New design method for composite columns including high strength steel

When drafting EN1994-1-1 the simplified design method for composite columns in ENV1994-1-1 was changed using second order analysis and equivalent initial bow imperfections taking into account the effects of residual stresses and geometrical imperfections. These equivalent geometrical imperfections were determined for different types of cross sections, for steel grades S235 (fy = 235 N/mm2) and S355 (fy = 355 N/mm2) and concrete grades from C30 (fck = 30 N/mm2) to C60 (fck = 60 N/mm2). In addition, the method was checked for the use of high strength steel, so as to determine whether the basis of the simplified design method and the full plastic interaction between normal force and bending moment can also be applied. The paper deals with the necessary modifications of the simplified design method for the use of high strength steel, the classification into buckling curves, and the background of the determination of the equivalent initial bow imperfections. Some comments for special sections for composite columns are also given.