How to define the interaction between the core steel and the encasing concrete? I used the surface-to-surface interaction and tried to refine the mesh but it always ended with convergence problem.
you should modeling yielding core , concrete core and steel tube .
Interaction between Yielding core and concrete core is contact without friction ( friction less , normal behavior : Hard contact )
interaction between concrete core and Steel tube is contact with coefficient of friction 0.3 (friction coefficient is 0.3 , normal behavior : Hard contact)