Bilinear Model of Behavior Reinforced Concrete Column under High-intensity Lateral Loads after Fire

Abstract

The experience of destructive earthquakes shows that the problem of determining the response of RC frames under seismic loads after a fire is relevant. Calculation models for individual elements and buildings as a whole should reflect nonlinear behavior in this case. A brief review of models for describing reinforced concrete elements under low-cycle loads is given. It is noted that such models for elements damaged by fire do not exist at present. A model based on a bilinear diagram for calculating an eccentrically compressed RC column damaged by fire is proposed. Only three parameters are required to describe the model: the ultimate moment, the ultimate curvature, and the effective initial stiffness. The ultimate moment and curvature are determined by analyzing the distribution of stresses and strains in the column cross-section during the fracture stage. The column cross section is represented by a set of separate layers heated to different temperatures. The effective stiffness is defined by a straight line passing through the point in the curvilinear diagram where the initial yielding in the reinforcement or concrete occurs. The model takes into account different levels of axial loading, indirect reinforcement by transverse hoops, second-order effects and non-uniform distribution of stresses in the compressed zone of concrete. On the basis of the proposed model, bilinear deformation diagrams of RC columns, which are subjected to standard fire of different duration, are compared. The calculation results have shown a significant decrease in bearing capacity and stiffness of the damaged columns and an increase in their plasticity. The resulting model is simple enough to be used and suitable for most engineering calculations. This 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.