Compression Tests and Bearing Capacity Calculation of Reinforced Square Steel Tube Concrete Short Columns

Literature Overview

This paper by Zhang Xinlong, Chen Yong, Liu Qingdong, and Zhang Yuzhuo from China Northeast Design and Research Institute and Shenyang Jianzhu University presents experimental and analytical studies on reinforced square steel tube concrete (CFST) short columns under axial compression. Published in the journal "Concrete" in 2020 (Issue 12, pages 11-14), the research was supported by the National Key R&D Program of China (2017YFC0703301). The study investigates the influence of reinforcement ratio on the axial compression behavior of square CFST short columns and proposes a simplified calculation formula for bearing capacity based on the Chinese national standard GB 50936-2014.

Core Technical Content and Key Findings

The research conducted axial compression tests on square CFST short columns with different reinforcement ratios, specifically comparing specimens with reinforcement ratios of 1.45% and 3.26%. The test results demonstrate that increasing the reinforcement ratio from 1.45% to 3.26% results in approximately 5% improvement in ultimate bearing capacity, a substantial improvement of nearly 20% in axial compression stiffness, and a moderate improvement in ductility.

The 5% bearing capacity improvement with a more than doubling of reinforcement ratio highlights an important engineering consideration: the efficiency of additional reinforcement in CFST columns is limited by the composite action mechanism. In CFST columns, the steel tube and concrete core share the applied load, and the internal reinforcement contributes additional load-bearing capacity. However, the confinement effect of the steel tube on the concrete core already provides significant strength enhancement, which limits the incremental benefit of additional reinforcement.

The 20% improvement in axial compression stiffness is a more significant finding, as stiffness directly affects serviceability performance, including deflection limits and vibration characteristics. For structures where stiffness is a governing design criterion, such as high-rise buildings or long-span structures, the reinforcement ratio optimization has more practical significance than the bearing capacity improvement alone.

The study also discusses the bearing capacity calculation methods from various national and international codes and standards, and proposes a simplified calculation formula based on the Chinese standard GB 50936-2014. This simplification is particularly valuable for engineering practice, where computational efficiency and ease of application are important considerations.

Technical Parameters and Test Results

The reinforcement ratio values tested (1.45% and 3.26%) are within the typical range for CFST column design. The lower ratio represents a minimum practical reinforcement level, while the higher ratio approaches the upper limit commonly used in practice. The study does not investigate very high reinforcement ratios, which may be relevant for special applications such as nuclear facilities or blast-resistant structures.

Engineering Practice Implications

From a steel pipe manufacturing perspective, the reinforcement ratio study has implications for the selection of steel tube dimensions in CFST column design. When higher reinforcement ratios are required to achieve target stiffness, the steel tube dimensions may be optimized to a smaller size while maintaining overall structural performance through increased internal reinforcement. This optimization can reduce steel tube material consumption while maintaining structural adequacy.

The welding quality of the steel tube is critical for the performance of reinforced CFST columns. Any defects in the steel tube, such as weld porosity, incomplete fusion, or microcracks, can initiate premature failure under compressive loading. The confinement action of the steel tube on the concrete core depends on the tube's structural integrity, and any local weakness in the tube wall can lead to premature local buckling and loss of confinement.

For fabrication, the internal reinforcement cage must be carefully positioned within the steel tube to ensure uniform concrete cover and proper bond between the reinforcement and concrete. The reinforcement cage fabrication should comply with relevant standards for bar bending, tying, and spacing tolerances. The positioning of the cage within the tube should be controlled using spacers or positioning devices to prevent displacement during concrete placement.

Quality Control and Inspection Requirements

The quality of the steel tube material should be verified through mechanical property testing, including yield strength, tensile strength, elongation, and impact toughness. For CFST columns, the steel tube should meet the requirements of relevant standards such as GB/T 3091 for welded steel tubes or GB/T 8162/8163 for seamless steel tubes. The chemical composition should be verified to ensure appropriate carbon equivalent for weldability and toughness.

The welding of the steel tube, whether ERW, LSAW, or other process, should be inspected through appropriate NDT methods. For square CFST columns, the longitudinal and transverse welds require particular attention, as these welds are potential weak points under compressive loading. Ultrasonic testing (UT) and radiographic testing (RT) are suitable for weld inspection, with the specific method selection depending on wall thickness and access conditions.

Study Insights and Reflections

The most practically significant finding of this research is the quantification of the reinforcement ratio effect on CFST column performance. The finding that a 5% bearing capacity improvement requires more than doubling the reinforcement ratio from 1.45% to 3.26% has direct cost implications for structural design. Engineers should carefully evaluate whether the stiffness improvement justifies the additional reinforcement cost, particularly in applications where bearing capacity is not the governing design criterion.

The simplified calculation formula proposed based on GB 50936-2014 represents a valuable contribution to engineering practice. The standard's original calculation method may involve iterative procedures or complex expressions that are difficult to apply in preliminary design stages. A simplified formula that captures the essential behavior while maintaining reasonable accuracy can accelerate the design process and improve design efficiency.

The study's focus on square CFST columns is particularly relevant for practical applications, as square sections are commonly used in building construction due to their architectural compatibility and ease of connection to other structural members. The research findings can be directly applied to the design of reinforced square CFST columns in commercial and residential buildings, where stiffness and ductility are important performance criteria.