Experimental and numerical study of slender concrete-encased CFST columns under combined compression and biaxial bending

Over the past few years, concrete-encased concrete-filled steel tube (CECFST) columns have seen widespread adoption in engineering practice. Many studies have been conducted on this innovative type of short column, exhibiting excellent structural performance. Given the growing use of slender columns in practical applications, this work aims to experimentally and numerically investigate behaviour of CECFST slender columns under combined compression and biaxial bending. Six CECFST columns with varying relative slenderness ratio, load eccentricity and moment ratio were tested under pin-ended boundary conditions. Primary test results, including load–deflection curve, failure mode analysis, moment–curvature relationship and strain distribution analysis, indicated a negative correlation of structural performance with increasing slenderness ratio and load eccentricity. Subsequently, a finite element (FE) modelling approach was developed and validated against test results. The validated FE model was then leveraged to analyse structural behaviour, such as longitudinal stress distribution and contact stress between concrete and steel of the composite columns. Additionally, applicability of European code and American Specification was assessed using available test and FE data. The latter provided a more precise set of predictions by considering the varying influence of axial load on column load-carrying capacity and steel contribution on effective flexural stiffness. The analysis of this study offered in-depth insights for evaluating CECFST slender columns under biaxial bending, particularly underscoring the importance of member slenderness effect.