Concrete-encased CFST columns under combined compression and torsion: Analytical behaviour

This paper presents the analytical behaviour of concrete-encased concrete filled steel tubular members (concrete-encased CFST members for short) under the combined effects of compression and torsion, which is a typical loading condition for structural members such as bridge piers under earthquake. A finite element model (FEM) is established to account for the complex material nonlinearity and interaction, accuracy of which is verified by a set of test data. Full range analysis of the composite members under combined compression and torsion is then presented. Typical failure modes are investigated, whilst the behaviour of RC and CFST components in the composite members are compared with those of individual RC members and CFST members under the same loading condition. Parametric analysis is carried out as well to evaluate the influence of significant factors, including the material strengths, arrangement of rebars, steel ratio of inner CFST component and CFST ratio. Torsion-compression relations of concrete-encased CFST are investigated. A simplified calculation method is validated using the simulation results in order to predict the torsional capacity of axially loaded concrete-encased CFST.