Study On Fire Resistance Of Reinforced ECC-encased CFST Composite Columns

With the characteristics of high bearing capacity,good integrity and excellent seismic performance,concrete-filled steel tubular(CFST)has gradually become a more common structural form.However,its steel tube is directly exposed to the outside,thus its durability and fire resistance are poor.This paper proposed to wrap reinforced Engineering Cementitious Composites(ECC)in the outer layer of CFST to form a new type of reinforced ECC-encased CFST composite column.ECC has the characteristics of no burst at high temperature and high residual strength after fire,which can greatly improve the fire resistance of CFST.Concrete is easy to burst at high temperature.Compared with concrete--encased CFST composite columns with concrete poured outside CFST,the fire resistance of ECC-encased CFST composite columns would be greatly improved.In this research,distribution of temperature field and mechanical properties of ECCencased CFST composite columns under standard fire were studied.The main research contents are as follows:(1)The standard fire test and post fire axial compression test of 15 ECC-encased CFST composite columns were completed.The temperature field distribution and post fire axial compression performance of ECC-encased CFST composite columns and concrete-encased CFST columns were studied.The fire test showed that ECC-encased CFST composite columns remained intact after the standard fire,and there were no burst and exposed reinforcement unlike concrete-encased CFST columns,and the axial compression bearing capacity increased by 32%,the ductility and energy dissipation coefficient increased by 10.4% and56.3% respectively.The increase of heating time,fire sides and stirrup spacing could reduce the axial compressive mechanical properties of composite columns after fire.(2)The axial compression behavior of ECC-encased CFST composite columns after fire was simulated by finite element method.After selecting appropriate material high-temperature constitutive model,the temperature field model of ECC-encased CFST composite columns under ISO-834 fire and the axial compression mechanical analysis model after fire were established.Through mechanism analysis and parameter analysis,it could be seen that ECC had better thermal insulation effect on the inner CFST than concrete,which made the inner CFST experience lower historical maximum temperature;ECC-encased CFST composite columns had temperature-lag effect in the process of ISO-834 fire,and the temperature distribution was hierarchical.The increase of stirrup spacing,heating time and number of fire surfaces could reduce the residual bearing capacity coefficient,and the increase of outer ECC thickness,steel pipe thickness,steel pipe yield strength,diameter of longitudinal bar and yield strength of longitudinal bar could increase the residual bearing capacity coefficient.In addition,based on parameter analysis,a feasible calculation method of axial compression capacity of ECC-encased CFST composite column after fire was proposed in this research.(3)The standard fire test and post fire eccentric compression test of 8 ECC-encased CFST composite columns were completed.The temperature field distribution and post fire bias performance of ECC-encased CFST composite columns and concrete-encased CFST columns were studied.ECC-encased CFST composite columns still maintained complete and high residual bearing capacity and continue to work after standard fire.The peak load of ECC-encased CFST composite columns under the same fire conditions and eccentricity was 1.58 times that of concrete-encased CFST columns,while the maximum lateral deflection of ECCencased CFST composite columns was only 59.2% of that of concrete-encased CFST columns.The increase of heating time,stirrup spacing and eccentricity could reduce the bias performance of composite columns;the increase of longitudinal reinforcement ratio could improve the eccentric compression performance of composite columns.Finally,based on the basic assumptions,a calculation method of eccentric bearing capacity of ECC-encased CFST composite columns after fire was put forward.(4)The eccentric compression behavior of ECC-encased CFST composite columns after fire was simulated by finite element method.After selecting appropriate material high-temperature constitutive model,the temperature field model of ECC-encased CFST composite column under ISO-834 fire and mechanical analysis model under eccentric pressure after fire were established.Through mechanism analysis and parameter analysis,it could be seen that under different eccentric compression states,the stress of longitudinal reinforcement and steel pipe on the tension side and compression side were different,and the internal steel pipe could provide different restraint effect.Longitudinal reinforcement ratio,pipe ratio,heating time and wrapped cement-based materials had a great influence on eccentric compression performance of ECC-encased CFST composite columns after fire,while the change of stirrup spacing had little influence.