Zhuojin Pipe Fitting Co., LtdExperimental Study on Central Local Bearing Capacity of Recycled Concrete-Filled Square Steel Tube Short Columns with End Plates
Overview and Research Background
The paper by Zhang Zhaoqiang and You Shiguang, published in World Information on Earthquake Engineering in 2014, presents an experimental investigation into the central local bearing capacity of recycled concrete-filled square steel tube (RCFST) short columns with end plates. This research is supported by the National Natural Science Foundation of China (grant 51308479), the Sichuan Provincial Science and Technology Department (2010JY0174), and the Sichuan Provincial Department of Education (13ZB0179). The study addresses an important practical issue in structural engineering: the behavior of CFST columns when subjected to concentrated loads at their ends, a condition commonly encountered in beam-column connections and load transfer nodes.
The use of recycled concrete in CFST columns is driven by sustainability concerns and the need to reduce the environmental impact of construction. Recycled concrete, produced by incorporating crushed concrete waste as a partial replacement for natural aggregate, typically exhibits lower strength and stiffness compared to conventional concrete. Understanding the local bearing behavior of recycled concrete-filled columns is therefore essential for ensuring structural safety and serviceability.
Core Technical Content
Experimental Programme and Specimen Configuration
Six RCFST short column specimens were tested under axial local loading, with two primary variables: the local compression area ratio (the ratio of the loaded area to the total cross-sectional area) and the end plate stiffness. The specimens featured square steel tubes filled with recycled concrete, with end plates of varying stiffness attached to the loaded end to simulate different connection conditions.
| Specimen Parameter | Variation Range | Effect on Local Bearing |
|---|---|---|
| Local compression area ratio | Small to large | Larger ratio reduces elastic modulus and ultimate capacity |
| End plate stiffness | Low to high | Higher stiffness increases local stiffness and ultimate capacity |
| Concrete strength | Recycled concrete | Lower than conventional concrete |
| Steel tube geometry | Square cross-section | Uniform confinement in both axes |
The test setup applied a concentrated load at the center of the top end plate, simulating a typical beam-column connection condition. Load-displacement curves were recorded throughout the test, and strain gauges were installed at critical locations to monitor the strain distribution on the steel tube surface and within the end plate.
Failure Modes and Load-Displacement Behavior
The experimental observations revealed that the failure modes of the RCFST specimens were fundamentally similar to those of conventional CFST columns. The loading process progressed through distinct stages: an initial elastic stage, a plastic stage with progressive yielding of the steel tube and crushing of the concrete core, and a post-peak softening stage characterized by concrete crushing and steel tube deformation.
The load-displacement curves exhibited a characteristic shape with a steep ascending branch in the elastic stage, a gradual transition to the plastic stage, and a descending branch after the peak load. The rate of stiffness degradation in the descending branch was influenced by both the local compression area ratio and the end plate stiffness.
Parametric Analysis Results
The experimental data revealed clear trends in the influence of the two primary variables:
Effect of local compression area ratio: As the local compression area ratio increased (i.e., the loaded area became a larger fraction of the total cross-sectional area), the local elastic modulus decreased and the ultimate bearing capacity was significantly reduced. This is attributed to the increased stress concentration and the reduced effectiveness of the stress diffusion mechanism within the column.
Effect of end plate stiffness: Increasing the end plate stiffness led to a significant increase in the local stiffness of the column. The load-displacement curve in the descending branch became more gradual, indicating better post-peak deformation capacity. The ultimate bearing capacity also increased with end plate stiffness, although the rate of improvement diminished at higher stiffness levels.
Combined effect: When the local compression area ratio was small and the end plate stiffness was high, the behavior of the specimen approached that of a fully loaded column, with the stress distribution becoming more uniform across the cross-section.
Analytical Model Development
Based on the experimental data, the authors developed a calculation formula for the central local bearing capacity of RCFST short columns with end plates. The formula incorporates the effects of the local compression area ratio, the end plate stiffness, the concrete strength, and the steel tube geometric parameters. The calculated values showed good agreement with the experimental results, validating the proposed analytical approach.
Key Design Considerations
The research findings have direct implications for the design of beam-column connections in RCFST structural systems:
- The local compression area ratio should be kept as small as practicable to maximize the local bearing capacity and ensure efficient stress diffusion within the column.
- The end plate should be designed with adequate stiffness to provide effective load distribution and to prevent premature failure at the connection interface.
- The recycled concrete strength should be properly characterized and accounted for in the design calculations, as it is typically lower than conventional concrete.
- The composite action between the steel tube and the recycled concrete core should be verified through appropriate testing or analysis.
Integration with Engineering Practice
The practical relevance of this research extends to the broader context of sustainable construction and the use of recycled materials in structural applications. As the construction industry increasingly adopts recycled concrete to reduce waste and carbon emissions, understanding the structural behavior of recycled concrete-filled members becomes critical for ensuring safety and serviceability.
Engineers designing RCFST structural systems should consider the following practical aspects derived from this research:
- Material characterization: The mechanical properties of recycled concrete should be determined through comprehensive testing, including compressive strength, elastic modulus, and splitting tensile strength.
- Connection design: The end plate stiffness should be designed to ensure effective load transfer from the beam to the column, with particular attention to the stress distribution at the steel tube-concrete interface.
- Quality control: The concrete placement and consolidation within the steel tube should be carefully controlled to ensure full composite action and uniform concrete quality.
- Inspection and testing: Non-destructive testing methods such as ultrasonic testing should be employed to verify the quality of the concrete fill and the integrity of the steel tube.
Critical Reflection and Study Insights
This research contributes valuable experimental data to the understanding of RCFST column behavior under local loading conditions. The systematic investigation of the effects of local compression area ratio and end plate stiffness provides a clear basis for design recommendations. The development of an analytical formula based on the experimental data offers a practical tool for engineers to predict the local bearing capacity of RCFST columns in design applications.
One notable finding is that the failure modes of RCFST columns are fundamentally similar to those of conventional CFST columns, despite the lower strength and stiffness of the recycled concrete. This suggests that the composite action between the steel tube and the concrete core is robust enough to compensate for the reduced material properties, at least within the range of parameters investigated. However, further research with a wider range of recycled concrete strengths and steel tube geometries would be beneficial to confirm the general applicability of these findings.
The study also highlights the importance of end plate design in ensuring the structural performance of CFST columns under local loading. The end plate acts as a load-spreading element that distributes the concentrated load over a larger area of the column cross-section. The research demonstrates that adequate end plate stiffness is essential for achieving optimal structural performance, and that the benefits of increasing stiffness diminish at higher stiffness levels, suggesting an optimal design range.
For the structural engineering community, this research reinforces the viability of recycled concrete in CFST applications and provides the experimental evidence and analytical tools needed to confidently incorporate recycled concrete-filled columns into design practice. The findings contribute to the broader goal of sustainable construction by demonstrating that recycled materials can be used in structural applications without compromising safety or performance, provided that appropriate design considerations are incorporated.