Cutting Bottom Flange of Steel Beam (In Place)

[jack36]

I have a project under construction and have a beam that was specified as a W12x50, but they installed a W21x50. The other way around would have been a lot worse, but there are two ducts that need to pass under this beam and a certain ceiling height is required. This is an interior beam that supports a floor and is not part of the lateral system. The concrete deck has already been poured. The architect has asked if the bottom flange could be cut in the area the ducts need to go, so I'm looking into the possibility.

They want to cut out 5 1/2" from the bottom of the W21x50. That would leave enough depth to place horizontal stiffener plates at 14" below the top of beam. My quick preliminary check shows it wold work with that depth and appropriate stiffener design, but I realize there are a lot of things to consider: cutting heat, temporary shoring, preheating, welding inspection, stiffener design, etc.

I would make them shore the floor to take load off the beam and shore the beam on each side of the cut prior to work. I have enough depth to work with that I could add stiffener plates away from the cut edge about 1.5 inches. That will move the stiffener plates away from the heat-affected zone of the cut edge. All welds would be inspected.

I would appreciate any additional thoughts on this situation.

 

[KootK]

You're saying all the right stuff regarding shoring, preheating, etc. I think that you've got this down cold. KootK approved.

If you're going with only horizontal stiffeners, I actually think that you could get away with no shoring / load removal here. You could install the stiffeners and then make the cut. I guess it depends how agressive and contractor friendly you want to be.

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.

 

[BAretired]

Definitely, the beam should be supported each side of the cut before commencing the work. You could weld an angle each side of the web just above the duct and extend them well past each edge of cut. The angles would replace the cut flange and stiffener plates may not be required.

BA

 

[KootK]

Questions:

1) simple span beam?

2) where in the span(%) will the reduced section occur?

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.

 

[Triangled]

I don't see shoring by itself necessarily removing existing stresses from beam, just preventing additional stresses.

I like BA's concept of welding the new bottom flange, ie angles each side, prior to cutting off the existing bottom flange, as the reverse order would allow further accumulation of compressive stress in the top flange should there be any movements in the shores.

 

[Archie264]

This might be a dumb idea but I'll mention it anyway. That is, if you do shore it, at that point might it be easier/cheaper to just replace the beam with the W12 originally specified? Just something to consider.

 

[JStructsteel]

@ Archie, It might have form deck welded or composite floor, making it hard to remove. Also, might be hard to get out of the room/building

 

[Archie264]

Ah yes, good point. Also, getting the new beam in there. Getting the old one out could be helped with a cutting torch, though.