Stiffness Analysis Of Partially Filled Narrow Steel Box-concrete Composite Beams Under Negative Bending Moment

In order to solve the problem of section bearing capacity and concrete flange crack control in the negative moment area of continuous composite beam,a new type of composite structurepartially filled narrow steel box-concrete continuous composite beam is proposed on the basis of square steel tube concrete and traditional narrow steel box-concrete composite beam.In this paper,the stress characteristics of the negative moment zone of the continuous composite beam are simulated by the mid-span reverse loading method of the partially filled narrow steel box-concrete composite simply supported test beam,and the working characteristics of the whole process of the test beam are studied experimentally.Through theoretical analysis,the influence of slip effect concrete flange cracking on the section stiffness of composite beams is studied,and a reasonable method for calculating the section stiffness of composite beams in negative moment zone is proposed.The finite element software is used to establish the numerical analysis model of the test beams,and the finite element analysis model verified by the test results is used to analyze the influence parameters of the structural stiffness under the bending moment of the composite beam.(1)The test composite beams were finally destroyed due to excessive deflection,and the bearing capacity level in the plastic working stage was not significantly reduced,showing good ductility.The ultimate bearing capacity of PSCB2 and PSCB3(with reinforcement ratio2%)is improved by 47% and 43% than PSCB1(with reinforcement ratio 1%),respectively.The crack distribution of PSCB2 and PSCB3 is denser than that of PSCB1,and the crack width is smaller under the same load.Increasing the reinforcement ratio can improve the crack distribution of composite beams and improve the stiffness of concrete flanges.(2)The analytical formula of additional deflection of partially filled narrow steel boxconcrete composite beams considering slip effect under negative bending moment is established by slip analysis of micro-element model.The effectiveness coefficient of cracked concrete is introduced to explore the cracking law,and the deflection calculation method of partially filled narrow steel box-concrete composite beam in normal stage is obtained.The deflection value obtained by the calculation method considering the slip effect and the contribution of cracked concrete is in good agreement with the experimental data.The average ratio of the calculated value to the experimental value is 0.99,and the mean square error is 0.2,which is less than the traditional calculation method.The calculation deflection accuracy is high,and it can better reflect the change characteristics of the section stiffness of the composite beam in the normal use stage.(3)The finite element model is used to analyze the structural parameters.The results show that when the shear connection degree is less than 1,the increase of the shear connection degree improves the bending stiffness of the composite beam.However,when the shear connection degree exceeds 1,the specimen is completely sheared,and the increase of the shear connection degree has little effect on the bending stiffness.Filling concrete can improve the stability and bearing capacity.With the increase of filling concrete height,the section stiffness of composite beam increases,and the ultimate load can be increased by about 3% for per 100 mm increase of filling height.The thickness of the bottom plate has a significant effect on the ultimate bearing capacity of the composite beam.When the thickness of the bottom plate is thickened by 10 mm,the ultimate bearing capacity of the composite beam can be increased by about 4 %.