I use cohesive behaviour to simulate the interaction between two material, but I don't know how to calculate stiffness coefficients knn, kss, ktt. I found some papers which calculate stiffness coefficients for resin material ( this material contact between FRP plate and concrete), stiffness( k=αE/t). But for my case FRP bars embed in concrete, there is no material at interface between FRP bars and concrete.

How to calculate stiffness? what is the equation? what is the difference between Knn, Kss and Ktt?

Regards