When doing section classification to calculate buckling of a plate subjected to some bending and compression on its edges, do we consider the plate width as outstand of flanges "c"? What are the limiting values for plate and bar in section classification?
For a flat bar - there is no local buckling, only global buckling.
For a plate you follow the rules for plated sections. In Europe that would be EC3-1-5.
The local design code should define the limits at which a flat bar becomes a plate. (I can't remember off the top of my head where in the eurocodes this would be found.)
I haven't seen any method of calculation to determine the cross section classification of a plate but SCI advisory note AD 391 & also design manual for steel frame for SAP 2000 all indicate/assume the cross section classification of a plate as class 3.
See below link for AD 391: http //www.newsteelconstruction.com/wp/wp-content/uploads/TechPaper/AD391.pdf
Hope this help.