Beam to HSS Column Connection

[cgstrucg]

Hello All,

I need to design an in-plane moment connection plate between wide flange beams to HSS tube columns. Please see the attached sketch. I went through chapter K of the AISC steel manual amd also design guide 24 but I can't seem to find any good examples for this situation. I could have changed this connection to a simple angle brace but because of post height and flashing restrictions, that's not possible. I am actually interested in knowing how the forces/moments would be transferred in this type of connection and what limit states to consider. I got a basic idea after reading the design guide but I want to make sure I am not headed in the wrong direction.

Thanks a lot for the help.

 

[human909]

I'd be extending the HSS tube and create direct moment connections between the beams and the columns rather than this convoluted approach.

Detailing of connections still isn't easy but it is easier than what you have proposed.

 

[KootK]

Is this a two way moment connection? In and out of the page? If so, I'd likely do as human909 suggests and run the column through the joint.

 

[cgstrucg]

Yes, it's going to be a two-way moment connection. Same plate for beams in 2 directions. Actually the reason to do what I have shown is that there is a walking platform on top of these beams and hence metal grating because of which I never extended the HSS.

 

[CANPRO]

Are the two beams the same section? If they're different sections, your connection could get messy. You are not currently showing a moment connection in your sketch, the top flange has to be connected. Which could potentially complicate the grating attachment. If you provide your beam(s) and column sizes along with approximate loads, you will get some better suggestions for connection types. I have a couple thoughts, but need the above info first.

 

[JoshPlumSE]

The image below is from the AISC design guide on HSS connections (a great resource). I believe this is a similar type of connection to what Human and KootK are describing.

 

[JoshPlumSE]

Another option is the proprietary connection mentioned in AISC 358... ConXtech. The only HSS column connection, I believe, pre-approved for IMF or SMF.

https://www.conxtech.com/conx-systems/

 

[cgstrucg]

Hey Canpro, my beam size is W12x45 (same on both sides) and column is HSS 6x6x1/2. Shear at the end of the beam is as high as 13 kips, axial 9 kips and moment going through that plate around 4.5 kip*ft in individual worst-case scenarios. I can share in my STAAD model if you want to have a clear picture of the structure.

JoshPlum, thanks for the recommendation but I can't extend the column because I need a walking platform above and hence metal grating.

 

[CANPRO]

A 3D screenshot from your model would go a long way here. When you say axial and moment going through the plate - does that mean that the beams are continuous over the column in both directions? Or is this a corner column with beams on two sides only?

 

[cgstrucg]

For some reason, staad is not showing 3D of plate and steel elements together. See attached connection attachment. Green things are plates I was talking about and the circles are moment releases at beam ends. One beam sits on top of the column and another beam attaches to the beam which sits on top of the column.

 

[CANPRO]

If your loading allows you to use a reasonable flush end plate moment connection, I would shop weld stub sections of beams to the top of the HSS (stiffen stub beams as required above the HSS) and move your moment connection away from the column. The flush end plate won't interfere with your grating above.

EDIT: Alternatively, could you replace the moment connections with knee braces? A bolted knee-brace between the column and beam would be much easier than a beam-HSS moment connection.

 

[skeletron]

Use knee-brace connections to the column. I think your overall stability will be much better regardless if you can get the analytical/rational load path to work out with a moment connection. It would also potentially have benefit for erecting the structure.

What's happening in the upper left corner of the structure near the ramp? Is that beam running continuous and cranked? Or is it two separate beams?

I'm not a huge fan of the double cope on beams because of the reduced stability.

 

[cgstrucg]

Because of some restrictions, we can't put a knee brace.

skeletron, do you mean left to the ramp? There are 2 separate beams there.

 

[skeletron]

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1569959798/tips/ramp_hiekrr.tiff[/url] ... just wondering about this joint.

Is shop welding/fabricating either the platforms or the end frames an option? There's not a lot of force going through these beams which is why I would focus on making sure the stability (both for erecting and service) is looked after.

I agree with the flush-end moment connection as CANPRO is suggesting as well. There's a few options, but I don't think running the beam over the column is the most reliable one.

 

[KootK]

If the proportions shown in your model are anywhere close to accurate, then your answer here is surely to NOT create moment connections between the columns and the beams. Rather, consider the columns to be fixed at their bases and flag poling up to stabilize the steel deck with pinned beam to column connections. With beams that long, and columns that short, there's no way in hell tht this things acting like a portal frame system unless the columns are mounted on ball joints or something.

 

[human909]

Yes, it's going to be a two-way moment connection. Same plate for beams in 2 directions. Actually the reason to do what I have shown is that there is a walking platform on top of these beams and hence metal grating because of which I never extended the HSS.

Again you can still achieve you going while extending the HSS. Sure it won't be a normal connection but it should be far better than what you propose IMO. You will likely be able to the HSS with access from the top before placing the floor plate. Welded face plates on the HSS may or may not be needed depending on wall thickness.

However KootK has a very good point.

If the proportions shown in your model are anywhere close to accurate, then your answer here is surely to NOT create moment connections between the columns and the beams. Rather, consider the columns to be fixed at their bases and flag poling up to stabilize the steel deck with pinned beam to column connections. With beams that long, and columns that short, there's no way in hell tht this things acting like a portal frame system unless the columns are mounted on ball joints or something.

Hey Kootk. I'm goint to try to go on a tangent if you'll tolerate it. In practical terms how do you usually manage base plates and the normal binary decision of pinned vs moment? Because as we all should recognised this is far from binary, in particular a real pinned joint needs pretty much needs a literal pin. I've often toggled my models between pinned and fixed to ensure appropriate behaviour at both extremes.

Likewise if you have a thick base plate with 'pin' style anchor bolts, you can still have moment connection behaviour if your axial loads are enough.

 

[KootK]

Hey Kootk. I'm goint to try to go on a tangent if you'll tolerate it.

Oh, I'm all about the tangents. My take on this tends to be somewhat unique. In my opinion, the particular detailing of the base connection mostly affects joint strength and, in many cases, doesn't have all that much impact on joint stiffness. Obviously, there are exceptions as with anchor prestressing etc. I base most of my determination considering what's going on around the neighboring structure. Something like this:

1) If this base connection rotation is not part of the lateral system, what is?

2) How much deformation would it take to fully engage the intended lateral system?

3) When the lateral system is fully engaged, how much rotational deformation would that imply at this base connection if it behaved as my model assumed.?

In OP's case, because the beams are so long and the columns so short, pinned columns would have to undergo significant rotation at their bases before the beam portal frame system really kicked in. And that makes me believe that cantilevered posts is the prudent choice for the lateral system.

 

[human909]

Thanks for the response. I appreciate the community feedback. I would still consider myself a 'green' engineer. But I just don't have enought inquisitive and thoughtfull peers around me.


I'm completely with you regarding the OP's case. One look at that and my conclusion was the same as yours.

My question partially arrose out of a recent project. A tall, slender steel tower that was designed with 'pinned' connnections. The thing is the uplift requirements on the pinned connections required quite a few anchors which once you meet that requirement then the connection largely becomes a moment connection which due required K bracing on one column significantly reduced he comfort margin in the column as all of a sudden the column was likely carrying moments when the nice clean 'pinned' conection model had none of that.