Experimental Study on Local Bearing Capacity of Square Steel Tube Recycled Concrete Short Columns with End Plates

Literature Overview

This paper by Zhang Zhaoqiang and You Shiguang (2014, World Information on Earthquake Engineering, Vol. 30, No. 2, pp. 13–18), supported by the National Natural Science Foundation of China (Grant No. 51308479) and Sichuan Provincial science and technology programs, investigates the local bearing capacity of square concrete-filled steel tube (CFST) short columns using recycled concrete. The study examines the influence of local bearing area ratio and end plate stiffness on structural performance.

Core Technical Content

Six square CFST short columns with end plates were tested under axial local loading. The experimental parameters were:

The local bearing area ratio is defined as the ratio of the loaded area to the total cross-sectional area. A smaller ratio indicates more concentrated loading, while a larger ratio approaches full-section loading.

Test Results Summary

The key findings from the experimental program are:

1. Failure mode similarity: The loading process and failure modes of recycled concrete CFST columns are fundamentally similar to those of conventional concrete CFST columns. This is significant because it validates the use of recycled concrete in CFST applications without requiring fundamentally different design approaches.
2. Effect of local bearing area ratio:

• As the local bearing area ratio increases, the local elastic modulus decreases.
• The ultimate bearing capacity decreases significantly with increasing local bearing area ratio.
• This counterintuitive result (larger loaded area → lower capacity) is explained by the stress redistribution mechanism: when the loaded area is large, the stress concentration is less pronounced, but the overall stress distribution becomes more uniform, reducing the beneficial confinement effect.

1. Effect of end plate stiffness:

• Increasing end plate stiffness significantly increases local bearing stiffness.
• The descending branch of the load-displacement curve becomes more gradual with stiffer end plates.
• Ultimate bearing capacity increases with end plate stiffness, but the rate of increase diminishes at higher stiffness levels.
• When the local bearing area ratio is small and end plate stiffness is large, the structural behavior approaches that of full-section loading.

Bearing Capacity Formula

Based on experimental data analysis, the authors derived a bearing capacity calculation formula that considers the primary parameters. The calculated values showed good agreement with experimental results, providing a practical design tool for engineers.

Standards and Material Considerations

Recycled concrete, also known as recycled aggregate concrete (RAC), uses crushed concrete waste as aggregate replacement for natural aggregate. The use of RAC in CFST structures is motivated by:

• Environmental sustainability: Reducing construction waste and natural resource consumption.
• Economic benefits: Lower material cost for recycled aggregate.
• Technical feasibility: Experimental evidence shows acceptable structural performance.

However, RAC has different properties compared to conventional concrete:

The ITZ (Interfacial Transition Zone) between recycled aggregate and cement paste is a critical microstructural feature. The ITZ in RAC is typically weaker and more porous than in conventional concrete due to residual old mortar on the recycled aggregate surface. This affects the overall mechanical properties and durability of RAC.

Integration with Engineering Practice

The local bearing capacity of CFST columns is a critical design consideration in several engineering scenarios:

1. Column-beam connections: Beam flanges bear locally on column faces, creating local bearing conditions.
2. Bracing connections: Diagonal braces apply concentrated forces to column nodes.
3. Foundation connections: Column bases transfer concentrated loads to foundations.
4. Equipment mounting: Heavy equipment mounted on CFST structures creates local bearing demands.

The experimental findings have direct implications for connection design:

• End plate design: Stiffer end plates improve load distribution and increase bearing capacity. In practice, end plate thickness should be optimized to balance stiffness benefits with material cost.
• Loaded area optimization: The relationship between loaded area ratio and bearing capacity is non-linear. Engineers should not assume that increasing the loaded area always increases capacity; the confinement effect must be considered.
• Recycled concrete viability: The similarity of failure modes between RAC-CFST and conventional CFST columns provides confidence in using recycled concrete, subject to appropriate strength reduction factors.

Key Questions and Reflections

A significant question arising from this study is the long-term durability of recycled concrete in CFST members. While the short-term mechanical performance is acceptable, the higher permeability of RAC may lead to accelerated corrosion of the steel tube in aggressive environments. The confinement effect of the steel tube on the concrete is beneficial for mechanical performance but may trap moisture and chlorides within the concrete core, potentially accelerating corrosion.

Another important consideration is the concrete-steel interface behavior under local loading. When a concentrated load is applied to a CFST column, the stress distribution is highly non-uniform. The interface between concrete and steel tube experiences complex shear and normal stresses that may differ significantly from the interface behavior under full-section loading. The bond strength and slip behavior at the interface under local loading conditions require further investigation.

The end plate stiffness effect raises practical fabrication concerns. Achieving very high end plate stiffness requires thick plates and substantial weld connections, which increase fabrication cost and complexity. The diminishing returns at high stiffness levels suggest an optimal stiffness range that balances performance and cost. Engineers should perform parametric studies to identify this optimal range for specific applications.

The study also raises questions about the applicability of the derived bearing capacity formula to different column geometries and loading conditions. The formula was developed based on a limited number of test specimens with specific parameters. Its applicability to other column dimensions, concrete strengths, steel grades, and loading configurations requires further validation.

Study Insights and Implications

This research makes a valuable contribution to the sustainable development of CFST structures by demonstrating the feasibility of using recycled concrete. The finding that RAC-CFST columns exhibit failure modes similar to conventional CFST columns is encouraging for the construction industry, which faces increasing pressure to reduce waste and resource consumption.

The parametric study of local bearing area ratio and end plate stiffness provides practical design guidance. The non-linear relationships revealed by the experiments challenge simplified design assumptions and highlight the importance of detailed analysis in connection design. The derived bearing capacity formula offers a practical tool for engineers, although its application should be limited to the parameter range covered by the experimental data.

For the broader engineering community, this study exemplifies the importance of experimental research in validating innovative structural systems. The combination of recycled materials, composite action, and local loading creates a complex structural system that cannot be fully understood through analytical methods alone. Physical testing remains essential for building confidence in new structural systems and materials, particularly when sustainability objectives drive the adoption of unconventional solutions.