Post-fire Behaviors Of Axially Loaded Reinforced Concrete Stub Columns Confined By Square Steel Tubes

The steel tube confined reinforced concrete(STCRC)column is a new kind of composite member in which the outer steel tube mainly offers confinement to the inner concrete due to its discontinuity at the member ends.The STCRC column has been widely applied in structural engineering for its high load-bearing capacity and ease in construction of the connection to reinforced concrete beams.To date,extensive researches on the behaviors of STCRC column at room temperature have been conducted.However,few researches hasve been reported on its post-fire performance.A few researches on post-fire behaviours of members in circular section were reported.Considering the differential confinement effect between STCRC columns in square section and circular section,their behaviors after exposure to fire may differ significantly and it’s necessary to investigate the post-fire behavior of the square STCRC stub column.The main contents and conclusions of this paper are listed as follows:(1)Eighteen square STCRC stub columns,including twelve exposed to ISO834 standard fire and six at ambient temperature,were tested under axial loading.The considered parameters were heating time(0min,45 min,90min)and cross-section dimension(200mm,250mm).Firstly,the specimens were heated following the ISO 834 standard fire curve.The temperatures in furnace and specimens were recorded during the heating and cooling phases.After cooling to ambient temperature,the specimens were tested under axial compression load.The loda-displacement relationships as well as strains in the corner and flat of the steel tube were measured during the loading process.Test results show that the residual load-bearing capacity and residual stiffness decrease significantly with the increase of heating time while the ductility increases correspondingly.(2)Finite element models were developed using the program ABAQUS to investigate the post-fire performance of square STCRC stub columns and were validated by comparing the test results and the predictions of the FE model.(3)Parametric studies were conducted with the validated FE model.The heating time,cross-section dimension,material strengths,steel tube to concrete area ratio and reinforcement ratio were studied to identify their influences on the residual capacity ratio of square STCRC stub columns.The results of the parametric studies show that heating time and cross-section dimension affect the residual capacity ratio obviously.Steel tube to concrete area ratio and reinforcement ratio have linear influence on the residual capacity ratio.The material strengths,including concrete strength,yield strengths of rebars and steel tube,have slight influence on the residual capacity ratio.(4)Consistent with design method for unexposed specimens,equations for predicting the residual load-bearing capacity and residual stiffiness of speicmens after fire expsoure were proposed,which were validated with test and FE results.