Ultimate Load Carrying Capacity Analysis Of Box Section Beams And Beam-Columns

With the better torsion performance of box section Beams and Beam-Columns than members with open cross-section and the progress of the manufacturing technology of box section members,the box section members find wider and wider application,especially in high rise buildings.First,the stability of box section beams under pure bending is studied based on two different torsion theories on members with closed cross-section by Umansky and Gjelsvik in the elastic phase,and the critical moments are obtained. Comparisons show that these two different theories give nearly the same critical moments. Because of its simplicity of the critical moment and the good precision,the solutions based on Gjelsvik Theory are used in the elastic-plastic phase. The ultimate load carrying capacity of box section beams are calculated considering the effect of plasticity and residual stresses. The effect of different sizes of sections,modes and values of residual stresses are studied. Through the analysis,the conclusion is arrived at that the universal slenderness ratio should be used to better correlate the stability of box section beams with the member length instead of the out-of-plane slenderness,and the stability coefficient is taken to be 1.0 for practical used box girder.Then,the elastic critical loads of box section beam-columns are obtained based on the Gjelsvik Theory,and the relationship between axis force P and moment M is given.Finally,the box section beam-columns in plastic phase are studied through the general FEA program ANSYS with the Beam 189 element. The initial curvature and the residual stress are considered,also the material plasticity is considered in the solution process. With the program,many members with different section sizes and slenderness ratios are analyzed. When the axis force P is much greater than the moment M,the out-of-plane column -instability mode is dominated. Otherwise,the in-plane instability mode of the beam-column is dominated. At the end of this part,the practical formula of box section beam-columns issuggested which avoid the incorrect concept of .