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Steel members using nonsymmetric sections are often used for their structural efficiency in contemporary construction. However, traditional inelastic analysis methods are mostly derived from regular hot-rolled steel sections with symmetrical shapes; the adoption of those methods may lead to significant differences when analyzing members with nonsymmetric sections. In view of such a need, this research proposed an innovative method for the Geometrically and Materially Nonlinear Analysis with Imperfections (GMNIA) of steel frame with symmetric and nonsymmetric sections. An improved line-element formulation derived based on its principal axis is adopted. The tangent stiffness matrix of the element is formulated based on the nonsymmetric section assumption, explicitly modeling the noncoincidence of the shear center and centroid for considering the Wagner effects. The proposed line-element can capture complex buckling behaviors of the members with nonsymmetric sections. Then, the Concentrated plasticity (CP) model is integrated into the element tangent stiffness matrix to consider the material nonlinearity, where the full-yield criterion using the yield surface is adopted. As part of this, a rigorous cross-section analysis method has been developed to generate the yield surfaces of arbitrary steel sections regardless of shapes. Such yield surfaces will be used to evaluate the full-yield condition, and the gradients to the yield surfaces will be calculated and used to control the plastic flow. The development of this GMINA for steel frame with nonsymmetric sections is presented in detail, as well as the derivation of the mathematical formulations. At …