Study On Fire Resistance Of Concrete-filled Steel Tube Column Considering Initial Defect

Among many architectural forms,the concrete-filled steel tube structure has always been considered as one of the structures with high bearing capacity and outstanding fire resistance.The research on the fire resistance of concrete-filled steel tube(CFST)columns started earlier,and there are many research achievements at home and abroad.However,the existing researches mainly focus on the size effects,different end restraints and material types.The influence of initial geometric defects and residual stresses caused by construction on the performance of CFST columns in fire is well studied limitedly.The research on the fire resistant mechanism of CFST subject to the multi-factor coupling is not enough.In addition,the accuracy of the method for predicting the ultimate bearing capacity of CFST columns with initial geometric defects and residual stresses remains to be improved.In view of the above contents,this paper has completed the following work:(1)According to the residual stress of steel pipe caused by welding and other factors,the actual residual stress and its distribution state of concrete filled steel tube column were measured by means of blind hole method by improving drilling equipment and measuring point arrangement.It is found that the residual stress distribution on the steel pipe is larger near the weld seam of the end plate and smaller in the middle of the column.The maximum measured residual stress is about 0.1times the yield strength of steel.(2)After the residual stress is measured,a new measuring method is designed for the initial geometric defects caused by manufacturing process and transportation mode.That is,the theodolite is used to measure the coordinates of 11 measuring points on each side of the experimental concrete-filled steel tube column,so as to obtain its initial eccentricity and initial bending,and then its initial geometric defects.According to the measurement method in this paper,it is found that the initial geometric defects produce the maximum value at the end of the concrete-filled steel tube column.(3)After obtaining the residual stress and initial geometrical defects of the experimental CFST column,the fire test was carried out on 6 axially compressed CFST concrete columns and 6 eccentrically compressed CFST columns by means of constant load heating.At the same time,the displacement,temperature and other data samples were collected for comparative analysis.It is found that due to the difference of specific heat between the outer steel tube and the core concrete,the difference oftemperature between the two materials is large.The residual stress affects the strength of the steel tube,making it more prone to yield,resulting in debonding and local buckling.The existence of initial geometric defects leads to the increase of load eccentricity,which affects the overall stability of the members and makes the concrete-filled steel tube column produce a larger end Angle.When the steel tube is broken open,cracks are found between the concrete and the steel pipe wall inside,and the phenomenon of deboning occurs.In other words,during the process of heating up,the steel tube and the core are separated.(4)In order to further explore the effects of residual stress and initial geometric defects on the fire resistance of CFST columns.In this paper,the finite element analysis software ABAQUS6.14 is used to simulate the concrete filled steel tube column.Through the comparison with the real test,the model is tried to match with the test as much as possible,so the numerical model under fire is analyzed with the residual stress and initial geometric defect as variable parameters.It is found that the residual stress and initial geometric defects have adverse effects on the axial displacement,fire resistance and reaction force of CFST columns.The residual stress mainly affects the strength of the steel tube,so it is easy to produce local buckling at the large residual stress.The initial geometric defects had a significant effect on the compression deformation of the specimen and reduced the resistance of the column to deformation.(5)In order to predict the ultimate bearing capacity of CFST columns with residual stress and initial geometric defects more accurately.Based on the Japanese standard,this paper combines the existing formulas for predicting the ultimate bearing capacity of CFST columns under fire.At the same time,the residual stress and initial geometric defects are taken as the influencing factors,and the reduction coefficient is modified to obtain the prediction formula of ultimate bearing capacity of concrete-filled steel tube column under fire.Moreover,a typical example is selected and compared with the simulation results,the results show a reasonable gap and a good overall fit.