Abstract This paper presents a method for simulating composite steel–concrete columns of double-symmetrical, partially encased cross-sections, as equivalent steel columns of fictitious cross-section, for the purposes of linear elastic analysis. The latter consist of the actual steel cross-section and two additional pairs of plates, one perpendicular to the web at mid-height and one perpendicular to the flanges at mid-width. Plate dimensions are chosen to match the compression resistance and principal bending stiffness of the composite section. Section equivalence results in three algebraic equations for the dimensions of the additional plates, in terms of the geometry of the actual section and the material properties of steel, concrete, and reinforcement bars. These equations are solved in closed form by neglecting a small contribution to major-axis stiffness. The proposed method combines accuracy, efficiency, and convenience; it takes advantage of widely available software for linear elastic steel structure analysis and obviates the need for finite-element simulation.