Analytical Calculation Method for the Axial Compressive Bearing Capacity of Steel Tube Confined Concrete Considering the Additional Hoop Effect of Temperature

Objective Steel tube confined concrete (STCC) members operate in external environments for extended periods. Ambient temperature variations alter the material properties of steel and core concrete. Due to the significant differences in their thermodynamic properties, interface debonding, and void formation in STCC are likely to occur, leading to substantial changes in bearing capacity. These changes have a considerable impact on the structural safety of STCC members. Investigating the influence of ambient temperature on STCC columns is crucial for understanding its effect on axial compression bearing capacity and developing a calculation method that accounts for ambient temperature.Methods This study independently designed and developed an axial compression test device for variable temperature steel tube confined concrete to address the significant influence of ambient temperature on the mechanical behavior of STCC structures. Using this test device, 26 axial compression tests of STCC short columns, including two types (M-type and S-type), were conducted under different ambient temperatures. The test phenomena and failure modes under various temperature conditions were analyzed, and the effects of ambient temperature on the axial compression mechanical properties of STCC columns were investigated. In addition, the mechanism of ambient temperature influencing the axial compression bearing capacity of STCC columns was identified. Given the limitations of existing theoretical calculation methods, which do not consider the effects of ambient temperature, a theoretical calculation method for the axial compression bearing capacity of STCC short columns was developed by integrating thermodynamic theory and limit equilibrium theory. The concept of "temperature-induced additional constraint effect" was introduced for the first time, and the accuracy of the proposed theoretical method was validated through experiments. This analytical approach systematically revealed the mechanism of influence of ambient temperature on the axial compression bearing capacity of STCC.Results and Discussions The external failure modes of the specimens under different temperature conditions were similar, all exhibiting typical shear failure. However, after cutting the outer steel tube, it was observed that the damage degree of core concrete under low-temperature conditions was significantly higher than that under normal and high-temperature conditions. The load-displacement curves of specimens under different temperature conditions were similar, consisting of the elastic, elastic-plastic, and descending stages. When the ambient temperature decreased from 40 ℃ to 20 ℃, 0 ℃, –20 ℃, and –40 ℃, the bearing capacity of M-type (S-type) specimens increased by 2.6% (12.0%), 7.8% (20.0%), 26.0% (22.7%), and 36.4% (43.2%), respectively. This result indicated a significant negative correlation between the axial compression bearing capacity and temperature of STCC short columns, and the larger the diameter-thickness ratio, the more pronounced the relationship. Under low-temperature conditions, the ambient temperature increased the specimen's bearing capacity and the brittleness of the core concrete and steel tube. Hence, the ductility of the specimen decreased significantly. The current calculation method of the axial compressive bearing capacity of STCC columns did not consider the influence of temperature, resulting in the maximum deviation between the calculated and experimental values ranging from 23% to 51%. However, the theoretical calculation method in this study accounted for the 'temperature-induced additional constraint effect' and the material variability of steel tube core concrete caused by temperature. The average deviation between the calculated and experimental values was only 2.0%, with a standard deviation of 0.06, demonstrating high accuracy and reliability. The bearing capacity of steel tube confined concrete columns under different ambient temperatures comprised three components: N1 (bearing capacity under normal temperature conditions), N2 (additional bearing capacity caused by material strength variation), and N3 (additional bearing capacity caused by the temperature-induced additional constraint effect). The temperature-induced additional constraint effect referred to the mechanical behavior in which the steel tube exhibited 'diameter shrinkage' due to ambient temperature changes. This resulted in applying circumferential temperature stress to the core concrete, which altered the confinement effect of the core concrete. The temperature-induced additional constraint effect had a significant impact on the mechanical properties of the specimen. When the ambient temperature was –40 ℃, the influence ratio on the bearing capacity reached 34.1% of the temperature-induced additional bearing capacity.Conclusions The temperature-induced additional constraint effect significantly influences the axial compression bearing capacity of STCC. This effect exhibits strong nonlinearity, and the influence amplitude of the bearing capacity of the specimen is affected by the temperature difference between the ambient temperature and the normal temperature, the temperature-dependent elastic modulus of the material, Poisson's ratio, the linear expansion coefficient, and the diameter-to-thickness ratio of the component. The theory proposed in this study considers the temperature-induced additional constraint effect and the material variability caused by temperature. The calculation results demonstrate high accuracy. Based on the theory presented in this study, a theoretical basis is provided for quantifying the influence of ambient temperature on the bearing capacity of steel tube-confined concrete. In the design of steel tube confined concrete structures in alpine regions, the effects of the temperature-induced additional constraint effect and temperature-induced material property variation on the mechanical behavior of structures should be comprehensively considered.