Experimental and numerical study on nonlinear creep of square concrete-filled steel tube under high-stress level

Previous studies have indicated that the core concrete experiences a relatively high stress level under quasi-permanent combination loading, exhibiting nonlinear creep behaviour. Current research on the nonlinear creep behaviour of concrete-filled steel tube (CFST) mainly focuses on circular concrete-filled steel tube (CCFST). However, under high stress levels, the nonlinear creep behaviour of square concrete-filled steel tube (SCFST) may differ from that of CCFST due to the different confinement effects. This paper investigates the nonlinear creep behaviour of SCFST and its difference from that of CCFST. Five groups of sustained load tests were carried out at stress levels, which means the ratio of the stress applied to the core concrete at loading to its 28-day cylindrical compressive strength, ranging from 0.32 to 0.76 and compared against test results of CCFST. The tube strains were measured to understand how the confinement effect changed over time. To study the uneven passive confinement of SCFST, the strain states at mid-span and corner locations of specimens at a stress level of 0.76 were simultaneously monitored. A nonlinear finite element model considering the confinement effect was developed to model the nonlinear creep behaviour of SCFST, and its accuracy was validated. Experimental results show that under the influence of nonlinear creep behaviour, using a linear creep model underestimated the continuing deformation by 31 %–105 %. The stress increase in the steel cross-section due to the time-dependency was 14 %–53 % of the instantaneous stress condition. SCFST and CCFST exhibit highly similar time-dependent behaviour when the stress level is no more than 0.76 (≤ 4 % creep difference; ≤ 7.1 % time-dependent response difference), because the confinement effect is trivial with the maximum transverse tensile stress only about 3 MPa. Therefore, the nonlinear creep model for CCFST can be employed to predict the creep deformation of SCFST in this condition. The proposed finite element model achieves reasonable accuracy in evaluating the longitudinal and transverse deformations considering the confinement effect. The findings establish a theoretical basis for predicting long-term deformations of SCFST and provide a foundation for future research on instability and failure mechanisms under overload.