Axially loaded rectangular FRP-concrete-steel hybrid multi-tube concrete stub columns: Performance, confinement mechanism, and design-oriented model

This paper presents an experimental investigation of rectangular glass fiber-reinforced polymer (FRP)-concretesteel hybrid multi-tube concrete columns (MTCCs) subjected to monotonic axial compression to explore their mechanical behavior and confinement mechanism. Sixteen rectangular MTCCs and eight concrete-filled FRP tube columns (CFFTs) were tested. Four experimental variables, namely aspect ratio, FRP thickness, steel volume ratio, and steel tube configuration, were taken into consideration. The experimental results demonstrated that rectangular MTCCs' load-bearing capacity and deformability diminished with increasing aspect ratio, as it reduced the effective confinement area of FRP tube to concrete. Additionally, the confined concrete's compressive strength and axial deformability of rectangular MTCC (with a 6.4% steel volume ratio) enhanced by 21.5% and 32.9%, respectively, compared to the corresponding CFFT. This demonstrated the effectiveness of additional steel confinement. Furthermore, the confinement mechanism of rectangular MTCCs was clarified based on the full-range compressive behavior and dilation property of confined concrete. It included unconfined stage, transition stage, and hardening stage. Finally, a stiffness-based design-oriented model for rectangular MTCCs was established with favorable accuracy.