Research On Residual Load Bearing Capacity Of Post-fire Circular Steel Tubes Under Axial Compression And Practical Design Method

Performance evaluation and reinforcement of space grid structures after fire is a hot issue in the field of civil engineering.Analysis of residual mechanical properties of steel components is the basis of the evaluation of the performance of the post-fire structures.In this paper,the residual load bearing capacity of the axial compressive circular steel tube is studied.The residual load bearing capacity of the axial compressive circular steel tube depends mainly on the residual lateral deformation after fire.The larger the residual lateral deformation is,the smaller the residual load bearing capacity is.In this paper,the lateral deformation of axial compressive circular steel tubes during the whole process of heating and cooling is studied by means of numerical simulation and theoretical derivation.(1)The accuracy of finite element analysis was verified for the determination of mechanical properties of compressive components under fire and after fire by comparing with test results in literatures.(2)Based on parameter analysis,the effects of slenderness ratio,axial restraint stiffness ratio,axial compression ratio and maximum temperature on the residual lateral deformation of axial compressive circular steel tubes were discussed and the practical calculation formula of the residual lateral deformation was obtained.(3)With the strain and lateral deformation as unknown quantities,the lateral deformation under fire and after fire were obtained for axial compressive circular steel tubes by theoretical derivation,considering the influence of the plastic deformation,axial temperature deformation,material properties in high temperature and axial constraint.The results of theoretical derivation were in good agreement with the finite element analysis results,which verified the correctness of the theoretical derivation.Based on the analysis of the ultimate load bearing capacity of the circular steel tube with different residual lateral deformation,a practical design method for the residual load bearing capacity of the axial compressive circular steel tube after fire was presented.The influence of residual lateral deformation is considered by the calculation of equivalent slenderness ratio.An additional calculation length coefficient is introduced to determine the equivalent slenderness ratio and residual load bearing capacity of the axial compressive circular steel tube after fire.The practical design method was applied to a fire scene of a warehouse in Qinhuangdao.A simplified calculation method for the axial constraint stiffness ratio of different types of members was presented.The weak position and residual load bearing capacity of the post-fire structure were evaluated through the analysis of the residual load bearing capacity of the post-fire members.