Experimental and numerical studies of concrete-encased concrete-filled steel tube stub columns under uniaxial and biaxial eccentric compression

Abstract

Experimental and numerical studies were conducted to investigate the structural performance and resistances of concrete-encased concrete-filled steel tube (CECFST) stub columns under uniaxial and biaxial eccentric compression. A test programme, consisting of 1 axially-loaded stub column, 3 uniaxial-eccentrically-loaded stub columns and 11 biaxial-eccentrically-loaded stub columns, was firstly conducted. The effects from uniaxial and biaxial loading eccentricities and end moment ratios on the structural performance of CECFST stub columns were studied. The key test results, including the failure load and mode of each specimen, crushing and spalling of the concrete cover, load–deflection and moment–curvature curves, were presented. It was found that the loading eccentricity and end moment ratio affect the ductility and failure mode of the specimen. The mid-height moment capacity of each specimen was maintained by the inner CFST after the outer concrete encasement was crushed. The test programme was followed by a numerical modelling programme, where finite element models were initially developed and validated against the experimental results, including the full load–deflection curves and end moment–rotation histories, and then adopted for conducting parametric studies to generate additional numerical data. The obtained test and numerical data were then used to evaluate the applicability of the codified design approaches in the American specification (Method 1 and 2), European code, and AIJ standard for composite columns under eccentric compression. The results of the evaluations revealed that the European code and American specification (Method 2) overestimate the cross-section resistances of CECFST stub columns under uniaxial eccentric compression, while the AIJ standard and American specification (Method 1) provide conservative resistance predictions. For CECFST stub columns subjected to biaxial eccentric compression, both the European code and American specification (Method 2) underestimate the cross-section resistances. Finally, new design methods were proposed for CECFST columns under uniaxial and biaxial eccentric compression, indicating a higher level of design accuracy over the design codes.