# Why does my, with RFEM simulated crosssection (class 3) fails at M_pl and not at M_el

I have designed a steel-plate with the c/t-Ratio of 124. The plate is a replacement system for a web. So the plate is 8,1 mm thick and 1000 mm wide. This implies, that the W_el=1343,33 cm³. The yield strentgh of the steel is fy=23,5 kN/cm².

-> M_el=W_el*fy=315,68 kNm

I have applied, as you can see in the picture, a line load of 315 kN/m over 1 meter, so overall 315 kN. The lever-arm is 1 meter. So the amount of momentum at the end of the plate is 315 kNm. That is pretty close to M_el. Unfortunally, the buckling load factor amounts 1,48 that means that you can multiply the load (315 kN) times 1,48 till the plate buckles. That ist ca the M_pl=473,53 kNm. But the crosssectionclass 3 implies that it will fail after reaching the M_el.

Anyone got an idea why it is like this?

The load is calculated non linear with the theorie third order. Imperfektions are added like the Eurocode 3 predefines. The Material behaviour is isotrop plastic.