In the case of steel structures we can use plastic properties of steel for design of structures. Plastic reserves are in beams loaded by bending. In the case of statically indeterminate structures we have additional plastic reserves. Finite Difference Method and Finite Element Method have been used for evaluation of deformation of elastic-plastic behaviour of structures. In the case of combination of load (bending moment, shear force and normal force) interactive formula has been used. HMH (Huber -- Misses- Hencky) criterii has been use for evaluation of plastic properties of steel.