Finite Element Analysis on the Concrete-Filled Square Steel Tubular Pure Bending Members Encased with CFRP Profile Under Cyclic Loading

Carbon fiber reinforced polymer (CFRP) has the characteristics of high-strength and good corrosion resistance. Encased I-shaped CFRP profile into concrete-filled square steel tubular structure (CFRP-CFSST) forming a new-typed composite member can not only improve the mechanical properties of the member, but also reduce the material consumption, the dead weight of the structure and the cross-sectional size of the member. It is more suitable for super high-rise, large-span and heavy-load structures. As a critical lateral force-resisting member in structures, CFST often determines the seismic performance of the whole structure when subjected to an earthquake, which is directly related to the safety of people's lives and property. At present, the relevant design codes and standards around the world are not suitable for the seismic design of new composite members, so it is necessary to carry out in-depth research on its seismic performance.
In this paper, finite element analysis software, ABAQUS, was used to study the seismic performance of the flexural behavior of CFRP-CFSST pure bending members. Firstly, considering the accuracy and applicability, the finite element model was verified with the existing literature, and a large number of refined models of CFRP-CFSST pure bending members were established based on the verified model. Then, on this basis, the whole process of stress analysis and stress analysis of each component at characteristic points were carried out based on the typical member. Finally, the effects of concrete compressive strength, steel yield strength and steel ratio on the flexural capacity and energy dissipation capacity of CFRP-CFSST pure bending members were studied.
The simulation results indicated that the load-displacement envelope curves of the CFRP-CFSST pure bending member can be defined as three stages: elastic stage, elastoplastic stage and descending stage. Through the whole process analysis of typical members, in the elastic stage and elastoplastic stage, the load is mainly borne by the steel tube compared to the core concrete and I-shaped CFRP profiles. In the descending section, the load-bearing ratio of the CFRP profile increases, which shows that the encased CFRP profile can effectively improve the bearing capacity and ductility of members in the later loading stage. Therefore, compared with ordinary CFST members, the better tensile performance of CFRP profile efficiently improves the flexural performance of new composite members. Based on the parametric analysis results, the steel ratio has a significant effect on the bearing capacity and energy dissipation capacity of CFRP-CFSST members. When the steel tube thickness increases from 4 mm to 7 mm with an increment of 1 mm, the flexural bearing capacity of CFRP-CFSST members increases by 13. 83%, 8. 99% and 9. 10% respectively, which shows that a 5 mm steel tube thickness is most economic and the cumulative energy dissipation capacities increased by about 16. 57% on average. The steel tube is the main component that bears pure flexural load in the whole loading process, and the change in its strength also has a great influence on the hysteretic behavior of the members. When the steel strength increases from Q235 to Q420, the flexural bearing capacity of the members rises by about 30. 13%, the cumulative energy dissipation increases by about 12. 45%, and the flexural bearing capacity increases linearly with the increasing strength. The compressive strength of core concrete has a minor influence on the bearing capacity and energy dissipation capacity of members, and with the increase of concrete strength, the increase of flexural bearing capacity gradually decreases. When it is increased from C30 to C60, its cumulative energy dissipation capacities only increase by about 3. 44% . Therefore, compared with ordinary CFST members, CFRP-CFSST members have a better seismic performance, and most economic measures recommended are to improve the bearing capacity of new composite members by increasing the steel yield strength or steel ratio.