Connection between Column and Base Plate 1

[azureblue83]

Designing the connection between the column and the base plate. The column is modeled as pin connected at the base. The column is designed for primarily gravity load, but need to resist some shear due to lateral loads.

There are a lot of design guides out there about how to design the base plate and anchor bolt (AISC Design Guide #1 for example), but not much about the connection between the column and the base plate.

I want to simply just weld the column to the base plate, designing the weld size to resist the shear? But would that weld count as fix connecting the column to the base?

Should I use AISC Manual Part 10 regarding Simple Shear Connections? or is this redundant?

 

[canwesteng]

That's the standard detail. Count is as pinned, assuming you have a suitable lateral load resisting system to take loads down to the foundation, since that will be much stiffer than the fixity at the column base unless you've designed it for moment connectivity.

 

[azureblue83]

By standard detail, you meant welding? and design the weld size for shear? Thanks.

 

[jayrod12]

Yes, design for shear.

Generally though, the column is fully welded to the base plate so the size is never a question. The shear capacity for the smallest weld is still more than enough when you have that much of it.

 

[azureblue83]

Thank all.

 

[WiSEiwish]

For a wide flange column, I include on my base plate detail that welds can be omitted on the flange toes and web fillets. My understanding is that those welds are sometimes tricky and they don't add much resistance.

If you wanted to get extremely technical, there will be a small amount of moment at the top of the base plate if the rotation takes place at the bottom of the base plate. As the moment approaches 0 at the connection because it is pinned it will have a certain miniscule value at the top of the base plate assuming the thickness of the base plate is not like 2ft or anything crazy like that.

 

[JeanLucPicard]

You must weld it. Then try to look the column and the plate as a singular connection, rigid. Thus, "working together. You must break the moment with anchor bolts. The anchor bolts, therefore, must be in the near proximity of the web. By doing so, the couple forces would minimize the moment (look out for the minimal distances). The weld itself, along with the anchors will resist moment, but not by much, so they can be neglected.

Try checking the weld with equivalent, mises hencky theory of failure: σe=√(σ²+3*τ²)

If two bolts are not enough to resist shear, try welding a smaller profile to the base plate (wedge).

I captured some connection details.



Live long and prosper!

 

[JeanLucPicard]

Did not upload, sorry, here it is!

Live long and prosper!

 

[graybeach]

Your column to base plate welds also need to be designed for the vertical force in the column - unless you mill the bottom to bear.

 

[azureblue83]

Thanks.

 

[SAIL3]

greybeach..that is an interesting point about milling the column base.....never thought of that...however, I would normally assume that ,without milling ,the base would be relatively square and that during the welding process, any minor gap would be closed.....if there is a significant gap, then the weld would probably yield and one would end up in a bearing situation anyway..it's selflimiting....there may be a problem with cyclic loading, though...I never skimp on welding the base pl to the col as there are many unanticipated loads that may occur(shipping, handling, etc)

 

[wannabeSE]

AISC FAQ

https://www.aisc.org/DynamicQuestion.aspx?Grpid=6&QueId=1546&ste=_Preview&id=2100&type=0

Question: Is it commonly necessary to mill bearing surfaces after sawing?
Answer: No . . .

 

[KootK]

Okay, I'm intrigued/confused myself. As a designer, do I need to do anything special to ensure that my columns are fit to bear directly for compression loads?

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.

 

[PittEng88]

Don't forget that OSHA requires that all column base plates must be designed and fabricated with a minimum of 4 anchor bolts. Unless the column weight is less than 300 lbs., or the column is not axially loaded and supported laterally at the top, then 2 anchor bolts my be provided.

 

[canwesteng]

I've never done any of those things, what's being suggested here falls into the category of
a- by inspection, does not need to be checked (shear in welds - obviously bolts will govern)
b- condition may exist on a sheet of paper but will never exist in the real world (welds in compression - any gap would be immediately filled by weld anyways)

Just call out base plates welded to the column and you're done OP.

 

[azureblue83]

Thank you all for the input.

 

[KootK]

I'm going to contest part of that Canwest. A fillet weld across a compressed gap is still a fillet weld in shear. Closing the gap may well rupture the weld in shear before yielding it in compression as welds are not particularly ductile. If a substantial gap were present, I would definitely design the weld to carry the column compression. That's why I'm anxious to confirm that default fabrication practices yield an appropriate "finished to bear" condition.

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.

 

[canwesteng]

I don't disagree a substantial gap would be an issue with shear in the weld. I don't think in fabrication you will ever see substantial gap though.

 

[KootK]

That's what I want to hear. Thanks Canwest. My expectation has always been that base plates would be fabbed square to columns and any angular tolerances would be taken up in the grout bed.

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.

 

[canwesteng]

Maybe if you get stuff fabbed in an uncertified shop. I've only seen work done by CISC/AISC certified fabricators. Even if the gap were to exist the weld will fuse into the gap and bear to some extent - see attached sketch. This kind of relate to my other pet peeve - being forced to check the base metal for welds. Since fusion occurs well past the assumed face of the weld, you never can't have a base metal failure there. In fact, the same U of A paper that recommended the orientation factors we have in S16 now recommended removing the base metal check.