The research reported herein aims to propose an accurate and efficient simplified numerical modelling approach for circular Concrete-Filled Steel Tubes (CFST) under flexural loading. Experimental tests were carried out to characterize the monotonic and cyclic behaviour of CFST members under bending. To assess the seismic performance of a composite structure with CFST members, both Distributed Plasticity (DP) and Concentrated Plasticity (CP) models were considered as potential simplified models for CFST members. The DP model was developed on the basis of a fibre discretization of the composite cross-section and displacement-based beam-column finite element. It was concluded that one could not accurately capture the development of local buckling of the steel tube and the development of multi-axial stress state effects (e.g. concrete confinement). Thus the DP model was found to be unsuitable for modelling of CFST members under cyclic flexural loading. Regarding the CP modelling, the modified Ibarra-Medina-Krawinkler deterioration model (with peak-oriented hysteretic response) was selected to define the behaviour of the plasticity spring associated with the plastic hinging region of the member. In order to accurately simulate the cyclic behaviour of the CFST section within the response of the spring, the deterioration model was calibrated, within a parameter-optimization framework, on the basis of 3D comprehensive numerical models in ABAQUS. The CP model was found to capture well the deterioration in both strength and stiffness of the hysteretic loops of the CFST members, which may be mostly associated with the development of local buckling phenomena. Furthermore, the elastic stiffness, the ultimate strength and the pinching effects of the hysteretic loops were also well simulated. Thus, the proposed CP model, coupled with the advanced calibration framework, was concluded to have a high level of accuracy in terms of simulating the cyclic flexural response of CFST members.
SIMPLIFIED MODELLING OF CIRCULAR CFST MEMBERS WITH A CONCENTRATED PLASTICITY APPROACH