Research On Fire Resistance Indicators Of PEC Column Based On The Whole Process Of Heating And Cooling Phases

Natural fires contain both heating and cooling stages,the cooling section of the natural fire has a great influence on the fire resistance indicators of the structure.Nowadays,fire resistance is based on the standard fire of heating conditions,which is incompatible with the process of the fire on actual structure.Experiments and analyzes indicate that PEC columns may also be damaged during cooling stage.When the outside temperature field is in the cooling stage,due to the thermal inertia of the concrete material,resulting in the internal temperature of the structure is keeping rising when outside is in cooling stage,the load-bearing capacity of the structures continue decreasing in the cooling stage,and ultimately may lead to structural damage in the cooling section.For a comprehensive analysis of fire resistance of PEC column in the whole process of nature fie,based on the whole process of fire,this paper focuses on the possible damage of the cooling section.The main research work is as follows:(1)By using ABAQUS program,a finite element model was proposed to investigate the fire performance of the Partially Encased Concrete(PEC)columns exposed to fire condition including the heating and cooling phases.The finite element model was validated by experimental data of PEC columns exposed to standard fire.(2)The material properties of the cooling section have been restored from the heating section,different material properties were used in heating and cooling phases.(3)Define field variables,prepare subroutines that enable automatic identification and conversion of material properties at different temperature stages of materials,to achieve the whole process of nature fire.(4)Define and analyze the fire resistance index under natural fire(duration of the heating phase,DHP),the post-verification model is used to analyze the possible delayed failure of the PEC column in the whole process of fire.(5)Considering the influence of five factors such as load ratio,section size,slenderness ratio,concrete strength and section reinforcement ratio on the DHP value of PEC column:the fire load ratio and the slenderness ratio of the cross section have a great influence on the fire resistance of the PEC column;the effect of section size on DHP has two processes:when the load is relatively small,DHP increases with the cross-sectional size;When the load is relatively large,the cross-sectional size of the impact of DHP becomes smaller,DHP does not produce significant changes with the cross-sectional size;The strength of concrete and the reinforcement ratio of section have little effect on the fire resistance.The empirical formula of DHP with these factors is fitted based on the calculated data.