Experimental and numerical analysis of square tubed steel-reinforced concrete columns: Confinement mechanism and modeling

To improve the confinement effect of square steel tube confined concrete members, embedding a steel section inside the specimen may be an effective measure. Based on that, this paper conducted an experimental and numerical study of square tubed steel-reinforced concrete (STSRC) columns to comprehensively investigate the composite confinement mechanism of the external steel tube and internal steel section. The main parameters considered in this study include the geometry, yield strength, and shape of the steel section, as well as the thickness and yield strength of the steel tube, and the concrete strength. The results show that the compressive performance of STSRC columns is significantly influenced by shape of steel section and thickness of steel tube. Compared with H-shaped steel, X-shaped steel exhibits more uniform and effective constraint on the core concrete due to the additional support of webs. The distribution of constraint stress is concentrated in the region near the flange-web junction, which is more heavily affected by geometric parameters of the steel sections compared with material strength parameters. Moreover, increasing the steel tube thickness improves the load-carrying capacity and ductility of specimens: taking the 2 mm-thick steel tube specimen as the control, when the thickness increases to 5 mm, the peak load rises by up to 21.1 % (for H-section specimens) and 17.8 % (for X-section specimens), and the ductility index ( DI ) increases by an average of 8.4 %, but it will weaken the influence of the shape of the steel section. Based on numerical analysis results, a method for calculating the constraint stress of steel sections is proposed in this study, and a new stress-strain model for concrete under composite constraint of the steel tube and steel section can be established. Finally, developing a load-strain relationship model for STSRC columns by using the cross-section analysis method. Comparing with the test results from this study and the existing literature, the proposed model exhibits relatively high accuracy.