I am conducting a pushover analysis of a 2D frame using varying rebar grades (400, 500, and 600 MPa). I use P-M2-M3 hinges in columns. The analysis was completed for all cases. However, it showed this warning (PMM Interaction surface is not convex ) for fy=500 Mpa and 600 MPa but not 400 Mpa. What is the reason for this? Should I use user-defined hinges?