Can you be more specific as to what kind of plates you want to weld? Are you welding plates to increase/beef up the top flange? Are you concerned with the heat generated by welding?
A coworker was warned by a supervisor about welding to the tension flange. I suspect the warning is due to heating the flange though I'm not completely sure (that's what I'm asking for actually). I've never been concerned about this before but since it was brought up I want to see if there are any codes or standards that talk about it.
I'm not sure of the specific sizes of members but the plate being welded to the top flange is a 3/4" base plate from a column.
Since you are not trying to transfer forces from the beam into the plate then your only concern is that you do not overheat the beam flange to a point where you might affect the allowable tensile stresses. Use minimum weld required for strength and plate thickness and have the welder mark the flange with a heat crayon.
