Wide Flange to HSS moment connection

[Ben29]

I want to do a moment connection of a wide flange beam to an HSS column whereby the beam flange is 6" wide and the workable flat of the HSS column is only 4 1/2" wide. The moment is relatively small (50 FT-K).

Design Guide 24 suggests that the workable gauge of the HSS must match the beam flange width for the Directly Welded Flange Moment Connection. But if I can develop the moment using only the 4 1/2" flange connectivity, am I OK?

 

[DrZoidberWoop]

A concept like this would work. The flange force resulting from your moment will vary greatly, depending on the depth of your beam. You could also do an end plate style connection.

 

[KootK]

Design Guide 24 suggests that the workable gauge of the HSS must match the beam flange width for the Directly Welded Flange Moment Connection. But if I can develop the moment using only the 4 1/2" flange connectivity, am I OK?

Can you direct us to the DG24 provision so that we can easily check it out? That connection type, possibly paired with a field bolted splice in the beam, can be pretty economical. The only problem that I could see cropping up with the beam wider than the column is the tendency for that to create kind of a fracture sensitive re-entrant corner where the flange meets the HSS wall. And even that could probably be addressed by tapering the width of the flange as it comes in (adds cost of course). I'll reserve final judgement until checking out what you're seeing DG24 however.

 

[Ben29]

DrZ: Thank you. My beam depth is 18 inches. My question is whether or not there is anything technically wrong with doing a direct connection of the flange to the column when the flange width is slightly wider than the workable flat of the HSS column. Here is a plan-view of the connection:

 

[Ben29]

@KootK

 

[DrZoidberWoop]

Ben29: You're going to have roughly 33 kips going into the HSS (50ft-kips*12/18)=33.3 kips. That's substantial. The force is going to go to the stiffest area, which is going to be the longer walls for your hss orientation. I might even size the welds to the longer hss walls by discounting the contribution of the shorter hss walls. That way, the load goes where you want it, and doesn't "pull/pry" on the hss wall. You have a lot of options to achieve your goal.

 

[KootK]

Thanks for the info Ben29. While DG24 doesn't strike me as being definitive on the subject, I personally still worry about the fracture mechanics aspect of it. To me, it feels as though you're creating natural points for the weld to unzip. If sticking with this connection typology, I'd either taper the flanges as I mentioned previously, or add some flange plates as shown below.

 

[KootK]

 

[agfengineer]

I have never been a fan of HSS to WF moment connections, they are just more involved than a WF col solution, HSS columns are so limited in their I vs. a WF col, and, typically, they completely violate the strong column / weak beam principle. However, we just had to use one for a canopy frame where the client really wanted HSS columns. SOMF is not permitted in this SDC (D), so we used an RBS connection to pull down the capacity of the beam and at least fulfill the intent of an intermediate steel moment frame. In for plan check now, we'll see if I get away with this. I know that most of you are well away from the seismic demands that constrain the use of an SOMF, but I would still try to keep the SC/WB condition intact if possible. Doesn't matter where the lateral load comes from, ductility = good. (Yes, I see the drafting errors at beam copes. )

 

[KootK]

However, we just had to use one for a canopy frame where the client really wanted HSS columns.

Speaking to your canopy, and probably Ben29's one story frame, there really is no logical reason to maintain the seismic strong column / weak beam business in a one story frame. It winds up being the same single story mechanism whether the hinges form in the columns or the beams. And it's not as though you didn't require plastic hinges at the base of your columns in any case in order to form a complete mechanism. If your plan check reviewer is any good, I suspect that you'll be okay on the canopy.