Zhuojin Pipe Fitting Co., LtdAxial Compression Bearing Capacity of Built-in Steel Section Circular Steel Tube Concrete Columns
Literature Overview
The paper by Mao Wenjing, Shi Yanli, and Wang Wenda, published in Progress in Steel Building Structures (2021, Vol. 23, Issue 11, pp. 37-46), investigates the axial compression behavior of circular steel tube concrete columns with built-in steel sections. Funded by the National Natural Science Foundation of China (Grants 51468037 and 51768038), this research addresses the growing demand for taller, heavier, and longer-span structures in modern construction. The authors employed ABAQUS finite element software to model the full stress-strain behavior of these composite columns.
Core Technical Findings
The study demonstrates that the built-in steel section provides a dual confinement effect on the core concrete, enhancing the concrete strength beyond what would be achieved by the steel tube alone. The interaction between the steel section, concrete, and outer steel tube creates a synergistic load-bearing mechanism that significantly improves both the load capacity and ductility of the column.
The authors systematically analyzed the influence of several key parameters on the structural performance: material strength, steel section steel ratio, steel tube steel ratio, and steel section shape. The findings indicate that the steel section steel ratio and the steel section shape have relatively minor effects on strain behavior, while the overall interaction between components is the primary driver of enhanced performance.
Key Parameters and Their Effects
| Parameter | Effect on Load Capacity | Effect on Ductility |
|---|---|---|
| Material strength (concrete and steel) | Significant positive effect | Moderate positive effect |
| Steel section steel ratio | Moderate positive effect | Minor effect on strain |
| Steel tube steel ratio | Significant positive effect | Significant positive effect |
| Steel section cross-sectional shape | Minor effect on strain | Minor effect |
| Component interaction | Primary driver of capacity enhancement | Primary driver of ductility improvement |
Mechanism of Dual Confinement
The concept of dual confinement is central to understanding the enhanced performance of built-in steel section circular steel tube concrete (BSS-CFST) columns. The outer steel tube provides conventional lateral confinement to the core concrete, while the built-in steel section introduces an additional confining mechanism through direct contact and mechanical interlock with the concrete. This dual confinement increases the triaxial stress state within the concrete, delaying concrete crushing and extending the post-peak load-deformation response.
From a welding and fabrication perspective, the connection between the built-in steel section and the steel tube is critical. Whether this connection is achieved through welding, mechanical fastening, or frictional contact, the integrity of this interface governs the effectiveness of the composite action. Welding engineers should pay particular attention to the weld quality at the steel section-to-tube interfaces, as poor weld quality can lead to premature separation and loss of composite action.
Simplified Calculation Formula
The authors propose a simplified calculation formula for the axial compression bearing capacity of BSS-CFST short columns. The formula is calibrated against experimental data and demonstrates good agreement with test results. This formula is designed to be practical for engineering design while capturing the essential mechanics of the composite behavior.
The formula likely incorporates the following components: the contribution of the steel tube in compression, the contribution of the built-in steel section in compression, and the enhanced concrete strength accounting for the dual confinement effect. The confinement enhancement factor is typically derived from the lateral confining pressure provided by both the steel tube and the steel section, considering the geometry and material properties of each component.
Engineering Practice Integration
The application of BSS-CFST columns is particularly relevant for high-rise buildings, long-span structures, and heavy-load industrial facilities where conventional reinforced concrete or steel tube concrete columns may be insufficient. The added steel section provides additional compressive capacity without significantly increasing the column diameter, which is advantageous in space-constrained design situations.
For fabrication, the key challenges include: (1) precise positioning of the steel section within the circular tube, (2) ensuring adequate concrete placement around the steel section without voids, and (3) maintaining the concentricity of the steel section during construction. Welding the steel section to the tube (if required) must be performed with careful attention to distortion control, as the asymmetric geometry can introduce significant welding residual stresses.
Study Insights and Implications
This paper makes a valuable contribution to the understanding of composite column behavior. The finding that the steel section shape has minimal effect on strain behavior is practically useful, as it provides designers with flexibility in selecting the most economical steel section profile. The proposed simplified formula offers a practical tool for preliminary design, though detailed finite element analysis remains necessary for critical applications. The dual confinement concept opens up new possibilities for optimizing composite column design by strategically combining different confinement mechanisms.