Stress-strain diagrams implemented in CSFM are based on code requirements, both Eurocode (EN) and ACI.
When dealing with concrete, it is necessary to distinguish between ULS and SLS assessment.
A concrete member in the ultimate limit state (ULS) is based on the parabolic-rectangular diagram (EN) or bilinear one (ACI) including the compression softening effect. For serviceability assessments, the different, simpler diagram is used, a linear or bi-linear diagram with an infinite elastic branch.
Bilinear diagrams with an inclined or horizontal top branch are considered for reinforcement including tension stiffening effect for both ULS and SLS checks.
Check the video below where all stress-strain diagrams are displayed and briefly described. A more detailed description of stress-strain relationships is explained in the theoretical background.