Comparative Study On The Second-order Flexural Effect Of Concrete-filled Square And Circular Steel Tube Slender Columns Subjected To Lateral Force

As primary frame columns,square and circular concrete-filled steel tube slender columns are commonly used in the composite system of primary and secondary structures,sandwich structures and other structural systems.It will be subjected to the combined action of eccentric compressive and lateral force.In order to investigate the rule and differences of the second-order effect of deflection on the concrete-filled square and circular steel tubular slender columns under the combined action of eccentric compression and lateral force,six square and two circular concrete-filled steel tube slender columns were tested under biaxial loading.The influence of the loading mode of lateral force,lateral force position,the ratio of end bending moment,and the hoop coefficient on the compression bending failure were considered.According to the test results,the ultimate bearing capacity and load-deflection curve of the column were obtained.Based on the comparative analysis of the test results,the stiffness reduction,moment amplification factor and equivalent bending moment factor of the column were further analyzed by ABAQUS finite element software.① Six square and two circular concrete-filled steel tube columns were tested under biaxial compression and lateral force.The results showed that the failure mode of the square column was basically the same as that of a circular column,but the failure modes are different.The local buckling and overall bending failure of the square column occurred,while only the overall bending failure mode was observed on the circular column.The fitting results of lateral deflection and sine curve were also different.The fitting effect of the circular column and sine curve was better than that of the square column.② Through ABAQUS analyses on the specimens under multiple parameter variables,it was found that the stiffness reduction of internal concrete would have a more significant impact on the development of flexural deformation when square and circular steel tubes were damaged under eccentric compression.Besides,the stiffness reduction degree of the square steel tube was greater than that of the cycular specimens due to local bulging deformation.According to the results of finite element simulation,it is suggested that the moment amplification factor of the concrete-filled square specimen should be taken as η=1/(1-Nl2/(π2(ESIS+0.3ECIC))),and that of circular specimen was η=1/(1-Nl2/(π2(ESIS+0.SECIC))).③Through the finite element simulation analysis and test data verification,it was found that for the compression member with lateral force,when the axial force remains unchanged and the lateral force or eccentricity increased,the equivalent moment coefficient βm1 of square and circular specimens would increase.When the column entered into the elasticplastic state after flexure,the results show that βm1 can be taken as 1.14 and 1.11 for circular steel tube;when the equal end moment column with lateral force at the middle point is equivalent to the equal end moment column without lateral force,the square steel tube βm1 can be taken as 1.12,circular steel tube can be taken as 1.10.When the column with unequal end bending moment is equivalent to the column with unequal end bending moment under the same condition under two kinds of lateral forces,βm2 is taken as 1,and the proposed value is applicable to both low cycle reciprocating load and monotonic load.