Nonlinear analysis of concrete columns under non-uniform and anisotropic confinement

Abstract

For the past twenty years, more and more attention has been drawn to the “performance-based design” of structures due to the concerns of earthquakes, dynamic impact, wind loads and so on. As a result, the ductility of a building has become an important aspect on par with its strength. As far as construction material is concerned, concrete structures have been the most prevalent in these years. As a matter of fact, the ductility of concrete structures is inferior to that of steel structures. However, recent studies have proved that the confinement effect will improve the strength and the ductility of concrete. In traditional concrete construction, confinement is provided by transverse steel hoops placed inside the concrete members with certain spacings, i.e. discontinuously. New forms of concrete structures, such as concrete columns confined by fibre-reinforced polymer (FRP) and concrete-filled steel tubular (CFST) columns, are therefore proposed to enhance the efficiency of confinement, since the concrete in these structures is confined continuously.

The mechanisms of uniform and isotropic confinement effect in FRP-confined concrete columns and CFST columns have been well studied, and the existing theoretical models can be applied to those typical load cases, such as FRP-confined circular concrete columns and circular CFST columns under axial compression. But the research on the mechanisms of non-uniform and anisotropic confinement effect is still evolving, in order to predict the behaviour of confined concrete columns with sections of various shapes under combined axial loads and bending moments. Usually finite element (FE) method is used to analyse such complicated problems. However, in a lot of current studies using FE method, the dilation angle of plastic flow of concrete is often treated as a constant, which does not reflect the observed behaviour of concrete regarding the variation of volumetric strain under triaxial compression. In reality, the volume of concrete will eventually expand due to the propagation of splitting cracks, and the true dilatancy of concrete should be non-linear. In this study, a new FE model is developed through Fortran 90 to account for the nonlinearity of the dilatancy of concrete, and verified against several load cases with non-uniform and anisotropic confinement effect, i.e. eccentrically loaded FRP-confined circular concrete columns, axially loaded FRP-confined rectangular concrete columns and eccentrically loaded CFST columns. Moreover, a simplified method is also proposed to provide quick estimations on the behaviour of circular CFST columns under eccentric compression.