Experimental and theoretical studies of seismic resistance parameters of reinforced concrete columns damaged by fire

Abstract The consequences of destructive earthquakes show that the problem of analyzing the response of reinforced concrete frames under seismic loads after a fire is urgent. Calculation models for individual elements and buildings as a whole must accurately reflect the nonlinear properties of concrete and reinforcement. The development of hysteresis models that reflect the nonlinear behavior of the structure is necessary due to the low‐cycle nature of seismic impact. However, no theoretical model that describes the nonlinear resistance of reinforced concrete columns under low‐cycle loads after fire has been proposed at present. The development of theoretical models must be based on the results of experimental studies. Six reinforced concrete columns underwent standard fire tests of varying durations (15, 30, and 45 min). Temperature field distributions along the cross‐section of the specimens were obtained, taking into account the heating and cooling stages. Furthermore, eight specimens were tested for the impact of vertical static load and horizontal low‐cycle load. The article presents an analysis of failure patterns in reinforced concrete columns, utilizing experimental results to construct hysteresis diagrams and analyze the parameters of column seismic resistance. The study reveals a significant degradation of seismic characteristics with increasing duration of standard fire: the load‐bearing capacity of columns decreased by 4.5%, 25%, and 33% following 15, 30, and 45 minutes of standard fire exposure, respectively; the initial stiffness of the columns decreased by 10.4%, 11.2%, and 11.5% for the corresponding fire duration; the ductility coefficient decreased by 11.1%, 20.6%, and 28.5% for the corresponding fire duration. A theoretical model is proposed based on a bilinear diagram for calculating off‐center compressed reinforced concrete columns damaged by fire. The model only requires three parameters: ultimate moment, ultimate curvature, and effective initial stiffness. A reasonably good convergence with the experimental results is obtained. The change of mechanism of reinforced concrete columns failure from plastic to brittle has been revealed. After the fire, the degradation of the concrete core structure is observed. Refinement of calculation expressions in accordance with this mechanism is proposed. The obtained expressions can be used in FE modeling the columns as part of a reinforced concrete frame damaged by fire.