Fire Behaviours Of T-shaped Stiffened CFST Slender Columns

T shaped and L shaped concrete-filled steel tube(CFST)columncolumns are more reasonable for structures to obtain more utilize space compared with circular CFST columns.However,the constraint of steel tube to concrete core is weak and easier to occur local buckling for the ordinary special-shaped CFST columns.The stiffening steel ribs uniformly arranged in the longitudinal section of the T-shaped column can delay or reduce the local buckling of the steel tube and enhance the confinement capacity of the steel tube under ambient temperature.The mechanical properties of the steel tube can still meet the requirements of the building structure even if the thickness of the steel tube is greatly reduced.However,there has no research on the fire resistance of stiffened thin-walled T-shaped concrete-filled steel tubular long columns to date,which restricts its further development.In order to accurately predict the fire resistance limitation of such specimens,a series of study on the fire resistance of thin-walled T-shaped CFST long column with stiffened steel ribs under fire are carred out.The main research work and results are as follows:(1)Six thin-walled T-shaped stiffened CFST lonlong columns and one non-stiffened specimen are subjected to uniform fire on all sides based on ISO-834 standard fire were carried out to investigate their fire resistant limitation,and the time-temperature curves at key positions,the axial and lateral deformation curves of the specimens as well as the failure mode under fire were measured and obtained.The test results show that,compared to the non-stiffened specimen,the fire resistant limitation of the columns with the stiffened steel ribs at a spacing of 80 mm increased up to 275% with better fire resistance.The fire resistance of the specimen is significantly affected by the load ratio,and the fire resistance decreases exponentially with the increase of the load ratio.(2)Finite element models of temperature field and fire resistant limitation of thin-walled Tshaped stiffened CFST long columns under ISO-834 were established by using the sequentialthermodynamic coupling method.The reliability of the models were verified by the experimental data and the experimental results of relevant literatures.Then,the mechanial properties was analyzed.The analysis results showed that the temperature distribution of T-shaped specimen section is unisymmetric,and the heating ring zone is formed due to the stiffening steel ribs.The heating of concrete core has significant hysteresis effect.Setting of stiffening steel bars lead to the increasing number of buckling waves,meanwhile,decreasdecrease the height of buckling waves and the distribution of stress is more uniform,finally optimize the fire resistance.(3)Based on the thermodynamic coupled finite element model,the influence of different parameters on the fire resistance limit of thin-walled T-shaped stiffened CFSTs were analyzed.The results showed that the fire resistance of the specimens can be improved by decreasing the spacing of stiffening steel rib,steel ratio,the yield strength of steel tube or increasing the strength of concrete core.A simplified formula for calculating the fire resistant limitation of thin-walled Tshaped stiffened concrete-filled steel tubular long columns under ISO-834 is proposed.When the fire resistant limitation over 10 mins,the simplified formula has higher accuracy,which can provide a reference for the engineering application of this kind of specimen.