Why is interaction check still 100% even with a higher load?

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Interaction check is performed for a combination of shear force and torsion, but also for their combination with bending moment. Even after internal forces have been increased, the interaction check can still be at 100%. The reason is in reaching the plastic branch of either reinforcement bar or concrete. Let´s show it on example:

The cross-section is loaded by shear force 80kN and bending moment -180kNm. The top layer of rebars just reached yield point – shown in stress along the height of cross-section and stress-strain diagram of rebar.

Let´s increase the bending moment to -200kNm and we will see, that we are increasing strains on the plastic branch for the top layer. Layers under the top one will start to plastify but in the top layer, 100% of yield stress will remain. That is why the check of interaction gives 100%.