Strength of concrete-filled steel box columns with buckling effects

This paper studies the ultimate strength and ductility of short concrete-filled thin-walled steel box columns with local buckling effects using a nonlinear fibre element analysis technique. New effective width formulas for determining the ultimate strengths of steel plates with geometric imperfections and residual stresses in concrete-filled thin-walled steel box columns are incorporated in the fibre element analysis program to account for the effects of local buckling. The progressive local and post-local buckling is simulated by gradually redistributing the normal stresses within the steel box. Performance indices are employed to quantitatively evaluate the section and ductility performance of composite columns. The effects of local buckling, steel ratios and steel yield strengths on the strength and ductility of concrete-filled steel box columns are investigated. It is demonstrated that local buckling significantly reduces the ultimate strength of thin-walled high-strength steel box columns filled with concrete. The 4% limit on the steel ratio or the 0.2 limit on the steel contribution ratio imposed on composite column sections in current design codes leads to the use of very slender steel plates in concrete-filled steel box columns. It is suggested that the section performance index of concrete-filled steel box columns should be as high as 0.5 for a better structural performance. The use of high strength steels in concrete-filled steel box columns significantly increases the ultimate strengths of the columns but reduces their ductility.