Creep behaviour and confinement effects of circular steel tube confined reinforced concrete (STCRC) stub columns under high level of sustained loading

Steel tube confined reinforced concrete (STCRC) columns demonstrate superior static/dynamic performance and efficient beam-column joint construction, showing promising future in long-span structures, heavily loaded constructions and high-rise buildings. However, the non-linear creep behaviour of STCRC columns under possible high sustained loads for scenarios such as the upper limit release of the axial load ratio or long-term overloading remains insufficiently investigated, while it differs fundamentally from traditional concrete-filled steel tubular (CFST) columns: the enhanced confinement effects in STCRC columns effectively suppress concrete cracking, while less prone cross-sectional stress redistribution may occur in STCRC columns owing to the less tube stiffness contribution caused by the disconnection at the beam-column joints. Hence, this study experimentally investigates the non-linear creep of circular STCRC stub columns under varying stress levels, concrete strengths, reinforcement ratios and steel tube ratios. Key mechanisms were explored, including the time-varying passive confining pressure and vertical tube stiffness. By comparing creep deformation of STCRC, CFST, and plain concrete specimens, the influence of confinement effects and tube stiffness were then quantified. A non-linear creep model incorporating concrete strength and confinement effects was proposed alongside a validated prediction analysis method. Results indicate that the non-linear creep increases the long-term deformation and longitudinal rebars stress by 57.0 %–85.5 %, rendering conventional linear creep analyses non-conservative. The confinement effects were non-negligible in STCRC columns under high sustained loading, causing obvious lower creep than that of plain concrete. The vertical tube stress caused by interfacial bond and friction always shows relatively small influence on the non-linear creep of STCRC stub columns, leading to 21.2 % higher creep than that of CFST columns. The proposed non-linear creep model and analysis method demonstrate high accuracy in predicting non-linear creep deformations of STCRC stub columns.