Abstract: The extended end-plate joint is mainly used to transfer gravity and seismic effects. However, in extreme states, connecting joints also have a significant impact on the overall performance of the structure and the prevention of continuous collapse. Among various disasters, fire is still one of the disasters with the highest frequency and the widest range of influence. At present, research on fire resistance performance mainly focuses on the heating stage of a fire, but materials, components, and structures may exhibit different characteristics during the cooling stage compared to the heating stage. During the entire process of a fire, steel joints will be subjected to the tension generated by the "catenary effect" caused by the large deflection of the beam span, as well as the tension generated by the cold shrinkage of the steel. Under the combined action of tension and bending, the steel joints will be damaged, leading to the collapse of the entire steel frame structure. At present, there is relatively little research on the mechanical properties of extended end-plate joints of steel frames during the entire fire process, both domestically and internationally. Therefore, it is urgent to conduct in-depth research on them to provide technical supports for performance-based fire protection design. This ensures that the main structural components are not damaged within a certain period of time after a fire, allowing sufficient time for personnel in the building to escape and for firefighters to carry out their rescue operations.
In the present study, nonlinear thermal-mechanical coupling analysis of extended end-plate joints under the combined action of tension and bending was conducted by using the ABAQUS finite element software. Firstly, a three-dimensional thermal analysis and structural response nonlinear analysis model was established for the extended end-plate joint in the fire heating stage. The accuracy of the modeling method was verified by using the results of heating tests under constant bending effect in existing literature. On this basis, the joints’ temperature field distribution during the entire temperature rise and fall process in a fire was obtained through transient thermal analysis. The thermal-structure coupling analysis were performed to obtain the mechanical properties of joints under the combined action of tension and bending, while the parametric analysis was conducted on the influence of factors such as the fire’s temperature rise and fall history and the magnitude of tensile loads on the mechanical behavior of joints. The results indicated that throughout the entire fire process, the deflection of joints would continue to increase during the cooling stage due to the influence of temperature hysteresis, leading to their failure in the early stage of cooling. At the same time, due to the influence of the catenary effect and the tensile force generated by the cold shrinkage deformation of steel beams, bolt tension was detrimental to the fire resistance time and residual deformation of the joint. In addition, parameter analysis of different tensile forces on joints showsed that when the bending moment of the joint remained constant, the tensile force would have a significant impact on the mechanical behavior of the joint. When the tensile for increased, the deformation and residual strain of the joint gradually increased during the cooling stage, eventually leading to joint failure. Therefore, in fire analysis, the entire process of the fire and the influence of tensile forces on the mechanical behavior of joints should be comprehensively considered, and a rational assessment and detailed calculation analysis of the additional tensile forces exerted on these joints should be conducted.