This paper introduces a numerical analysis method to evaluate the residual fire-resistance of reinforced-concrete (RC) structures and proposes considerations in designing RC structures on the basis of a comparison between numerical results and design codes. The proposed analysis method consists of two procedures, a transient heat transfer analysis and a non-linear structural analysis. To precisely simulate the structural response with temperature, the material properties of concrete and steel according to two representative temperature conditions, "under-fire" and "after-cooling", have been taken into account. Furthermore, non-mechanical strains of concrete and steel such as thermal strain, transient strain, and creep strain, which change with temperature variation induced by fire, are implemented into the formulation. Upon validation of the introduced numerical method through a comparison between experimental data and numerical results, the importance of an exact consideration of the non-mechanical strains as well as material properties of concrete and steel corresponding to the changing temperature has been emphasized. Moreover, the importance of the after-cooling analysis to ensure the safety of fire-damaged RC structures has been shown. Finally, through a comparison of the numerical results with the design code EN1992-1-2, it has been concluded that the design code should consider the influence of temperature decrease after experiencing high temperature to ensure the safety of fire-damaged structural members.