A new confined concrete model considering the strain gradient effect for RC columns under eccentric loading

In this paper, a new confined concrete model is developed for reinforced-concrete (RC) columns under eccentric loading. The model is based on the well-known Mander model, but considers the strain gradient effect for RC columns under eccentric loading. The confining mechanisms of RC columns under concentric and eccentric compression are firstly compared and the influence of strain gradient under eccentric loading on the equivalent lateral pressure is analysed. Then an experimental programme is performed to investigate the actual lateral confining pressure of RC columns under eccentric loading. According to the experimental results, an eccentricity coefficient based on the equivalent confinement volume is proposed as an amendment to the original Mander model to account for the section strain gradient effect. Moreover, the proposed confined concrete model is implemented within the fibre beam–column element, so that the varying sectional eccentricity at different positions and times can be represented during the analysis procedure. The model is validated through moment–curvature analysis of several critical column sections with different eccentricities and confinements, and the results indicate that the model can accurately reflect the influence of eccentricity on the confinement effect. Meanwhile, the model is also used to simulate RC columns under lateral cyclic loading, showing the capability of the proposed model in seismic analysis of RC columns under combined axial-bending interaction.