Stability Capcity of Circular Steel Tube Beam-Columns Under Uniaxial Bendings

In the current Standard for Design of Steel Structures(GB 50017-2017), the clause 8.2.4 is specifically devoted to the buckling strength of beam-columns with circular pipe cross-section, but the only difference from the clause 8.2.1 for the beam-column with H cross-section is in the equivalent uniform moment factor. The cross section of circular steel tube has its unique characteristics compared with the section of the I-beam, therefore, it is necessary to revisit the buckling strength of beam-columns with circular pipe and if possible to propose a design formula that can better reflect the real relationship between the axial force and the bending moment of circular steel tube in unidirectional bending beam-columns.
The paper uses finite element software to calculate the nonlinear buckling capacity of unidirectional bending beam-columns with circular steel tube, and compares the FEM results with the results of the current coded formulas, and discusses whether the current code related formulas are suitable for calculating buckling capacity of circular steel tube beam-columns. Starting from the sectional plastic axial force-bending moment interactive equations of the pipe, substituting the elastic second order bending moment including the geometric imperfection into the cross-sectional interactive equation, the present paper obtained a new equation as upper bounds for the in-plane stability capacity with the codified equivalent moment coefficient, for which a coefficient is modified to make the theoretically derived equation be able to agree with the FEM results.
The following conclusions are drawn:the calculation results of the relevant formulas of the current code are generally conservative compared with the FEM results, especially when the bending moment changes linearly and is in double-curvature bending; when the end bending moment ratio is-1, the axial force-bending moment interactive curve of the relevant formula of the current code and the curve of the FEM result show fully different trends, indicating that the relevant formula of the current code cannot well reflect the actual axial force-bending moment correlation of circular steel tube beam-columns. Comparing the proposed formula of this paper with the FEM results, it is found that the new formula can more accurately calculate the buckling capacity of circular steel tube unidirectional bending beam-columns when the end bending moment ratio is 1, and, after using the equivalent bending moment coefficient of the I-beam members, the buckling capacity of beam-column under different end bending moment ratios can also be calculated more accurately.