Numerical investigation and design recommendation for thin-walled octagonal concrete filled steel tubes

Abstract

With the growing applications of high strength steel in modern constructions, thin-walled steel tubes attract many attentions. Local buckling failure is the most typical failure mode in a thin-walled steel tube, and concrete filled steel tubes (CFSTs) are excellent composite members in which the occurrence of local buckling is mitigated by the infilled concrete. This paper aims to investigate the compressive strength of concrete filled thin-walled octagonal steel tubes, which have the advantages of confinement efficiency over rectangular CFSTs, and constructability over circular CFSTs. Finite element models are established and validated by the experimental results from existing literatures. A parametric study is subsequently conducted and the result shows that the buckling strength of octagonal tube is significantly improved by the infilled concrete and the enhancement percentage increases with the increase of section slenderness coefficient, and the local buckling behaviour has a minor influence to the effect of confinement. In addition, a database consisting of 37 experimental specimens and 37 FE models are developed and used to assess the design methods in current codes of practice. It is found that the design methods from ANSI/AISC 360-16 and GB 50936-2014 underestimate the load capacity of octagonal CFSTs while the design formula from EN 1994-1-1 which considers the effect of confinement overestimates the capacity. Finally, a design model considering the buckling strength of octagonal steel tube, the influence of infilled concrete core and the effect of confinement is proposed and this model can well capture the compressive strength of octagonal CFSTs.