Research On Flexural Behavior Of Rectangular Concrete-filled Tubular Flange High-strength I-Beams

High-strength steel structures have many advantages,such as higher bearing capacity,smaller section size,less steel consumption,economic and environmentally friendly.However,structural components made by high-strength steel tend to be slender and thin,so they are more prone to stability problems.To solve this problem,this paper proposed a new type of high-strength steel composite beams with good stability performance,named rectangular concrete-filled tubular flange high-strength I-beam(RCFTFHSIB),in which the open thin-walled plate top flange of traditional I-section beams was replaced by the closed rectangular concrete-filled steel tube with high torsional rigidity.This new type of composite beam has high bearing capacity and good stability,and it hence has high engineering application value.Considering the research on it still remains scarce,this paper systematically studied the flexural behavior and design method of RCFTFHSIBs by means of experimental investigation,numerical simulation,and theoretical derivation.The main contents are as follows:(1)A total of 6 flexural behavior tests on RCFTFHSIBs were conducted under lateralunrestrained mid-span concentrated load.The deformation characteristics,strain developments and capacity of RCFTFHSIBs with the failure modes of flexural yielding and lateral-torsional buckling were explored.The results indicated that the lateral-torsional buckling resistance of I-beams could be effectively improved by replacing the top flat flange by concrete-filled tubular flange.(2)Three-dimensional nonlinear RCFTFHSIBs finite element(FE)models were established by using commercial FE software.The FE models considered material properties,initial geometric imperfection and instability,and were carefully validated against test results.The validated FE models were then used to carry out a comprehensive parametric study to investigate the influences of several parameters on the flexural strength and lateral-torsional buckling behavior,such as span length,initial geometric imperfections,number of stiffeners,geometric dimensions,steel grades and concrete grades.(3)Section stress and strain analysis and elastic buckling analysis were performed,and the calculation method of yielding moment,full plastic moment and critical elastic lateraltorsional buckling moment of RCFTFHSIBs were proposed.Based on the theoretical,experimental and numerical results,together with current design specifications of steel structures,the design method of flexural strength and lateral-torsional buckling strength of RCFTFHSIBs was developed.The mean value and coefficient of variation of the predictionto-resistance ratio are 1.001 and 4.28%,respectively.(4)The elastic shear buckling analysis were carried out,and the formula for calculating the critical elastic shear buckling stress of the web of RCFTFHSIBs was derived.The postbuckling shear resistance of RCFTFHSIBs was investigated based on the validated FE models,and the effects of flange dimensions,number of stiffeners,initial geometric imperfections,and steel grades on the post-buckling shear behavior of RCFTFHSIBs under different width-to-height ratio and width-to-thickness ratio of webs were studied.Based on the theoretical and numerical results,together with current design specifications of steel structures,the design method of shear strength of RCFTFHSIBs was developed,and the reliability of the design equations is verified by the comparison with the numerical results.