allowable bending stress

huangyhg

allowable bending stress
i am reinforcing a curtain wall with a piece of plate steel .5in x 5 in.  i attached the reinforcing to the mullion  wall with screws to pick up the horizontal shear so i now have a transformed section.  i found the stress in the steel to be 4 ksi....now i am unsure what to check this against....i want to say .6 fy but i can't back that up with anything....if someone could help me out i would be much obliged.  thanks
if it is in tension, you could probably justify 0.6 fy x 1.33 for the allowable stress.  with wind working both ways, you would also have compression as well.
for the compression, i would check my plate against local buckling (compact vs. non-compact) per aisc asd table b5.1.  then, for the "global" mullion "beam" i would look at lateral torsional buckling.  are their horizontal elements that can brace the built-up mullion against lateral twist or translational movement of your plate?  what is then, the unbraced length of your composite section and treat it as a beam under moment with unbraced length l.
also...have you checked the material of the mullion against compression?  it may be a weaker material and actually control your design vs. the steel plate.