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

Engineering structures are highly susceptible to seismic loading. Under strong earthquakes, these structures fail and collapse, causing catastrophic events resulting in loss of life and properties. The seismic collapse capacity of many structures is primarily influenced by their nonlinear behaviors under cyclic loads. Reinforced concrete (RC) structures are widely used in buildings due to their remarkable properties, low cost and ease to satisfy the required design and technical specifications. Different experimental investigations have revealed that most RC components’ stiffness and lateral strength are gradually degraded with the increase in the loading displacement. The RC components first undergo progressive degradation at lower displacement at 1 – 6.3% drift ratio and later catastrophic degradation (sudden loss of strength) at ultimate displacement. The two types of degradation differently affect the dynamic analysis of RC components, meaning the force-displacement in each case should be considered for accurate prediction of collapse capacity.

Current research on the degradation of RC structures has mainly concentrated on progressive degradation. And the available theoretical and finite element models can be used to predict the progressive degradation with high accuracy. Whereas there are limited studies on the catastrophic degradation, accurate prediction of the collapse capacity of RC structures is still challenging. At present, the ultimate drift ratio based on empirical models is the main method for evaluating the catastrophic degradation of these structures at ultimate displacement. Unfortunately, while the existing models for evaluating the ultimate drift ratio are generally effective, they fail to account for the physical mechanisms causing catastrophic degradation of RC columns, which are also yet to be fully understood.

Lately, the fibre beam-column model commonly used in the simulation of structural behaviors has been identified as a promising solution for simulating progressive degradation. Using this technique, however, requires more investigation to validate its practicability in simulating catastrophic degradation. To this note, Dr. Lei Li, Mr. Wentao Wang, and Professor Pengpeng Shi from Xi’an University of Architecture and Technology investigated the cause of catastrophic degradation in RC structures to develop a more feasible fibre beam-column model for accurate prediction of catastrophic degradation at ultimate displacements. The authors focused mainly on flexural dominated RC structures. Their work is currently published in the research journal, Journal of Building Engineering.

In their work, the authors commenced by describing the de-confinement effects of kernel concrete based on the stress analysis and experimental observation of RC columns. During loading, the kernel concrete was transferred from confinement to a de-confinement state. Next, considering the de-confinement effect and associated behavior of the concrete kernel, an updated fibre beam-column model was proposed and validated by comparing the model results to the experimental results. The advantages and limitations of the proposed model were also discussed in detail.

The researchers reported that the proposed updated model reasonably predicted the catastrophic degradation of RC structures at ultimate displacement because it included the state shift of the kernel confinement. This confirmed that the state shift is one of the main factors causing catastrophic degradation in RC structures and plays a vital role in accurate prediction of catastrophic degradation. The model was also determined the ultimate drift ratio that induces catastrophic degradation. Unlike the empirical and classical fibre beam-column models, the updated model has the advantage of high prediction accuracy because it considers the confinement effects.

In summary, the study proposed an updated fibre beam-column model to simulate and model the catastrophic degradation of flexural-dominated RC columns at ultimate loading displacement. The confinement effects on the ductility and strength of peripheric concrete were fully established, allowing for detailed validation of the proposed model against the existing models. Unlike the existing models, the proposed model enabled the authors to find the main cause of catastrophic degradation, namely, the state shift of kernel concrete from confinement to dec-confinement state. In a statement to Advances in Engineering, Dr. Lei Li and Professor Pengpeng Shi explained that the updated model based on the local failure mechanism enables a more accurate prediction of the seismic collapse capacities of RC components, and also provides a mechanism-based-model strategy for the seismic collapse and performance evaluation problems of engineering structures.