Studies On Behaviour Of Axially Loaded Square Tubed Steel Reinforced Concrete Columns After Fire Exposure

As a kind of steel-concrete composite structural member,the tubed steel reinforced concrete(TSRC)column has been widely used in civil engineering because of its excellent bearing capacity and good seismic performance,which inherits the advantaged mechanical properties of CFST and SRC columns.Until now,there are comprehensive studies about the mechanical properties of TSRC columns at room temperature.About the fire and post-fire behaviors of TSRC columns,the existing researches mainly focus on CTSRC stub columns.The square columns are frequently used in practical engineering to supply the demand of architectural space design.However,few researches have been reported on post-fire behaviors of square tubed steel reinforced concrete(STSRC)slender columns.Therefore,it is necessary to investigate the post-fire behaviors of STSRC stub and slender columns with axial compression.The main contents are listed as follows:(1)The temperature field experiment and axial compressive experiment after fire exposure were carried out for 19 specimens of STSRC stub columns and 6specimens of STSRC slender columns.The parameters included heating time,concrete strength,steel tube to concrete area ratio and slenderness ratio.During the temperature field experiment,the ambient temperature in the heating process was controlled according to the ISO 834 standard temperature rising curve.After reaching the predetermined time,the heating is stopped to make the test furnace coo l naturally.The temperature in the test furnace and the temperature s at each measuring point of the specimens were recorded during the experiment.In the process of axial compression experiment,the load-displacement relationships and the strains on the surface of the tube were recorded.According to experimental results,the ultimate bearing capacity of STSRC stub column increases with the increase of steel ratio and concrete strength,decreases with the increase of the heating time.The buckling capacity of STSRC slender column decreases with the increase of slenderness ratio and heating time.(2)Using the finite element software ABAQUS,the finite element analysis model of temperature field and post-fire mechanical properties of STSRC columns are established.The post-fire behaviors of STSRC columns are verified by comparing FE and test data.The results show that the finite element model established in this article is able to accurately predict the ultimate bear ing capacit y and the load-displacement relationships of STSRC columns after fire exposure.(3)Based on the finite element analysis model of STSRC columns,the working mechanism of axially loaded STSRC slender columns after fire exposure was studied.The load bearing ratio of concrete first decreases and then increases in the whole process of axial compression,while the change trend of load bearing ratio of steel is contrary to that of concrete.The influences of 8 parameters on the buckling capacity of STSRC slender columns are analyzed,which include heating time,cross-section dimension,slenderness ratio,axial compressive strength of concrete,yield strength of steel,yield strength of tube,steel ratio and steel tube to concrete area ratio.Formulae for calculating the flexural rigidity and buckling capacity of STSRC slender columns after fire exposure were proposed,which is influenced by the global buckling and the deterioration of materials.