Modelling catastrophic degradation of flexural-dominated RC columns at ultimate displacements based on fibre beam-column model

Abstract Catastrophic degradation of the mechanical strength of reinforced concrete (RC) columns at ultimate displacements is one of the critical factors for the seismic collapse of structures. Accurately predicting this degradation is critical to assess the seismic collapse capacity of structures. However, the mechanism and theory about the degradation of RC columns are insufficient at present. This study is devoted to revealing the cause of catastrophic degradation and establishing an updated fibre beam-column model for predicting the catastrophic degradation of RC columns at ultimate displacements. First, the de-confinement effect of kernel concrete at ultimate loading displacements is described based on experimental observation and the stress state analysis of the confined concrete. The kernel concrete in the confined zone is shifted from confinement state to de-confinement state during the loading cycles. Next, an updated fibre beam-column model is proposed where a uniaxial stress-strain relation considering the de-confinement effect is introduced to represent the behaviour of kernel concrete during the loading cycles. The validity of the proposed model is confirmed by comparing experimental results and model results. Compared with the classical fibre beam-column model, the detailed discussions about the load-displacement response, moment-curvature, and strain distribution during loading cycles showed that the updated model achieves reasonable prediction of catastrophic degradation of RC columns by capturing the state shift of kernel concrete. At last, the applicability of the proposed model is discussed. Existing models including Panagiotakos and Fardis model, Eurocode 8 model, and ASCE 41 model are used as comparative models to evaluate the model accuracy of predicting the ultimate drift ratio of RC columns.