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RE: Backing Bars at CJP welds on Special Moment Frames Joints

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>Backing Bars : remove after CJP welds
>are completed or leave in place?  If
>removed,  is it only the backing bar
>below the bottom flange that needs to be
>removed? If anyone has an answer, I
>would appreciate your response. Thanks

Most of the testing with notch-tough CJP groove welds to column flanges on
SMF reported in FEMA 350 used backing removed at the bottom flange (i.e.,
when the bar was on the outside of the beam flange) and remaining in place
at the top flange  (i.e., when the bar was on the inside of the beam
flange).

At the top flange when backing remained in place, a fillet weld was normally
used on the bottom edge of the backing bar to the column flange. The idea
there was to make whatever natural notch effect remained at that location
(due to the unfused part of the backing bar) an internal defect, which is
much less sensitive to fracture than an external defect. I'm not convinced
that this makes any real difference in the performance, though.

The front edge of the backing bar should not be welded to the beam flange
because that would make it a part of the force transfer at the flange and
create a load on that unfused portion, which could initiate a fracture.

Charlie

P.S. Remember: Preregistration deadline for the 2002 North American Steel
Construction Conference is March 27. For more information visit
www.aisc.org/nascc.html or call 312/670-2400.

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