Longitudinal Allowable Stress of Steel Tube in Steel Tube Concrete

Literature Overview

This paper by Liu Yongjian, Jiang Lei, and Zhang Ning from Chang'an University and Northwest A&F University provides a theoretical framework for calculating the longitudinal allowable stress of steel tubes in steel tube concrete (CFST) short columns under axial compression. Published in Journal of Architecture and Civil Engineering (Vol. 32, No. 6, 2015, pages 1-7), the research addresses a fundamental question in CFST design: how the interaction between steel and concrete affects the stress state of the steel tube at ultimate load. The authors introduce a steel tube longitudinal allowable stress reduction coefficient and a concrete compressive strength enhancement coefficient to account for the multi-axial stress state that develops at the ultimate bearing stage. Supported by National Natural Science Foundation grants (51178051, 51378068) and Shaanxi Provincial Transportation Department funding, this work provides theoretical depth to a practically important design problem.

Core Technical Findings

The research establishes that at the ultimate bearing stage of CFST short columns under axial compression, both the steel tube and the core concrete experience a multi-axial stress state due to the lateral expansion of the concrete and the resulting confining pressure. The steel tube longitudinal allowable stress reduction coefficient accounts for the fact that the steel tube does not achieve its full uniaxial compressive strength because of the multi-axial stress condition. Conversely, the concrete compressive strength enhancement coefficient captures the beneficial effect of lateral confinement on the concrete's compressive capacity. The calculated values using the proposed method show good agreement with experimental results, validating the theoretical approach.

Key Coefficients and Their Significance

Theoretical Framework

The theoretical analysis begins with the equilibrium of a CFST column under axial compression. At the ultimate stage, the concrete has expanded laterally, exerting a confining pressure on the steel tube. This confining pressure creates a multi-axial stress state in the steel tube, with longitudinal compression and circumferential tension. The steel tube's longitudinal stress is therefore less than its uniaxial compressive strength, and the reduction coefficient quantifies this difference. Simultaneously, the confining pressure increases the concrete's compressive strength beyond its unconfined value, and the enhancement coefficient captures this improvement.

Technical Interpretation and Engineering Implications

From a steel pipe engineering perspective, this research has direct implications for the design and specification of steel tubes used in CFST structures. The steel tube geometry, particularly the diameter-to-thickness ratio (D/t), significantly influences the confinement effectiveness and the resulting stress state. Thinner-walled tubes provide more effective confinement per unit of steel but may buckle prematurely under high confining pressures.

Steel Tube Design Considerations

• The D/t ratio must be selected to balance confinement effectiveness against local buckling resistance, with typical values ranging from 30 to 60 for common CFST applications.
• Steel tube material grade affects both the confinement capacity and the longitudinal allowable stress, with higher-grade steels providing greater confinement but potentially requiring larger reduction coefficients due to earlier yielding in the multi-axial stress state.
• The steel tube fabrication quality, including weld quality at circumferential joints, directly impacts the confinement effectiveness, as any geometric imperfection or weld defect can initiate premature local buckling.
• Surface treatment and coating of steel tubes must account for the interaction with concrete, ensuring that the bond between steel and concrete is not compromised by corrosion or interface degradation.

Design Code Implications

The introduction of the steel tube longitudinal allowable stress reduction coefficient represents a refinement over conventional design approaches that often assume the steel tube achieves its full uniaxial compressive strength. This refinement leads to more accurate bearing capacity predictions and, potentially, more economical designs. For engineering practice, the proposed coefficients provide a rational basis for steel tube selection in CFST structures, enabling designers to optimize the steel tube geometry and material grade for specific structural requirements.

Comparison with Existing Design Approaches

Study Insights and Reflections

This research contributes significantly to the theoretical understanding of steel tube concrete behavior and provides practical tools for design optimization. The steel tube longitudinal allowable stress reduction coefficient is a conceptually simple but practically important refinement that acknowledges the complex multi-axial stress state that develops at ultimate load. For steel pipe manufacturers and engineers, the key insight is that the steel tube in a CFST member does not behave as a simple uniaxial compression element but rather as a confined pressure vessel that simultaneously resists axial load and lateral expansion. This dual role means that steel tube specifications for CFST applications must consider both axial strength and hoop tension capacity, which has implications for material selection, wall thickness, and fabrication quality. The good agreement between calculated and experimental values validates the theoretical approach and gives confidence in its application to practical design problems. This study reinforces the importance of fundamental mechanical analysis in steel pipe engineering and demonstrates how theoretical insights can lead to more efficient and reliable structural designs. The proposed coefficients provide a rational framework that can be incorporated into design codes and standards, improving the technical basis for CFST structural design worldwide.