The nonlinear structural analysis is considered as a basic design procedure, which is used for checking of the structural robustness in accidental design situation. It is explained by following reason: a nonlinear structural analysis based on realistic constitutive relations for basic variables (average values) makes possible a simulation of a real structural behavior. It should be pointed that, implementation of the nonlinear structural analysis in design of concrete structures requires an alternate approach to safety verification. The paper presents a new approach to safety format for nonlinear analysis of RC structures subjected to accidental loads.