Research On Calculation Method Of Ultimate Moment For Simply Supported Steel I-section Beams Failed By Flexural-torsional Buckling

Steel beams are prone to flexural-torsional buckling under load and are damaged.Among them,for the ideal steel beam,the critical moment is an important parameter to characterize its bearing capacity;for the practical steel beam,due to the initial geometric defects and the presence of residual stress,it will be bent and buckled before reaching the critical moment,and the corresponding moment is the ultimate moment.However,due to the difficulties in the study of the influence of initial torsion on the ultimate moment of steel beams failed by flexural-torsional buckling,the design standards of various countries are generally based on the critical moment of the ideal steel beam failed by elastic flexural-torsional buckling,and the test data is used to correct it to obtain the calculation method of the ultimate moment.On the basis of summarizing the research results of predecessors and the research shortcomings in the relevant provisions of the current standards,the thesis takes uniaxial symmetrical I-shaped cross-section steel beams as the research object,adopts the technical route of combining theoretical analysis and numerical simulation,and conducts research on the calculation method of the bending moment of the bending limit of steel beams considering the effects of initial geometric defects and residual stress.The main research content and conclusions are:(1)For the problems that the loads corresponding to bending and torsion respectively are not distinguished in the critical moment formula of ideal steel beams failed by flexural-torsional buckling,the critical flexural buckling load Pcr,y about yaxis,and the critical torsional buckling load Pcr,z about z-axis,are employed to express the critical moment.The results shown that,the formula of the critical moment,expressed by Pcr,y and Pcr,z is of simple form and of clear physical significance,which lays the foundation for considering the initial bending and initial torsion separately.(2)For the problems that the formula for the critical moment of ideal steel beams is not provided by the current national standard“Standard for design of steel structures"(GB 50017—2017),a formula for determining the critical moment of ideal steel beams,was proposed on the basis of the concept of global stability cofficent provided by the“Code for design of Steel structures"(GB 50017—2003).The finite element software LTBeamN is used to calculate the critical moment of 1010 ideal steel beams,and the numerical results are compared with values determined by the theoretical calculation formula of Mcr,x and the proposed formula corresponding to current national standard“Standard for design of steel structures"(GB 50017—2017).The results shown that,for the proposed formula that corresponding to GB 50017—2017,when φb>0.6,the corrected φb’ cannot be used instead of φb to calculate the critical moment Mcr,x;and for singly symmetric I-beams,the accuracy of the proposed formula that corresponding to GB 50017—2017 is insufficient,among which the simplification of geometric parameters and unnecessary equivalent bending moment coefficients are the important reasons for the insufficient calculation accuracy of the Mcr,x practical calculation formula in GB 50017—2017.(3)For the problems that the early experimental datas of steel beams failed by flexural-torsional buckling were scattered,a "database" of steel beams failed by flexural-torsional buckling was established,and the characteristics of the early experimental of steel beams was summarized.The results shown that,simply supported steel beams,simply supported steel beams with lateral restraints,cantilever and overhangs with lateral restraints,were studied together in the early literatures.However,the results of the critical moment for simply supported steel beams with lateral restraints were not accurate,which may cause the calculation formula to have a wide application range,but the calculation results are insufficient.(4)A formula for determining the ultimate moment of steel beams failed by flexural-torsional buckling,was proposed by insteading the Pcr,y multiplied by the coefficient Ci in the in the Mcr,x formula,by the flexural buckling formula of columns about y-axis.Precision of the proposed formula was then verified by the experimental results in the literatures.The results shown that,the proposed formula is simple and convenient to use.Moreover,the proposed method is more accurate and safer than the design approach provided in GB 50017—2017.For load cases that which are not included in GB 50017—2017,and for symmetrical I-section high-strength steel welded steel beams,the proposed formula is of high accuracy.