Bearing Capacity Analysis of Square Hollow Sandwich Steel Tube Concrete Columns

Literature Overview

The paper by Li Lianlian, Zhao Junhai, Zhang Changguang, and Feng Hongbo, published in Sichuan Building Science (2008, Vol. 34, Issue 6, pp. 46–48), presents a theoretical analysis of the bearing capacity of square hollow sandwich steel tube concrete (SHS-STC) short columns under axial compression. The research, supported by the Ministry of Education Doctoral Program Fund (Grant No. 20040710001) and the Shaanxi Provincial Natural Science Foundation (Grant No. 2005E204), applies the unified strength theory of double shear to derive an analytical expression for the ultimate bearing capacity. This study addresses a specialized structural system that combines the advantages of steel tube concrete with the benefits of hollow sandwich construction, offering potential for improved efficiency and performance.

Core Technical Content and Key Findings

Structural Concept of Hollow Sandwich Steel Tube Concrete

The hollow sandwich STC column consists of:

An outer square steel tube
An inner square steel tube
Concrete confined between the two tubes (sandwich layer)
A hollow core in the center

This configuration provides several advantages:

Enhanced confinement effect from both inner and outer tubes
Reduced self-weight due to the hollow core
Improved ductility and energy dissipation
Potential for internal reinforcement or service integration

Theoretical Framework: Unified Strength Theory of Double Shear

The unified strength theory of double shear (USTDS) was developed by Yu Maochun and provides a comprehensive framework for describing the strength behavior of materials under complex stress states. The key features of this theory include:

Feature	Description	Advantage
Unified formulation	Covers various yield criteria	Flexibility and generality
Double shear criterion	Based on double shear stress state	More accurate for concrete
Parameterized	Single parameter for material characterization	Simplified calibration
Applicable to confined concrete	Captures triaxial behavior	Relevant for STC members

Derivation of Bearing Capacity Formula

The theoretical analysis involved several key steps:

Equivalent conversion: The square hollow sandwich STC column was converted to an equivalent circular hollow sandwich STC column using appropriate conversion factors.
Thick-walled cylinder theory: The unified strength theory for thick-walled cylinders was applied to the equivalent circular column.
Confinement effect incorporation: Two reduction coefficients were introduced:

Equivalent confinement reduction coefficient (accounting for thick-edge ratio)
Concrete strength reduction coefficient (accounting for confinement effectiveness)

Bearing capacity derivation: The ultimate bearing capacity was derived by combining the unified strength theory with the STC unified theory.

Reduction Coefficients

The introduction of reduction coefficients is a key contribution of this paper:

Coefficient	Symbol	Function	Influencing Factors
Equivalent confinement reduction	α	Accounts for thick-edge ratio effect	Wall thickness, geometry
Concrete strength reduction	β	Accounts for confinement effectiveness	Concrete strength, confinement pressure

These coefficients allow the theoretical model to account for the practical limitations of confinement in real structures, improving the accuracy of predictions.

Parametric Analysis and Validation

The derived formula was validated by comparison with experimental results from the literature. The comparison showed good agreement, confirming the applicability of the unified strength theory to square hollow sandwich STC columns. The parametric analysis examined the influence of:

Concrete strength
Steel tube strength
Wall thickness ratio
Hollow core size
Aspect ratio

Engineering Practice Implications

Manufacturing Challenges for Hollow Sandwich STC

The fabrication of hollow sandwich STC columns presents several manufacturing challenges:

Challenge	Description	Solution Approach
Inner tube positioning	Maintaining concentricity of inner and outer tubes	Precision fabrication, alignment fixtures
Concrete placement	Filling the annular space between tubes	Pumping techniques, vibration methods
Welding connections	Connecting inner and outer tubes	Pre-welding, post-weld inspection
Hollow core formation	Maintaining the hollow space during concrete placement	Core forms, temporary inserts
Quality control	Ensuring uniform concrete fill	NDT methods, density testing

Welding Considerations

The welding of hollow sandwich STC columns requires careful planning:

Inner tube welds: The inner tube may require longitudinal welds if fabricated from plate, and these welds must be inspected for quality.
Outer tube welds: Similar to inner tube, the outer tube welds must meet applicable standards.
Connection welds: If the inner and outer tubes are connected by transverse welds or stiffeners, these welds must be designed for the expected stress levels.
Weld sequencing: The welding sequence should be planned to minimize distortion and residual stress.
Post-weld treatment: Stress relief may be required for thick sections or high-strength materials.

Design Recommendations

Geometry optimization: The hollow core size should be optimized to balance weight reduction with confinement effectiveness.
Wall thickness ratio: The thick-edge ratio should be controlled to ensure adequate confinement and prevent local buckling.
Material selection: The steel grade and concrete strength should be selected based on the required bearing capacity and ductility.
Connection design: If the inner and outer tubes are connected, the connections should be designed to transfer the expected shear and normal forces.
Quality control: Rigorous quality control is essential to ensure uniform concrete fill and proper weld quality.

Comparison with Conventional STC Columns

Aspect	Conventional STC	Hollow Sandwich STC	Advantage
Self-weight	Higher	Lower	Reduced gravity load
Confinement	Single tube	Dual tube	Enhanced confinement
Ductility	Good	Potentially better	Improved seismic performance
Fabrication	Simpler	More complex	Manufacturing challenge
Cost	Lower	Higher	Cost consideration
Application	General structural	Specialized applications	Niche market

Key Questions and Reflections

Several questions arise from this study:

How does the hollow core affect the long-term durability of the STC column, particularly regarding moisture ingress and corrosion?
What is the effect of the hollow core on the fire resistance of the column?
Can the hollow core be utilized for internal reinforcement or service integration without compromising structural performance?
How does the hollow sandwich configuration perform under combined loading (axial + bending + shear)?

Study Insights and Outlook

This research provides a theoretical foundation for the design of square hollow sandwich STC columns, demonstrating the applicability of the unified strength theory of double shear to this specialized structural system. The introduction of reduction coefficients for confinement and concrete strength is a practical contribution that improves the accuracy of theoretical predictions. For engineers and manufacturers, the key insight is that the hollow sandwich configuration offers potential advantages in terms of weight reduction and enhanced confinement, but requires careful attention to manufacturing quality and connection design. The welding of inner and outer tubes, as well as any connection elements, must meet rigorous quality standards to ensure the structural integrity of the column. Future research should extend to experimental validation, combined loading conditions, and practical fabrication techniques to support the wider adoption of this innovative structural system.