Study On Axial Load Behavior Of Stiffened Thin-walled Concrete-filled Square Hollow Steel Stub Columns After Non-uniform Fire

Stiffened concrete-filled square hollow steel(SHS)column is a new type of structural member.Transverse stiffening bars are set at certain intervals along the longitudinal direction of the column,which delays and restricts the buckling of steel tubular,thereby improving the restraint effect of steel tubular on concrete.At present,the mechanical properties of stiffened concrete-filled SHS columns at room temperature have become more and more mature,but there is no report on the mechanical properties of stiffened concrete-filled SHS columns after fire.In actual fires,most building structures or components are subjected to non-uniform fire.The temperature distribution of component cross-section is different from that of uniform fire on all sides.Different fire boundary conditions have different effects on the overall temperature of components,resulting in different mechanical properties and damage degree of materials.Therefore,different fire modes make stiffened concrete-filled SHS columns produce additional eccentricity,and the stress mechanism and fire resistance of components are different from those of four sides.There are great differences in uniform fire conditions.In order to reasonably evaluate the damage degree of reinforced stiffened concrete-filled SHS structures after fire,and to provide suggestions for damage assessment,repair and reinforcement of such members after fire,the study on the axial load behavior of stiffened thin-walled concrete-filled SHS stub columns after non-uniform fire was carried out.The main research work and achievements include :(1)The axial compression test of stiffened thin-walled concrete-filled SHS stub column after one normal temperature and seven ISO 834 standard fires is carried out.The main parameters are heating time,fire mode and reinforcement.The temperature-time curves,load-displacement curves,load-longitudinal and transverse strain curves,residual load-carrying capacity and failure modes of stiffened thin-walled concrete-filled SHS stub column sections during ISO 834 standard heating and cooling were measured.The test results show that the residual bearing capacity of the specimens decreases gradually with the increase of heating time and fire surface,and the setting of stiffening bars can effectively improve the bearing capacity and ductility of the members.(2)The finite element software ABAQUS was used to establish the temperature field and mechanical field model of stiffened thin-walled concrete-filled SHS stub columns after non-uniform fire,and the reliability of the model was verified.The influence of heating time,fire mode,longitudinal spacing of stiffening steel bar,section edge length,material strength and steel content on the residual mechanical properties of stiffened thin-walled concrete-filled SHS stubcolumns after fire was analyzed.(3)Based on the parameter analysis,a simplified formula for calculating the ultimate bearing capacity of stiffened thin-walled concrete-filled SHS stub columns at room temperature is proposed,and a formula for calculating the residual bearing capacity of stiffened thin-walled concrete-filled SHS stub columns after uniform fire on one side,relative two sides,three sides and four sides is proposed.