In RM Bridge, inside OpenBridge Designer, the user can select the Non-linear Material option at the Non-linear Analysis options inside the Basic Settings Tab of the Analyzer Recalc Window for the consideration of the non-linearities in the modeled structure. The option Non-linear Material activates the nonlinear material properties by using the stress-strain diagram "Non-lin" and existing reinforcement in any cracked concrete elements. The nonlinear material properties are considered according to the definitions at Analyzer > Properties > Material > Non-linear > Stress-strain diagram.
The structural analysis is performed as a 1st iteration that results in node displacements and the respective element forces. Then, these element forces as distributed for the equilibrium on the cross-section level. The software takes into account the non-linear relation between stress and strain according to the arrangement of the different materials in the section database, like a concrete section with reinforcement, and/or pre-stressing steel, a composite section like steel and concrete, or concrete-concrete composite with pre and post-tension.
The finite element model of the cross-section and the material properties in the program database are used for the different materials with their complete non-lin diagrams resulting in different strain, stress redistribution, and the respective iteration internal forces.
If the equilibrium from external and internal forces is not fulfilled in the current iteration the calculation goes back to the system for the next iteration. The element tangent stiffness is used and an integration over the elements as a force/load is added as a coupled Normal and Moment forces that cause additional strain in the element's cross-section. A modified Newton-Raphson method is used and the process is repeated until convergence is achieved.
In Analyzer > Structure > Elements > Checks, the consideration of non-linear material properties should be switched on for the required individual elements and it will only be calculated if the Non-linear material has been selected in the Recalculate Window in the Analyzer, as shown in the images below.
Hint: When stresses are displayed in diagrams, they are recalculated from the stored strain plains. In order to display non-linear material stresses correctly, the RMSET definitions have to contain Plot Parameter settings for the non-linear stress-strain diagram.
A non-linear material example can be found at the link below, with the dataset and document as part of the installation.