Zhuojin Pipe Fitting Co., LtdLoad-Bearing Capacity of Steel-Encased Steel Tube High-Strength Concrete Combined Columns Under Combined Compression and Bending
Literature Overview
This 2005 paper by Zhao Dazhou, Wang Qingxiang, and Guan Ping investigates the load-bearing capacity of steel-encased steel tube high-strength concrete (steel-encased STC) combined columns subjected to combined axial compression and bending moments. Published in the journal Industrial Building, Volume 35, Issue 9, pages 84–85, the research was supported by the National Natural Science Foundation of China (Grant No. 50078008). The study employs numerical integration methods to simulate the load-deformation behavior and derives simplified calculation formulas for practical design applications.
Structural Configuration and Material System
The steel-encased steel tube high-strength concrete column represents a hybrid composite column that combines three structural components: an inner steel tube containing high-strength concrete, and an outer steel encasement (typically in the form of steel angles or a steel jacket). This configuration leverages the confinement effect of the steel tube on the concrete core while providing additional flexural and shear capacity through the outer steel encasement.
High-strength concrete (HSC) typically refers to concrete with a compressive strength exceeding 50 MPa, and in this study, likely extends to 60-80 MPa or higher. The use of HSC in composite columns allows for more compact column dimensions while maintaining adequate load capacity, which is particularly valuable in space-constrained industrial and commercial buildings.
Numerical Integration Methodology
The authors adopted a numerical integration method to simulate the load-deformation relationship of the combined column. This method involves the following computational sequence:
- Discretize the column cross-section into numerous elemental areas, each assigned to a specific material (steel tube, HSC, outer steel encasement).
- Apply a curvature increment to the cross-section and compute the resulting strain distribution using the plane-section-remains-plane assumption.
- Determine the stress in each elemental area using the material-specific stress-strain constitutive relationship.
- Integrate the stresses across the cross-section to obtain the resultant axial force and bending moment.
- Increment the curvature and repeat until the section reaches its ultimate capacity.
This fiber-based numerical integration approach captures the nonlinear material behavior of all constituents, including the confinement effects on the concrete core, the strain hardening of steel, and the interaction between the different materials. The computed load-deformation curves were validated against experimental test results, confirming the accuracy of the numerical model.
Factors Influencing Load-Bearing Capacity
Based on the numerical analysis results, the authors identified several key factors that influence the load-bearing capacity of the steel-encased STC combined column:
| Factor | Influence Mechanism | Relative Importance |
|---|---|---|
| Concrete strength grade | Directly affects concrete contribution to axial and flexural capacity | High |
| Steel tube diameter and wall thickness | Controls confinement level and steel tube contribution to flexural capacity | High |
| Outer steel encasement geometry | Adds flexural capacity and provides additional confinement at the column periphery | Moderate |
| Axial compression ratio | Governs the transition from flexure-controlled to compression-controlled behavior | High |
| Cross-sectional shape | Affects the moment of inertia and the distribution of confined concrete | Moderate |
| Steel yield strength | Influences the plastic hinge capacity and post-yield ductility | Moderate |
The axial compression ratio was found to be a particularly critical parameter, as it determines the failure mode transition. At low axial compression ratios, the column exhibits flexure-controlled behavior with significant ductility. As the axial compression ratio increases, the behavior transitions toward compression-controlled failure with reduced ductility. The combined effect of the steel tube confinement and the outer steel encasement extends the ductile behavior to higher axial compression ratios compared to conventional reinforced concrete columns.
Simplified Calculation Formulas
A key contribution of this study is the development of simplified calculation formulas for the load-bearing capacity of the combined column. These formulas are derived from the numerical results and are intended for practical design use, where the computational efficiency of a simplified approach is preferred over the accuracy of full nonlinear analysis. The simplified formulas incorporate the major influence factors identified in the parametric study and provide engineers with a practical design tool.
The simplified approach typically involves: computing the axial force capacity as the sum of contributions from each material component using appropriate reduction factors; computing the moment capacity using the plastic section modulus of the composite section; and applying interaction equations that account for the combined effect of axial force and bending moment. The reduction factors account for the confinement effects on concrete, the material nonlinearity, and the interaction between materials.
Engineering Practice Implications
The steel-encased STC combined column offers several advantages for practical engineering applications: higher load capacity per unit cross-sectional area compared to conventional RC columns; improved ductility due to the steel tube confinement and outer encasement; suitability for high-rise and heavy-load industrial structures; and potential for reduced column dimensions in space-constrained projects. The simplified calculation formulas developed in this study enable engineers to incorporate these advantages into design practice without requiring extensive nonlinear finite element analysis.
For construction practice, the combined column requires careful attention to the connection between the inner steel tube, the concrete core, and the outer steel encasement. The welding or bolted connections between these components must be designed to ensure composite action throughout the loading range. Additionally, the high-strength concrete placement within the steel tube requires appropriate vibration methods to achieve adequate compaction without damaging the steel tube.
Key Reflections
This study contributes to the understanding of hybrid composite columns that combine multiple structural materials to achieve enhanced performance. The numerical integration method provides a rigorous analytical foundation, while the simplified formulas bridge the gap between research and practice. The steel-encased STC column concept represents an evolutionary step in composite column design, leveraging the confinement benefits of steel tube concrete while adding the flexural capacity of an outer steel jacket. For engineers involved in the design of high-rise buildings, industrial structures, and infrastructure projects requiring high-capacity columns, this research provides valuable analytical tools and design guidelines. The validation of numerical results against experimental data enhances confidence in the proposed design methodology and supports its adoption in practical engineering applications.