fb to use when strengthening an existing wf with a tee

huangyhg

fb to use when strengthening an existing wf with a tee
i am strengthening a variety of wide flange shapes made from  a7 steel (fy=33 ksi) by welding tees on the bottom.  when checking the combined superimposed stresses on the top flange of the strengthened section using asd methods, is there any justification for using a higher allowable fb than .66fy?  don't see it in the literature but was wondering what others do in this situation.
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not sure of your final configuration:
is the added "t" welded "upside down" with the bottom of its web to the middle (bottom flange) of the wf?  this would give you a combined beam with three separate flanges, each separated by a web.
or is the t welded right-side up so its web is sticking down into space?   this gives an upper wf flange, the wf web, the lower wf flange, the t flange, and the t web to make up the beam.   
no justification for changing fb in this situation.
no, use the appropriate fb, based on your strength and the spec.  that being said, if you determine mp you will likely get a boost over using fb.
are the tees the same material as the beam ?
if the tees have a higher fcy(fb) then you've got a rationale (allow the beam to work up to it's allowable fb (tees'll strain as well) but instead of the whole thing failing, the tees'll keep the flange stable and provide a stiffer loadpath for additional load.
are you reinforcing the tension side as well ?
agree that there is no justification for increasing fb.  in fact, depending on the unsupported length of the compression flange, you may have a decrease in fb.
thanks for the replies.  the wt's are a36 steel, attached web-up to the bottom flange of the existing beams.  the top flange of the existing beams are braced by an existing concrete slab which is to remain. structural eit, i will try calculating mp and see if that gets me greater capacity.  
the justification for going from 0.6 fy to 0.66 fy in asd is the average shape factor of 1.1 (that actually comes from plastic design). the criteria for allowing this is:
1) minimum width-thickness ratios (so local buckling does not occur with the higher strain levels)
2) unsupported compression flange not exceeding lc
3) symmetry
obviously, you do not meet the 3rd criteria.
jike-
if you use lrfd (i know that isn't the case when using fb=0.66fy) you can get much higher shape factors if the pna doesn't lie at the same location as the ena.  this is the reason that wt designs limit mp to 1.6my (to ensure elastic behavior under service loads, because shape factors for wt's can exceed 1.6).
i don't see anything wrong with checking it using lrfd and taking advantage of the higher shape factor.
fb of 0.6 fy is now listed as the asd flexure maximum.
civilperson-
where did that come from?