Confinement-based design and behaviour of concrete-filled stiffened steel tubular square slender columns.

This paper presents the details, analysis and results of an investigation into the compressive behaviour of concrete-filled stiffened steel tubular (CFSST) slender columns. These are members comprising an outer steel section with longitudinal stiffeners welded to the interior surface. A finite element (FE) model was developed and validated. An improvement in ultimate strength compared to conventional concrete-filled steel tubular column has been found due to the presence of stiffeners that reduce the width-to-thickness ratio of the steel tube, which in turn increases the effective cross-sectional area of the steel tube. Moreover, it was found that stiffeners increase the lateral confining pressure on the concrete. FE modelling was then employed to conduct a thorough parametric study to investigate the influence of a wide range of parameters. These included the slenderness ratio, concrete compressive strength, as well as the geometry and details of the stiffeners. Both medium-length and long CFSST columns, classified on the basis of slenderness ratio, were carefully studied and discussed, as well as their inherent differences. The strengths predicted from the FE simulations were compared with the design strengths obtained from various international design codes, and it was shown that the design expressions provide very conservative capacity predictions. Accordingly, a new three-stage confinement-based design model was proposed, which takes into account different confining mechanisms of columns based on their slenderness ratio. It is illustrated that this model provides a very accurate prediction of the square CFSST slender columns, and captures the key behavioural characteristics. • A confinement-based direct design model is developed for CFST slender columns with stiffened tubes. • EC4 and AISC design codes were found to be conservative. • Columns fail by elastic-plastic buckling show enhanced confinement of concrete, while it was negligible in elastic failure. • Plastic and elastic buckling slenderness limits have been provided. [ABSTRACT FROM AUTHOR]