Safety of Reinforced Concrete Columns: Effect of Initial Imperfections and Material Deterioration under Emergency Actions

Abstract

The effect of (1) initial imperfections and (2) material degradation of reinforced concrete columns on their safety in emergency situations was investigated. The research was limited to low- and medium-flexibility columns. Numerical modeling and proven regulatory methods of analysis were applied to determine the ultimate bearing capacity, taking into account supplementary dynamic loading by a longitudinal force and a bending moment in case of emergency. The numerical model, describing the column structure, has 3D elements simulating concrete, and rebars simulating reinforcement frames (cages). Imperfections are simulated by (1) the physical loss of elements, (2) unzip of nodal elements, and (3) unzip and further zip using nonlinear elements simulating gaps and cohesion between concrete and reinforcement. Implicit dynamics and an incremental method were employed to make computations. Within the framework of this computational scheme, a nonlinear problem was solved using the Newton–Raphson method with nodal forces convergence. The effect of imperfections, such as geometrical deviations and deterioration of mechanical characteristics, on the bearing capacity of compressed bending elements was identified under emergency actions. Risks of mechanical safety loss were analyzed to find that columns in the frame structures of highly hazardous, technically complex, and unique buildings and structures, subjected to supplementary loading, need an additional safety margin in the range of 3–21%. Rectangular cross-sections of columns are the most effective in terms of the safety criterion.