Zhuojin Pipe Fitting Co., LtdAxial Compression Bearing Capacity of Square Steel Tube Recycled Concrete Columns with Internal Steel Sections
Literature Overview
This paper by Liu Jian, Zhang Pengcheng, Liu Changjiang, Tian Yong, Ren Da, Zeng Rongsen, Bai Yuxiang, and Chen Panpan, published in Concrete, 2021, Issue 10, presents a theoretical analysis of the axial compression bearing capacity of square steel tube recycled concrete short columns with internal steel sections. The authors developed a calculation methodology by equivalent transformation of the square section to a circular section, introducing equivalent constraint reduction coefficient and recycled concrete strength reduction coefficient to account for the unique behavior of recycled aggregate concrete.
Theoretical Framework and Equivalent Transformation
The core approach of this research is the equivalent transformation of the square steel tube recycled concrete section with internal steel section into an equivalent circular steel tube recycled concrete section with equal cross-sectional area and steel ratio. This transformation simplifies the complex stress distribution in the square section by leveraging the well-established analytical solutions for circular sections.
Two key reduction coefficients were introduced to account for the differences between the actual square section with internal steel section and the equivalent circular section. The equivalent constraint reduction coefficient (ψ) accounts for the reduced lateral constraint effect due to the non-uniform stress distribution in the square section compared to the uniform constraint in a circular section. The recycled concrete strength reduction coefficient (γu) accounts for the reduced strength of recycled concrete compared to natural aggregate concrete, which is attributed to the weaker interfacial transition zone between the recycled aggregate and the cement paste.
| Design Parameter | Symbol | Influence on Bearing Capacity |
|---|---|---|
| Inner diameter to thickness ratio | D/t | Higher ratio reduces constraint effectiveness |
| Confinement coefficient | — | Higher confinement increases bearing capacity |
| Steel section ratio | — | Higher ratio increases bearing capacity |
| Recycled coarse aggregate replacement rate | — | Higher rate decreases concrete strength |
| Equivalent constraint reduction coefficient | ψ | Accounts for square section non-uniformity |
| Recycled concrete strength reduction coefficient | γu | Accounts for recycled aggregate strength loss |
Theoretical Analysis Methodology
The bearing capacity analysis employed the limit analysis method combined with the double-shear unified strength theory. This combination provides a rigorous framework for analyzing the complex stress state in composite columns under axial compression. The limit analysis method determines the upper and lower bounds of the collapse load, while the double-shear unified strength theory provides a more accurate description of the concrete failure criterion under multi-axial stress states compared to traditional Mohr-Coulomb or von Mises criteria.
The analysis considered the interaction between the steel tube, the internal steel section, and the recycled concrete core. The lateral constraint provided by the steel tube and the internal steel section enhances the compressive strength and ductility of the recycled concrete through the confinement effect. The internal steel section contributes directly to the axial load-bearing capacity while also providing additional lateral constraint to the concrete core.
Parameter Influence Analysis
The research systematically analyzed the influence of four key parameters on the axial compression bearing capacity. The inner diameter to thickness ratio (D/t) affects the constraint effectiveness: higher D/t ratios result in less effective lateral constraint because the steel tube wall is more flexible and provides less confinement to the concrete core. The confinement coefficient, which relates the lateral constraint pressure to the concrete strength, directly influences the enhanced concrete strength under confinement.
The steel section ratio (the ratio of the internal steel section area to the total cross-sectional area) has a positive effect on bearing capacity, as the steel section directly contributes to load-bearing and enhances concrete confinement. The recycled coarse aggregate replacement rate has a negative effect on bearing capacity because higher replacement rates result in lower concrete strength due to the weaker interfacial transition zone and the higher porosity of recycled aggregates.
Validation and Practical Application
The theoretical calculation results were compared with experimental data from relevant tests, and the agreement was found to be good. This validation confirms the effectiveness of the proposed calculation methodology and provides confidence in its application to design calculations. The good agreement suggests that the equivalent transformation approach, combined with the two reduction coefficients, adequately captures the essential behavior of square steel tube recycled concrete columns with internal steel sections.
For practical design application, the proposed calculation formula provides a theoretical basis for determining the axial compression bearing capacity of these composite columns. Designers can use the formula to evaluate different design parameters and optimize the column configuration for specific load requirements. The formula is particularly valuable for the design of structures using recycled concrete, as it provides a quantitative framework for accounting for the reduced strength of recycled aggregate concrete.
Study Insights and Engineering Implications
The most significant contribution of this research is the development of a practical calculation methodology for a structurally complex composite column type. The equivalent transformation approach, while introducing some simplification, provides a tractable analytical framework that captures the essential structural behavior. The introduction of two reduction coefficients to account for the specific features of the square section and recycled concrete is a pragmatic approach that balances analytical rigor with practical applicability.
The research has important implications for sustainable construction practices. By providing a reliable calculation methodology for recycled concrete composite columns, it supports the use of recycled aggregates in structural applications, contributing to waste reduction and resource conservation. The ability to predict the bearing capacity of these columns enables engineers to confidently specify recycled concrete in structural designs, promoting the adoption of sustainable construction materials.
In conclusion, this paper provides a rigorous theoretical framework for the axial compression bearing capacity calculation of square steel tube recycled concrete short columns with internal steel sections. The equivalent transformation approach, combined with the proposed reduction coefficients and the limit analysis method with double-shear unified strength theory, creates a practical and validated calculation methodology. The research supports the structural engineering community's efforts to incorporate recycled materials into structural designs while maintaining safety and reliability standards.