for a wide flange is very well tested.

2a) So, the b/t ratios for WF webs and flanges used in moment connections that go into the inelastic behavior are well understood.
2b) The b/t ratios for HSS members in the seismic manuals tend to be related to axial loaded members braces and columns.​

Note: I remember some early dog bone connections had some issues with the corners of the cuts. This is why they transitioned away from dog bone configurations towards a Radial Cut to get the beam reduction. They only realized that by testing.

3) HSS members tend to be thinner. So, the b/t issue is probably pretty important.
4) HSS members are cold formed from plate, not hot rolled. So, that creates some differences in the b/t behavior as well.

4a) I wonder about the potential for a fracture to be initiated at this seam. I wonder about this whether or not the seam is in the flange or the web of the HSS.​

5) Material differences. Remember that we as a profession totally changed the ASTM spec for WF members based on the Northridge failures. Realizing that the dual rated material we used in the past contributed to the lack of ductility of those connections. Therefore, we needed to have a CAP on the Fy to prevent this from happening in the future. We'd have to take a look at the HSS materials to see if they have a similar cap. Even if they do, I'd like to look at all the differences between the old and new WF ASTMs and see where the changes lie and how many of those could also be an issue with the HSS type of connection.

 

[KootK]

So, I'm still wondering "what is different?"

I never said that it was different, only that it needed to be done.

What is different that would make an HSS worse?

I, for one, don't know that an HSS would be worse. For all I know it might be better. The problem, for me, is simply that we have little by way of verification when it comes to the performance of the proposed connection. We lack:

a) Testing.

b) Code guidance.

c) A history of successful performance in the field.

If you would feel comfortable proceeding with this connection in the absence of such things, and doing so wouldn't run you afoul of your local regulations, have at 'er.

 

[sandman21]

I question why you would be considering a RBS for an OMF system anyways, its not necessary for an OMF. I have no idea how welds are sized greater than the thickness of the HSS. The fundamentals should be worked out before trying to "push" the envelop.

 

[dauwerda]

When a RBS for a wide flange beam is created you are left with a very similar section to that of the original wide flange, just narrower. It will still have the same I shape.

Cutting an oval in the top and bottom flanges will result in a completely different section - 2 "C's", rather than a square or rectangular tube. I imagine this would result in some unpredictable behaviors that the W section does not have an issue with.
To me, that is a major difference than can't be overlooked. Also, I imagine the tips of the ovals in the middle of the HSS would result in some funky stress risers that aren't an issue on the W section.

I believe it is a stretch to expect them to behave similarly without any testing to back it up.

 

[JoshPlumSE]

Good point Dauwerda.... the behavior of an open section vs a closed section is very different. So, in the local area of the cut, the HSS has greatly reduced torsional stiffness, making it potentially subject to Lateral torsional buckling at that location. I'm pretty sure some of the RBS testing saw that the RBS area was more susceptible to LTB, which is why we are required to have LTB bracing within some distance of the RBS. This might be more of a concert with an HSS that has been reduced like the OP would like to do.

 

[sbisteel]

Something slightly mundane that could make a big difference, the HSS are fabricated out of plate with a weld seam in one of the walls, parallel to the beam axis. If the "dogbone" was to cross the RBS, I'd be even more concerned with stress risers.

Concerning expected yield strengths, the new ASTM A1085 material is made with this in mind, putting tighter control on upper bound yield strengths.

 

[Althalus]

I question why you would be considering a RBS for an OMF system anyways, its not necessary for an OMF. I have no idea how welds are sized greater than the thickness of the HSS. The fundamentals should be worked out before trying to "push" the envelop.

I'm wondering why is it not necessary? Here's what I think you're talking about:

E1.6b(a) indicates that we have
[ul]
[li]maximum plastic moment [/li]
[li]maximum expected strength [/li]
[li]a total factor of 2*1.1=2.2[/li]
[/ul]
All increasing the required flexural capacity.
Then we have[ul]
[li]Divide by the clear length between column flanges (in inches)[/li]
[/ul]Which divides by a pretty large number.
I would think that wouldn't really be anywhere near the actual calculated moment. So, this looks like we wouldn't need any special loads. Just the unmodified Seismic would control.

E1.6b(b) indicates that we have
[ul]
[li]Eh*Ω[/li]
[/ul]Which is, of course, higher. But often not so high as to increase the weld beyond the thickness of the wall. Is there another requirement because it is a fillet instead of a full penetration weld?

How am I doing so far?

 

[Althalus]

When a RBS for a wide flange beam is created you are left with a very similar section to that of the original wide flange, just narrower. It will still have the same I shape.

Cutting an oval in the top and bottom flanges will result in a completely different section - 2 "C's", rather than a square or rectangular tube. I imagine this would result in some unpredictable behaviors that the W section does not have an issue with.
To me, that is a major difference than can't be overlooked. Also, I imagine the tips of the ovals in the middle of the HSS would result in some funky stress risers that aren't an issue on the W section.

I believe it is a stretch to expect them to behave similarly without any testing to back it up.

Something slightly mundane that could make a big difference, the HSS are fabricated out of plate with a weld seam in one of the walls, parallel to the beam axis. If the "dogbone" was to cross the RBS, I'd be even more concerned with stress risers.

Concerning expected yield strengths, the new ASTM A1085 material is made with this in mind, putting tighter control on upper bound yield strengths.

I, for one, don't know that an HSS would be worse. For all I know it might be better. The problem, for me, is simply that we have little by way of verification when it comes to the performance of the proposed connection. We lack:

a) Testing.

b) Code guidance.

c) A history of successful performance in the field.

If you would feel comfortable proceeding with this connection in the absence of such things, and doing so wouldn't run you afoul of your local regulations, have at 'er.

Valid concerns. Worth looking into. I think I'll contact the Steel Tube Institute and see if they have any desire to do research on the matter.

Thank you all. I appreciate your input.