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Removing a portion of a flange from a web tapered steel column

ipdBob (Structural) (OP)
2 Apr 12 14:36
I have a client who is attempting to place a ten pound load in a five pound bag. In order to fit a new section of electric switch gear in an existing building, the electric contractor would like to cut a section out of the flange of a web tapered steel metal building column. The strip removed from the flange would begin 2" above finish floor and extend up to 92" above finish floor.

I have been asked if this would work structurally. Although AISC Design Guide 25 covers the design of web tapered members, it does not give any guidance if the flanges are of unequal width. Am I too far off base if I weld a slightly wider flat plate to the affected flange if the built up section has greater section properties than the original section (I, Sx, r)?

Any suggestions would be appreciated.
ztengguy (Structural)
2 Apr 12 15:02
What would the detail look like at the 92" elevation? I would think you would have a stress increase due to the re-entrant corner there.
TLHS (Structural)
2 Apr 12 15:10
If I'm picturing this right you're reducing the width of the flange on one side and trying to replace the capacity by putting a thicker plate on top of the remaining section.  So you'd end up with a flange that extends further on one side than the other.

You'd be going from what is a doubly symmetric section to something that's asymmetric.  You're primary mechanism for failure had likely been flexural buckling, but after modification you've opened things up to torsional and flexural-torsional buckling as well.  You'll get a twist in your buckling failure.

The AISC spec has a section about this.  I've never used it, but it looks like it's pretty similar to the one in the CISC code.  You'll get to do some fun things like work out the torsional constants for your section.
ztengguy (Structural)
2 Apr 12 15:16
I was picturing a section cut out and not tapered, but vertical, then transitioning to a tapered section.

If you have to cut from one side, dont you have to cut from both, or is the compression side smaller?
ipdBob (Structural) (OP)
2 Apr 12 15:23
TLHS is correct. The plate would be added off center and the section is no longer symmetric. ztengguy is also correct that stress would be added to the re-entrant corners. I would most likely add stiffener plates at the corners to reinforce the column. I would extend the plate a distance beyond the 92" elevation to develop the weld. Not sure how to handle the flexural torsional buckling.

Also, I am not sure whether to shore the column before adding the reinforcing or add the reinforcing and then remove the flange section.

I may tell my client to relocate the equipment. That solution may be cheaper.
msquared48 (Structural)
2 Apr 12 15:42
If this is a PEMB Mainframe, I would be VERY leary of doing this.  

That being said, if you do add additional strips on top of the web, balance the strips to either side of the web, trying to balance the area on each side of the centerline, as well as re-providing the original S value.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 

JedClampett (Structural)
2 Apr 12 16:28
Agree with msquared wholeheartedly. If this is a PEMB, it's designed to the gnat's tushie. Any little tweak, no matter how insignificant, might send it over the edge. If they provided a 5 1/2 inch wide flange, they needed a 5 1/2 inch wide flange. Reinforcing it might change stiffness, torsional properties or length width ratios.
I hate to disappoint the sparkies, because if you ask them to change something, they're likely to make it worse, but even they understand that if you cut into a building column, it's a bad thing. Can they fit their gear into a slightly smaller box?

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