Why is 5% strain limit used in material diagram for EC
All steel plates like flanges or webs of profiles, stiffeners, ribs, cleats are modelled by finite elements. FEM is widely accepted in structural engineering and brings very good and trusted results.
The resulting equivalent stress (HMH, von Mieses) and plastic strain are calculated on plates. Since perfectly elasto-plastic material model is used, checking stress does not verify the plate (stress never exceeds Fy). Check of an equivalent plastic strain is performed. The limiting value of 5% is suggested in Eurocode (EN1993-1-5 app. C par. C8 note 1), this value can be modified by user in project settings. This limit value of equivalent plastic strain is taken into account in formulas in all other codes even though it is not explicitly mentioned there.
Plate element is divided into 7 layers and elastic/plastic behaviour is investigated in each layer separately. Output summary lists the most critical check from all 7 layers.