Stiffness of a fire-damaged reinforced concrete column during unloading after high-intensity horizontal impact

Introduction. 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. The calculation models applied in this case for individual elements and buildings as a whole should take into account the nonlinear properties of concrete and reinforcement. The low-cycle nature of seismic action leads to the necessity of developing hysteresis models, which would reflect the specific behaviour of the structure not only during loading but also during unloading and subsequent application of force in the opposite direction. A brief review of hysteresis models for reinforced concrete elements under low-cycle loading with emphasis on the stiffness properties during unloading is presented. Hysteresis models for reinforced concrete elements after fire have not been developed at present.

Materials and methods. The proposed model is based on a bilinear diagram for the calculation eccentrically compressed reinforced concrete columns damaged by fire. Only three parameters are required to describe the model: limiting moment, limiting curvature, and effective initial stiffness. In determining the unloading stiffness, a method based on direct consideration of the stress and strain distribution in the stress-strain stage corresponding to full unloading is used. The model takes into account different levels of axial loading, indirect reinforcement by transverse clamps, longitudinal bending and non-uniform stress distribution in the compressed zone of concrete.

Results. Based on the proposed model, a comparison of bilinear diagrams of deformation and stiffness at unloading for reinforced concrete columns subjected to standard fire of different duration was carried out. Calculation results showed a significant decrease in the bearing capacity and stiffness of the damaged columns and an increase in their plasticity. The unloading stiffness for reinforced concrete columns was less than the initial one. The decrease in unloading stiffness is the more intensive the longer the fire exposure was.

Conclusions. The obtained model is easy to use and suitable for most engineering calculations. The model can be used as a basis for constructing a hysteresis diagram for low-cycle impacts after a fire, which is necessary for seismic analysis of structures in the time domain.

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