Finite element analysis of square FRP-Concrete-Steel columns under concentric compression

The first objective of this paper is to propose a simple but rational and effective refinement to the concrete damage plasticity model (CDPM) used in the finite element (FE) analysis of confined concrete columns, in particular the damage parameter for columns with a post-peak descending path. The proposed damage parameter is based on an existing one, refined to account for the confinement effect. The FE analysis results demonstrate that the proposed damage parameter enables much more accurate tracing of the stress–strain paths of normal as well as high strength concrete columns confined by circular or square fibre-reinforced polymer (FRP) tube. The second objective is to use the resulting FE methodology to study the square FRP-Concrete-Steel columns tested previously by the first two authors, which had an outer FRP tube and an inner steel tube, both square. The FE analysis results confirm the experimental finding that the rotated inner steel tube typically led to better performance than the conventionally placed inner tube. At a given axial strain, the rotated configuration results in a larger area of high axial stresses in the concrete, corresponding to the area of higher confining pressures, but a lower maximum hoop strain (in the middle of the flat side) of the FRP tube