Bearing Capacity of Circular Hollow Sandwich Steel Tube Concrete Columns

Literature Overview

This paper by Zhao Junhai, Guo Hongxiang, and Wei Xueying, published in the Journal of Architecture and Civil Engineering in 2005 (Vol. 22, No. 1, pp. 50-54), investigates the ultimate bearing capacity of circular hollow sandwich steel tube concrete columns using the unified strength theory. The research was funded by the Shaanxi Provincial Natural Science Foundation (2003E215) and the Ministry of Education Doctoral Program Special Research Fund (20040710001). The authors analyzed the influence of the steel tube tension-compression strength ratio and the intermediate principal stress on the bearing capacity, and proposed a theoretical formula for the ultimate bearing capacity of this novel column type.

Core Technical Points

Unified Strength Theory Framework

The unified strength theory, developed by Yu Mao-hong, provides a unified framework for describing the strength behavior of materials under complex stress states. Unlike the classical von Mises or Tresca criteria, which are limited to pure shear or pure tension-compression conditions, the unified strength theory can accommodate various stress states through a single set of material parameters. For the hollow sandwich steel tube concrete column, the intermediate principal stress plays a crucial role in determining the overall bearing capacity, as the hollow geometry creates a unique stress distribution that differs fundamentally from solid concrete-filled steel tubes.

The key innovation in this study is the application of the unified strength theory to a hollow sandwich configuration, where the core concrete is confined by two concentric steel tubes with a hollow space between them. This configuration introduces a more complex stress state compared to conventional concrete-filled steel tubes, with the intermediate principal stress becoming a significant factor in the load-bearing mechanism.

Tension-Compression Strength Ratio of Steel Tube

The steel tube tension-compression strength ratio (B) is a material parameter that characterizes the difference between the tensile and compressive yield strengths of the steel. For structural steel, this ratio is typically close to 1.0, but it can vary depending on the steel grade and manufacturing process. The authors demonstrated that the value of B has a measurable effect on the predicted bearing capacity, with higher B values (indicating greater tensile strength relative to compressive strength) leading to slightly lower predicted capacities for the hollow sandwich configuration. This finding is consistent with the physical behavior of the column, where the outer steel tube is in compression while the inner tube experiences a combination of compression and tension due to the Poisson effect of the confined concrete.

Intermediate Principal Stress Effect

The intermediate principal stress in the hollow sandwich column arises from the interaction between the inner and outer steel tubes and the confined concrete core. As the concrete core expands laterally under axial compression, it exerts radial pressure on the inner steel tube, which in turn transfers part of this pressure to the outer steel tube through the hollow gap. The magnitude of this intermediate principal stress depends on the geometry of the column, including the diameter and wall thickness of both tubes and the height of the hollow space. The authors showed that the intermediate principal stress has a significant strengthening effect on the bearing capacity, particularly at moderate axial compression ratios.

Theoretical Formula Development

The proposed bearing capacity formula takes the following general form:

Comparison with Experimental Data

The theoretical predictions were compared with experimental results from existing literature on hollow sandwich steel tube concrete columns. The comparison showed good agreement between the calculated and measured values, with deviations generally within the range of 5-10%. This level of accuracy is acceptable for practical engineering design and confirms the validity of the proposed formula. The study also concluded that the hollow sandwich steel tube concrete column possesses similar advantages to conventional concrete-filled steel tubes, including high axial load capacity, good ductility, and efficient material utilization, while offering additional benefits such as reduced material consumption and improved constructability.

Engineering Practice Implications

From a fabrication and welding perspective, the hollow sandwich steel tube concrete column presents unique challenges. The inner and outer steel tubes must be precisely concentric to ensure uniform concrete confinement, which requires careful alignment during assembly. The welding of any connection elements to both tubes must be performed with attention to residual stress control, as the dual-tube geometry is more susceptible to local buckling than single-tube configurations. The hollow space between the tubes provides an opportunity for inspection and potential repair, which is a significant practical advantage over solid concrete-filled tubes. However, the hollow geometry also means that the concrete core is less effectively confined at the top and bottom ends, which may require additional confinement measures such as end plates or additional steel rings.

Study Insights and Reflections

This paper makes an important contribution to the theoretical understanding of hollow sandwich steel tube concrete columns, which are an evolving structural system with potential applications in high-rise buildings and bridge piers. The application of the unified strength theory to this novel configuration demonstrates the versatility and predictive power of this theoretical framework. One observation I have is that the study focuses primarily on the ultimate bearing capacity under axial compression, which is the most common loading condition for columns. However, in real engineering applications, columns are often subjected to combined axial compression and bending, which may require a more comprehensive design methodology. The proposed formula could serve as a basis for developing interaction curves for bending-compression scenarios, which would be highly valuable for practical design.

Another reflection is that the study does not explicitly address the welding quality requirements for the dual-tube configuration. In practice, the welding of the inner tube to the outer tube (if required for structural integrity) or the welding of connection plates to both tubes must meet stringent quality standards to ensure proper load transfer. The welding residual stresses in the inner tube can influence the concrete confinement behavior, and the welding procedure must be carefully controlled to minimize distortion of the tube geometry. Future research should integrate welding process parameters into the theoretical framework to provide more comprehensive design guidance.

Reference Value and Outlook

The research provides a solid theoretical foundation for the design of hollow sandwich steel tube concrete columns, with a validated bearing capacity formula that can be directly applied in engineering practice. The findings confirm that this column type offers comparable performance to conventional concrete-filled steel tubes while providing additional practical advantages. The unified strength theory approach is particularly valuable because it provides a physically meaningful framework that can be extended to other loading conditions and geometries. Future work should address bending-compression interaction, seismic performance, and the effects of welding residual stress on the confinement mechanism to establish a complete design methodology for this promising structural system.