Study On The Behavior Of Multi-limbs Built-up Cold-formed Steel Box Beam

Multi-limbs built-up cold-formed steel box beam is a common bearing componentconsisting of C-sections and U-sections connected by self-drilling self-tapping screws. In thispaper, the author completely and systematically researched on the mechanical performance ofdouble limbs, four limbs and five limbs built-up cold-formed steel box beam in the methodcombining test and numerical analysis, found out the main factors that influence the ultimatecapacity and deformation performance with pure bending failure, and utilized the result ofparametric analysis to regress formulas for calculating the bearing capacity and the overalldeformation in the pure bending of built-up box beams. The main research work andinnovation achievements can be summarized as follows:1．In this paper, the author has completed test and researched on total45specimensincluding double limbs, four limbs and five limbs built-up cold-formed steel box beams, andobserved the failure characteristics, the ultimate flexural capacity and deformation capacityof the specimens, meanwhile analyzed the influence of the spacing of screws, height to widthratio, width to thickness ratio and span to height ratio on the mechanical performance of thespecimens.The results of the tests showed that: for double limbs, four limbs and five limbsbuilt-up cold-formed steel box beams, failure modes were characterized by localbuckling,and the height to width ratio, width to thickness ratio of section, the spacing ofscrews and span to height ratio all had influenced on the flexural capacity.2．All the specimens were simulated by ANSYS finite element program to considergeometric non-linearity and material non-linearity. Based on the accuracy of the finite elementmodel, the variable parameters of the spacing of screw, height to width ratio, width tothickness ratio, span to height ratio and material yield strength were analyzed for theperformance of214double limbs,214four limbs and214five limbs built-up cold-formedsteel box beams under the pure bending. The results showed that the failure modes of doublelimbs, four limbs and five limbs built-up cold-formed steel box beams under the pure bendingwere characterized in local buckling, and the local buckling became more and more obviouswith increment of width to thickness ratio and height to width ratio. If the spacing of screwvaried from150mm to600mm, as the screws spacing was increased from150mm to300mm, there was little impact on the fleural capacity of beam, so it is recommended to take300mmas the spacing of screw of double limbs, four limbs and five limbs built-up cold-formed steelbox beams to meet construction measures and reduce cost in the engineering. Meanwhile, theultimate flexural capacity increased as the height to width ratio of section, width to thicknessratio of section and material yield strength increase. But the span to height ratio had littleimpact on the flexural capacity when the height to width ratio and width to thickness ratiowere small, and had great influence when the height to width ratio and width to thickness ratiowere large.3．The flexural capacity of double limbs, four limbs and five limbs built-up cold-formedsteel box beams with pure bending failure were caculated by both Chinese specificationGB50018-2002and American specification AISI2007, and the results were compared withthe finite element analysis results. The comparisons showed that with the increament ofsection size and material yield strength, the calculation results in the effective cross sectionmethod by AISI and GB50018-2002tend to be unsafe.4．The Bending Modulus Reduction Method formulas which are applicable to calculatethe flexural capacity of double limbs, four limbs and five limbs built-up cold-formed steel boxbeams with different materials and cross section sizes were obtained by finite elementparametric analysis results. The flexural capacity for all kinds of built-up cold-formed steelbox beam in the test has been calculated by Bending Modulus Reduction Method formulas,the calculation results agree well with the test results. In addition, certain amount ofspecimens were selected according to the common model in the engineering for BendingModulus Reduction Method formulas and finite element calculation respectively, and thecomparison results showed that the formulas have small calculation error. Then it furtherillustrates Bending Modulus Reduction Method is feasible and high precision.5．The method for calculating overall bending deflection of multi-limbs built-upcold-formed steel box beam was researched, and components with certain section parameterswere selected. It is found out that the height to width ratio and width to thickness ratio are thekey factors that can influence the overall bending deflection in the research on the influenceof flange width to thickness ratio, web height to width ratio on the bending deformation.Through comparing and analyzing the distribution of scattered points of every curve, the calculation formulas for the overall bending deflection of double limbs, four limbs and fivelimbs built-up cold-formed steel box beams with different cross section sizes with purebending failure were established. The overall bending deflection for all kinds of built-upcold-formed steel box beam in the test has been calculated by the calculation formulas, Thecalculation results agree well with the test results.