Residual Strength And Reinforcement Design Of CFST Columns After Exposure To Fire

The mechanical properties of steel and concrete are deteriorated by exposure to fire,which causes carrying capacity of concrete-filled steel tubular(CFST)columns decrease.Compared with steel and reinforced concrete structures,the repairability of concrete-filled steel tubular structures is better.Farther study on residual mechanical properties and reinforcement design of CFST columns after exposure to fire has an important theoretical and practical value.Residual strength and reinforcement design of CFST columns after exposure to the whole process of ISO-834 fire were inverstigated.The main achievements can be summarized as follows:(1)Previous test results were collected and regression analysis were carried out to determine the fitting formulas of mechanical properties of steel and concrete at ambient temperature,heating,cooling and post-fire stage.The reasonable stress-strain relationship models of steel tube and concrete core were developed,the simplified models were suitable for continuous analysis of the whole process of fire.(2)Fully coupled thermal-stress analysis model in finite element soft ABAQUS was developed to predict the residual strength of CFST columns after exposure to fire.Initial load,fire protection and end restraint were considered in the analysis model,the calculated results fit test ones well,so the feasibility of fully coupled thermal-stress analysis model was verified.Compared with traditional model,the maximum temperature during the whole process of fire was determined and work mechanism was inverstigated reasonably in the new model.(3)Work mechanism of CFST columns during the whole process of fire was inverstigated by fully coupled thermal-stress analysis model,then parametric analysis was carried out.It was concluded that the residual strength was significantly affected by sectional size,spray insulation thickness,heating time,and slenderness ratio.A set of simplified formulas were proposed to predict the residual strength of CFST columns after exposure to fire.Compared with existing formulas,the new simplified formulas are better.(4)Reinforcement mechanism of CFRP-strengthened concrete-filled circular steel tubular(CFCST)columns after exposure to fire was analysed by finite element method,then reinforcement effects were studied.It was concluded that CFRP-strengthened reinforcement method was not suitable for concrete-filled square steel tubes(CFSST),the reinforcement effects of CFCST stub columns were better,but not ideal for slender columns.Finally design recommendations of CFRP-strengthened CFST columns were proposed.(5)Concrete-filled double steel tubes is a special case of CFST-strengthened CFST columns.Twelve specimens were tested under axial compression to investigate the carrying capacity of concrete-filled double square steel tubular stub columns,the main experimental parameters were internal concrete strength,size of internal steel tube and section type.Test results can supply reinforcement design of CFST-strengthened CFST columns after exposure to fire with valuable reference.(6)Reinforcement mechanism of CFST-strengthened CFST columns after exposure to fire was analysed by finite element method,then reinforcement effects were studied.It was concluded that the carrying capacity of CFST columns after exposure to fire was improved significantly by CFST-strengthened reinforcement.The ultimate bearing capacity of CFST-strengthened CFST columns was superimposed by internal CFST columns after exposure to fire and external CFST.Finally design recommendations of CFST-strengthened CFST columns were proposed.