Stress-strain models for concrete in square hybrid multitube concrete columns

This paper presents the development of stress-strain models for the confined concrete in square fibre-reinforced polymer (FRP)-concrete-steel hybrid multitube concrete columns (MTCCs) subjected to uniaxial compression. A square MTCC consists of a square FRP outer tube and multiple inner steel tubes, with concrete filling the spaces inside all tubes. In comparison to traditional square concrete columns, square MTCCs have a number of advantages, such as ample ductility and excellent durability. The compressive behaviour of square MTCCs has been experimentally investigated to demonstrate its structural advantages in the existing studies. The experimental results confirmed that the concrete in square MTCCs is well confined despite its square cross-section. However, the complex confinement mechanism of square MTCCs is challenging to be completely understood experimentally due to the difficulties associated with the measurement of the nonuniform distributions of stresses across the cross-section. With the employment of the finite element (FE) method, the complex stress distribution and the interactions between the three components can be captured to fully explore the confinement mechanism of square MTCCs. In this paper, the development of three-dimensional FE models for square MTCCs is first presented. A parametric study using the validated FE models is then presented, which was used to generate a database for the establishment of stress-strain models for the confined concrete in MTCCs.