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Variable cross-section member in the portal frame plant is widely used, plane flexural torsional buckling the major buckling modes of thin-walled structures, instability is much more complex than single bending instability and reverse. BENDING MEMBERS plane stability of portal frame light steel structure housing the technical regulations (CECS 102:2002) checking the existence of many problems related formula central axis force term to take the small end of the internal forces and its cross-sectional properties , the internal forces and its cross-sectional nature of the moment items take big end, when TAPERED COLUMN degenerate into a section columns, \formula), resulting in lack of co-ordination between the specification. Coefficient for the overall stability of the beam is derived from the elastic critical moment formula must elastoplastic reduction, while the actual flexural members due to the presence of initial imperfections, residual stress, flexural torsional buckling is no longer a bifurcation buckling problem, but the instability of the extreme points. In addition, domestic and international norms the moment linear change the overall stability of the beam coefficient is calculated using a variety of ideas and methods, there is a big controversy, can not coordinate the views of experts and scholars. Section member, the plane stable formula is still room for improvement, for example, in M ??small / M = -1, βtx = 0.3, a great reduction of the bending moment, the axial force when the plane stable formula from the control; moment control from the intensity formula. This paper focuses on the components of variable cross-section plane elastic-plastic stable. First of all, on the basis of existing research based on the classic thin-walled theory, derived BENDING MEMBERS total strain energy formula, in the derivation of the axial force and bending moment two, and the whole process of derivation are consider to shear the mandrel and the centroidal axis the angle αcs the variable section members uniaxial symmetry, not only applies to the biaxial symmetry variable cross-section also applies to uniaxial symmetrical variable cross-section. Secondly, the use of the finite element method, the total strain energy formula stiffness matrix, and analysis using ANSYS finite element program for example, to verify the correctness of the theory in this article. In addition, the article focuses on the study of variable cross-section beams and BENDING rod-plane elastic-plastic, variable cross-section beam subjected to unequal bending moments, take the maximum stress ratio of the cross-section and the large end of the small end of the cross-section were -1.0, -0.5,0,0.5 and 1.0, consider components of the initial defects and residual stress mode, the results of the analysis by ANSYS fitting high precision overall stability factor formula of the same, BENDING lever with similar elastic-plastic analysis by a large number of numerical examples proposed a smooth axial force - bending moment curve of action, checking the formula to get the new plane.
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