Axial Compression Bearing Capacity of Steel Tube-Concrete Considering Environmental Temperature

Literature Overview and Research Significance

The research by Pan Qiren, Chen Yunfeng, Han Yun, Wang Li, Li Haiguo, and Liu Biao, published in China Safety Production Science and Technology in 2024, investigates the influence of environmental temperature on the axial compression bearing capacity of steel tube-concrete columns. This study is particularly relevant for structures located in cold regions with large temperature variations, such as those in northern China, Siberia, and other high-latitude areas. The research is supported by the Gansu Provincial Joint Research Fund and the Lanzhou Railway Bureau Group Science and Technology Research Program, underscoring its practical importance for railway infrastructure in cold environments.

Experimental Program and Temperature Range

The authors conducted axial compression tests on steel tube-concrete short column specimens at different environmental temperatures, ranging from room temperature down to -40 degrees Celsius. This temperature range is representative of the extreme cold conditions encountered in high-latitude regions of China, where railway bridges, tunnel portals, and other infrastructure must withstand severe winter conditions.

Temperature and Capacity Relationship

The experimental results demonstrated that temperature changes have a significant effect on the bearing capacity of steel tube-concrete columns. When the environmental temperature decreased from 20 degrees Celsius to -40 degrees Celsius, the specimen capacity increased by a maximum of 34.3 percent. This substantial increase is attributed to the combined effects of the temperature-dependent properties of both the steel tube and the concrete core.

Theoretical Analysis and Formula Development

Based on the limit equilibrium method, the authors derived a theoretical formula for the axial compression bearing capacity of steel tube-concrete columns that accounts for the influence of environmental temperature. The formula incorporates the temperature-dependent strength parameters of both the steel and concrete materials, as well as the confinement effect of the steel tube on the concrete core.

Theoretical Formula Validation

The theoretical formula was validated against the experimental results, and the relative error between the calculated and experimental values was found to be less than 10 percent. This level of accuracy is considered acceptable for engineering design purposes and demonstrates the effectiveness of the proposed analytical approach. The limit equilibrium method, while relatively simple compared to more complex numerical methods, provides a practical and transparent design tool that can be readily applied in engineering practice.

The theoretical analysis reveals that the increase in bearing capacity at low temperatures is primarily due to two mechanisms. First, the yield strength of the steel tube increases at lower temperatures, which enhances the confinement pressure on the concrete core. Second, the compressive strength of the concrete also increases at lower temperatures, particularly for ordinary Portland cement concrete, due to the continued hydration of unhydrated cement compounds and the reduced mobility of free water in the pore structure.

Engineering Practice Integration

For steel pipe manufacturers supplying tubes for cold-region infrastructure, this research provides important guidance on material selection and quality requirements. The steel tube must maintain adequate ductility at low temperatures to prevent brittle fracture, which is a critical concern for carbon steel grades in cold environments. The Charpy impact energy of the steel at the minimum design temperature must be verified to ensure that the material will not exhibit brittle behavior under seismic or impact loading.

The research findings also have implications for the design of railway bridges and other infrastructure in cold regions. The increased bearing capacity at low temperatures can be taken into account in the structural design, potentially allowing for more economical designs. However, engineers must also consider the thermal stresses induced by temperature variations, which can cause cracking of the concrete core and fatigue damage to the steel tube.

Quality Control Considerations

The manufacturing quality of the steel tube is critical for the performance of steel tube-concrete columns in cold environments. The steel tube must have a uniform wall thickness, smooth inner and outer surfaces, and no internal defects such as inclusions, seams, or cracks. The welding quality of any longitudinal or circumferential welds must also be verified, as weld defects can act as initiation sites for brittle fracture at low temperatures. Non-destructive testing methods such as ultrasonic testing and magnetic particle testing should be employed to ensure the structural integrity of the steel tube.

Key Questions and Reflections

An important question that arises from this study is how the temperature effect on bearing capacity would be influenced by the type of concrete used. The authors likely used ordinary Portland cement concrete, but in cold regions, special concretes such as low-temperature concrete or sulfate-resistant concrete may be employed. These special concretes may have different temperature-dependent properties, which could affect the accuracy of the proposed theoretical formula.

Another consideration is the effect of freeze-thaw cycles on the long-term performance of steel tube-concrete columns. While the research focuses on the static bearing capacity at different temperatures, in practice, the columns are subjected to repeated freeze-thaw cycles that can cause progressive damage to the concrete core. The cumulative effect of these cycles on the bearing capacity and the confinement effectiveness of the steel tube warrants further investigation.

Study Insights and Conclusion

This paper provides valuable insights into the behavior of steel tube-concrete columns under cold environmental conditions, offering both experimental data and a validated theoretical formula for design purposes. The 34.3 percent increase in bearing capacity at -40 degrees Celsius is a significant finding that can influence the economic design of infrastructure in cold regions. For steel pipe engineers, the key takeaway is that the temperature-dependent properties of the steel tube material must be carefully considered in the selection of steel grades for cold-region applications, and that the quality of the steel tube manufacturing is essential for ensuring the long-term performance of steel tube-concrete columns in severe environmental conditions.