I'm studying beam cross section classifications here.
It is explained that the difference between plastic and compact cross sections is that while both do develop their plastic moments, only plastic cross sections have sufficient rotation capacity to form a collapse mechanism.
I don't quite understand what happens when a compact cross section undergoes local buckling. In the text there is an example of a rigidly supported beam. When enough moment is applied, the ends develop plastic hinges and from there on the beam acts as a simply supported beam with moments on the ends. With more rotation, the middle also develops a hinge and the structure collapses.
So let's consider this scenario with both a plastic and a compact cross section. Plastic cross section develops the plastic hinge, and has enough rotation capacity to form the collapse mechanism. But what happens with the compact cross section? By definition it also forms the plastic hinge, but this one doesn't have the required rotation capability to form the collapse mechanism, so what happens with this one?
So how does the compact cross section form a plastic hinge and not a complete collapse mechanism?