Damage Characteristics And Residual Bearing Capacity Prediction Of Steel Endplate Joints Exposed To Fire

As components of steel buildings have not been completely failure after they exposed to a fire,the lost-capacity structure can be repaired to recover their resistances for unceasingly utilization.However,it is significant to estimate the residual bearing capacity according to the damage characteristics of steel beam-to-column connections.The study,therefore,focusing on the damage characteristics and residual bearing capacity of steel structures in fire,became more important for research and practice.Endplate joints,kind of typical semi-rigid connections,have been widely used both at home and abroad buildings.Domestic and foreign researchers have conducted many experiments and numerical simulations to investigate the fire responses and damage characteristics of endplate joints within considering some key parameters.However,these studies are still limit to describe the fire performances of endplate joints in fire,due to the lack of clear macroscopic damage index.Meanwhile,the researchers pay more attention,at ambient temperature,to the endplate joints residual bearing capacity rather than at elevated temperature.Therefore,it is significant to study the damage characteristics and residual bearing capacity of endplate steel joints in fire.In particular,the main works and research results completed in this paper include:(1)The finite element analysis software ABAQUS 6.13 was used to establish the endplate joints model with concrete slab.The temperature field distribution of the joints in fire was obtained with the ISO834 standard temperature curve to simulate fire zone.The accuracy of temperature distributions in joints have been validated between the test data and numerical results.So the methods of numerical thermal analyses can be used for the study in this paper.After that,the thermo-structure analysis model with the same size of fire test was established.The correctness of the modeling method was validated by the joint failure mode and the mid-span deflection-temperature curves.(2)Based on the modeling method in(1),this paper establishes the numerical model,and analyzes the effects of the column restraint stiffness,the thickness of the endplate,the load ratio,the pre-loading forces and the damage indexes on the joint fire responses.The FEA analysis shows that the restraint of column stiffness affects the fire resistances,which changes the development of mid-span deflections and internal forces of steel beams,and determines the foundational deformation of joints in fire.In addition,a thickness endplate can increase the joint fire resistance with little influence on the development of internal force and deformation.The large load ratios reduce the joint fire resistance significantly.However,pre-loadings have little effect on the fire resistance.When considering the parameter of ductile damages,the bolts would fracture because of a elongation in tension.(3)Based on the summary of the fire responses,the failure mechanism of the joints is analyzed.The failure of the joints in fire is the result of steel beam tension and bending moment in catenary actions,which always acts as the buckling of endplates and the fracture of bolts.The magnitude of the column stiffness and the load ratios have a great influence on the internal force evolution in the process of joint failure.(4)Combining the fire response characteristics and failure mechanism of the endplate joints,the macroscopic damage development of endplate joints in fire can be represented by the deformation of the T-stub in the joint domain.Therefore,the deformation of 1st T-stub is used as the damage variable.The damage index can be defined as a ratio that is the difference between the displacement of 1st T-stub at T temperature and yield displacement divided by the difference between the ultimate displacement and yield displacement.(5)The component method is used to modify the moment-rotation angle theory model defined by EC3 at ambient temperature.The theoretical calculation result is compared with the curve obtained by the finite element method,and the fitting is good.Carried out theoretical calculations according to a formula for calculating the bearing capacity of the endplate joints in fire.The damage index obtained in this paper is taken into the theoretical formula under the reasonable condition with the residual allowable angle of the joint used as the symbol of the joints residual bearing capacity.The rationality and correctness of the model is verified by compared the results with the finite element calculation and theoretical calculation without damage.