Finite Element Analysis Of Seismic Behavior And Reinforcement Of Reinforced Concrete Frame Joints After Fire

With the continuous progress and development of economy and science and technology,various fire-causing factors are increasing day by day,and building fires occur from time to time and the proportion is increasing year by year.After the fire,the mechanical indexes such as bearing capacity,stiffness and strength of reinforced concrete structures decrease significantly,but most of them still have certain bearing capacity and aseismic capacity after fire through proper reinforcement and repair,and can continue to be used.This approach is in line with the call of green environmental protection and sustainable development strategy.Due to the limitation of experimental equipment and means,there are few reports on the reinforcement and seismic performance of reinforced concrete structures after fire.In this paper,the stress model and reinforcement model of reinforced concrete frame joints after fire are used to simulate and analyze the seismic performance of the joints after fire and their reinforcement and repair by using finite element software.The main research contents are as follows:(1)Based on the basic knowledge of heat transfer,analysis principle and thermal performance of materials,the temperature field model of reinforced concrete is established by ABAQUS,and the whole process of fire temperature field is analyzed by finite element method.When the temperature rises under fire,the increase of temperature in the section of beam and column lags behind the outside of the section,which shows that the temperature decreases gradually from the outside of the section to the inside of the non-linearity.In the initial stage of cooling,the outside temperature of beam-column section decreases rapidly,while the inside temperature increases slowly.(2)Based on the finite element theory and material performance parameters,the seismic finite element model of post-fire joints is established.The hysteretic curve,skeleton curve,ductility coefficient,energy dissipation,strength degradation and stiffness degradation curve of reinforced concrete frame joints are calculated numerically.The results show that the elastic stage bearing capacity of the joints is almost unchanged without fire and at different time of fire,while the ultimate bearing capacity will decrease in varying degrees after fire.Comparing the calculated results with the experimental data,the simulation results are better.(3)The seismic performance of the joints strengthened with bonded steel and carbon fiber sheet after fire is simulated and calculated.The hysteretic curve,skeleton curve,ultimate bearing capacity,ductility coefficient,energy dissipation trend of the strengthened joints under different axial compression ratio,heating time and thickness are analyzed,as well as the stress distribution of concrete and steel plate.With the increase of reinforcement thickness(layers),the ultimate bearing capacity of the strengthened joints increases,but ductility and energy dissipation increase first and then decrease.The plastic failure of the joints gradually develops from the initial beam end to the local brittle failure of the joints.With the increase of axial compression ratio,the ultimate bearing capacity of joints strengthened with steel sheet and carbon fiber sheet increases first and then decreases,while ductility and energy consumption decrease.(4)The seismic performance of the joints strengthened by steel plate and carbon fiber sheet under different reinforcement parameters is compared and analyzed.The results show that steel plate should be used to reinforce the joints under the same axial compression ratio,and carbon fiber sheet should be used to reinforce the joints for a longer time of fire.The thickness of the joints strengthened under the two reinforcement methods should not be too large.Through comparative analysis,effective suggestions are put forward for practical engineering reinforcement.