Column on column support detail

[suzpari]

Hi! I have an existing steel structure on top of which another steel structure is being added at the roof level. In some cases I have new columns that will be supported by existing columns, and to connect the two, I'm going to weld the new column to it's baseplate, which is then welded to the existing column below. I think that's called a division plate..? What I'm unsure about is the design of the baseplate. I'm used to the typical scenario where the plate is fully supported by a concrete pedestal etc. But in cases like this, how do I determine the size and thickness of the plate when it's really only supported by the web and flanges of the column below? Also in particular, a case where the orientation of the new column is perpendicular to the existing column? Can anyone help me?

 

[msquared48]

Usually, a terminating column will have a cap plate on top, or the column will stop at the underside of a W shape above.

Which case do you have?

It should be no big thing to bolt a new column base plate to the structure below. Welding will get into alignment issues that bolting can adjust for.

Mike McCann, PE, SE (WA)

 

[cal91]

I'd be really conservative with the baseplate size if you're changing the orientation of the columns.

One way you could determine the thickness required would be to assume all the load is transferred through the column flanges in a yield line analysis.(ignoring the webs - conservative)

You would take the top column reaction force and divide it into two line loads, one at each flange (shown in red)

The plate would then have two "simply supported" lines at the bottom flanges. (shown in blue)

If you're not familiar with yield line analysis, here's a crash course. It's widely used for concrete but can be used even easier (easierly? ) with steel plates.

http://wsmurti.lecture.ub.ac.id/files/2012/10/Perencanaan-Praktis-Garis-Leleh1.pdf

 

[JLNJ]

First you need to decide if you are trying to carry moment through the connection. Moment-carrying connections will be much more difficult to analyze then gravity/axial connections. Think about shear, too.

And, like cal91 says, no point in saving a nickel to keep the base plate size down. Come up with a simple, conservative analysis and provide a beefy plate.

 

[KootK]

Any chance:

1) New column depth can match existing?
2) You have access to weld stiffeners to the exusting columns?

The key is to recognize that if any points of overlap exist, pretty much all of the load will transfer through those points and the base plate will essentially just be filler to accommodate minor offsets. Local column plate crippling becomes your key design issue.

If you're only adding a single story, it's probably no big deal. If the loads are heavy on that perpendicular column, I'd consider adding some stiffeners or WT to the new column to get a bit of overlap.

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.