Abstract: Considering the geometric and material nonlinearity, residual stress, and initial geometric imperfections, steel plates with different boundary conditions are analyzed under uniform compression and pure bending. The average stress-strain and moment-curvature curves are obtained. Based on the definition of ductility factors, the relation between ductility factors and plate slenderness are obtained and approximate formulas for the ductility factors are proposed. According to the values of the structural performance factor used to determine the base shear of a seismic action, the paper proposed a new classification for steel cross-sections oriented to the seismic design. Using the relation between the system ductility and the cross-sectional ductility, and based on whether an overstrength factor is included in the structural performance factor, the paper tentatively proposed the limiting values of the width-to-thickness ratio.