In this paper, a unified plastic model for concrete, in which only one criterion is considered to describe the behavior of concrete under different stress states, is used. This model can treat a major nonlinear phenomenon of concrete such as cracking; shear transfer degradation, tension stiffening and compressive strength reduction. The objective of this paper is to follow mentioned method together with presented formulation based on complementary matrix to analyze localized failure of RC panels as well as postpeak behavior while shear band is formed. This phenomenon occurs when the material acoustic tensor A(n) looses its positive definiteness and usually takes place when RC element fails in shear.