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In recent years, concrete-encased concrete-filled steel tube (CECFST) columns have gained popularity in structural engineering. Research studies have shown that this novel type of composite columns exhibits superior strength, ductility, stiffness, and resistance to fire and corrosion property when compared to alternative types of steel-concrete composite columns. Considering the increasing use of slender columns in practice, this study investigated structural behaviour of CECFST slender beam–columns subjected to combined compression and bending. In this paper, six pin-ended CECFST slender specimens with different section configurations, load eccentricities and slenderness ratios were tested under eccentric load. Key test results consisted of buckling mode analysis, load–deformation history, moment–curvature relationship and strain response at various locations. Results generally showed that P-δ effect deteriorated structural behaviour of CECFST slender beam–columns and this unfavourable effect should be addressed properly in the appropriate design code for composite columns. In this regard, design approaches in European code and American specification were evaluated in aspects of N–M interaction curve, moment magnification factor and effective flexural stiffness (EI) eff. It was identified that both codes tend to overestimate (EI) eff , resulting in an unconservative second-order analysis. Moreover, a parametric analysis was conducted which showed (EI) eff correlates negatively to eccentricity and slenderness, but positively to steel tube size and concrete strength. However, these relationships were not reflected in the predicted (EI) eff according to European code and American specification. • DIC was applied to analyse buckling failure modes of the tested CECFST specimens. • The moment–curvature relation of CECFST beam–columns was investigated, confirming plastic hinge formation at the midpoint. • AISC 360-16 unconservatively predicts the moment magnification factor for slender CECFST columns under eccentric load. • Parametric studies were performed through FEM to study the influence of multiple factors on effective flexural stiffness. [ABSTRACT FROM AUTHOR]

Concrete-encased concrete-filled steel tube (CECFST), an innovative type of concrete-steel composite structure, has been increasingly employed in mega construction projects due to its outstanding structural performance. However, although intermediate and sometimes slender columns have become commonplace in high-rise buildings and large-scale bridges, the research on member behaviour of CECFST slender columns barely exists in the current literature. In this paper, a comprehensive experimental and numerical modelling programme was conducted to study buckling resistance and structural behaviour of CECFST slender columns. Pin-ended concentric load column tests were carried out on four slender columns, together with a fixed-ended stub column as a reference specimen. Key test results of all specimens, such as failure modes and effects of slenderness on strength, ductility and stiffness, showed that buckling failure and P - δ effect degraded the performance indices of CECFST slender columns. Afterwards, a finite element (FE) modelling programme was performed, where a range of member imperfections was incorporated and validated against test results. Parametric studies were then conducted to supplement the test data bank, from which over sixty FE results were generated. The test and FE failure loads obtained were utilised to examine the applicability of current design methods in the European code, American specification and Australian standard. The buckling curve 'b' set out in the European code was found to result in the most accurate and consistent buckling resistance predictions, while the American specification and Australian standard generally yielded over-conservative and unsafe results, respectively. • Four slender and one stub CECFST columns were tested under concentric compression. • The compression strength, ductility and stiffness of CECFST columns are inversely correlated with slenderness ratio. • The confinement effect of CECFST slender columns is less significant than that of stub columns. • The applicability of the design methods specified in current standards to CECFST slender columns was examined. [ABSTRACT FROM AUTHOR]

The present paper reports an in-depth testing and numerical modelling programme, to study the post-fire local buckling behaviour and residual resistances of austenitic stainless steel circular hollow section (CHS) stub columns under combined compression and bending moment. The testing programme employed twelve austenitic stainless steel CHS stub column specimens and included heating and cooling of the stub column specimens, post-fire material testing and post-fire combined loading tests. The testing programme was followed by a numerical modelling programme, where finite element models were developed and validated against the test results and then employed to perform parametric studies to generate additional numerical data. It is worth noting that there are currently no design standards established specifically for stainless steel structures after exposure to fire. In this study, the relevant design rules (i.e. cross-section interaction curves) for stainless steel CHS under combined loading at ambient temperature, as set out in the European, American and Australian/New Zealand standards and given in the continuous strength method, were assessed for their applicability to austenitic stainless steel CHS under combined loading after exposure to fire, based on the obtained test and numerical data. The results of the assessments revealed that all the codified design cross-section interaction curves have certain shortcomings, including the lack of rational exploitation of material strain hardening and the use of linear interaction, and thus yield excessively high degrees of conservatism and scatter when used for austenitic stainless steel CHS under combined loading after exposure to fire. The continuous strength method was shown to lead to accurate and consistent post-fire residual resistance predictions for austenitic stainless steel CHS under combined loading, due principally to the rational consideration of material strain hardening. Finally, the reliability of the continuous strength method was confirmed by means of statistical analyses. • The post-fire local buckling behaviour of stainless steel CHS under combined loading was investigated. • A total of twelve post-fire combined loading tests were conducted. • FE models were developed and validated against the test results and then used to perform parametric studies. • The applicability of the codified and CSM design cross-section interaction curves was assessed. [ABSTRACT FROM AUTHOR]