Axial Compression Bearing Capacity Model of Circular Stainless Steel Tube Concrete Short Columns

Literature Overview

This paper by Tang Hongyuan, Fan Luyao, and colleagues from Xihua University (2020) investigates the axial compression behavior of circular stainless steel tube concrete (CFST) short columns. Supported by the National Natural Science Foundation of China (51768044), the research was published in Engineering Science and Technology (Vol. 52, No. 3, pp. 10-20). The work addresses the growing interest in stainless steel tube concrete structures for applications in marine platforms, bridges, and underground engineering where corrosion resistance is paramount.

Core Technical Content

The research involves axial compression tests on circular austenitic seamless stainless steel tube concrete short columns with different diameter-to-thickness ratios. The experimental program captures the complete load-deformation behavior, including load-axial deformation curves, load-hoop strain curves, and load-longitudinal strain curves. Based on the experimental results, a bearing capacity formula is derived through regression analysis and compared with existing design codes and models.

Experimental Program and Test Results

The test specimens consist of austenitic stainless steel tubes (typically 304 or 316 grade) filled with concrete. The diameter-to-thickness ratio (D/t) is varied to study the influence of confinement effectiveness on the structural behavior. The key experimental findings include:

• Failure mode: The typical failure mode is outward local buckling of the stainless steel tube, with the concrete core spalling outward as the tube deforms plastically.
• Load-deformation behavior: The columns exhibit an elastic stage, a plastic hardening stage, and a post-peak softening stage. The stainless steel tube provides progressive confinement to the concrete, delaying the onset of crushing.
• Confinement effect: The bearing capacity and ductility increase with increasing confinement coefficient, which is a function of the steel tube thickness, yield strength, and diameter.

Bearing Capacity Model Development

The authors derive a bearing capacity formula through regression analysis of the experimental data. The confinement coefficient ξ is defined as:

ξ = (A_s × f_y) / (A_c × f_c)

Where A_s is the cross-sectional area of the steel tube, f_y is the yield strength of the steel tube, A_c is the cross-sectional area of the concrete core, and f_c is the compressive strength of the concrete.

The derived formula is compared with:

• Existing design codes (GB 50935-2014, Eurocode 4, AISI S100)
• Mander model
• Li model
• Xiao model
• Teng model

The comparison shows that existing code formulas tend to underestimate the bearing capacity of stainless steel tube concrete columns, while the derived formula provides results closer to experimental values. Among the compared models, the authors' model, the Xiao model, and the Teng model provide the closest agreement with test results, with the authors' model having the smallest standard deviation.

Confinement Stress-Strain Model

Beyond the bearing capacity formula, the authors also develop a confinement stress-strain model for stainless steel tube concrete. This model describes the relationship between the confining pressure exerted by the steel tube and the lateral deformation of the tube. The model is compared with the Mander, Li, Xiao, and Teng models, and the authors' model provides the best fit to the experimental data.

Engineering Practice Integration

For engineers designing stainless steel tube concrete structures, several practical considerations arise from this research:

• Code applicability: The finding that existing code formulas underestimate the capacity of stainless steel tube concrete columns is important. Designers should be aware that using code formulas intended for carbon steel tube concrete may result in conservative (and potentially uneconomical) designs. The derived formulas provide a more accurate basis for design.
• Material selection: Austenitic stainless steels (304, 316) are the most common grades for structural applications due to their good combination of strength, ductility, and corrosion resistance. The lower yield strength of austenitic stainless steel compared to carbon steel (205-310 MPa vs. 235-460 MPa) means that thicker tubes may be required for equivalent confinement.
• Fabrication considerations: Stainless steel tubes require specialized welding procedures to maintain corrosion resistance. The welding process should use appropriate filler metals (e.g., ER308L for 304, ER316L for 316) and controlled interpass temperatures to prevent sensitization and intergranular corrosion.
• Quality control: Non-destructive testing of stainless steel welds requires careful selection of methods. Magnetic particle testing (MT) is not applicable to austenitic stainless steels due to their non-magnetic nature. Ultrasonic testing (UT) and radiographic testing (RT) are the primary methods for volumetric inspection, while liquid penetrant testing (PT) is used for surface defect detection.

Comparison of Bearing Capacity Models

Key Questions and Reflections

The primary question arising from this research is the generalizability of the derived formulas to different stainless steel grades, concrete strengths, and geometric proportions beyond the experimental range. The regression analysis is based on a specific set of test specimens, and extrapolation beyond the tested parameter ranges should be done with caution. Further experimental validation with a wider range of parameters is recommended.

Another important consideration is the long-term behavior of stainless steel tube concrete columns under sustained loading. The concrete core undergoes creep and shrinkage, which may affect the stress distribution between the steel tube and the concrete over time. The stainless steel tube, being more ductile than carbon steel, may accommodate larger deformations without fracture, but the long-term interaction with the concrete core under sustained loading requires further investigation.

From a corrosion perspective, the primary advantage of stainless steel tube concrete is the enhanced corrosion resistance of the steel tube in aggressive environments such as marine atmospheres, chemical plants, and de-icing salt environments. However, the concrete cover thickness and quality still play a role in protecting the steel tube from localized corrosion, and the interface between the stainless steel tube and the concrete should be carefully designed to prevent galvanic corrosion if carbon steel reinforcement is present.

Study Insights and Implications

This research makes a significant contribution to the structural engineering community by providing experimentally validated bearing capacity formulas and confinement models for stainless steel tube concrete columns. The finding that existing code formulas underestimate the capacity is particularly important for the practical application of stainless steel tube concrete structures, as it demonstrates the potential for more economical designs. For engineering practice, the key recommendations are:

• Use the derived formulas as a more accurate basis for the design of stainless steel tube concrete columns, with appropriate safety factors.
• Conduct further experimental research to extend the validated parameter ranges and to investigate the long-term behavior under sustained loading.
• Develop specialized fabrication and inspection procedures for stainless steel tube concrete structures, accounting for the unique material properties and corrosion resistance requirements.
• Promote the use of stainless steel tube concrete in applications where corrosion resistance is critical, such as marine platforms, coastal bridges, and underground structures in aggressive environments.

The research also highlights the broader trend toward the use of advanced materials in structural engineering, where stainless steel offers significant advantages in terms of durability and lifecycle performance, even if the initial material cost is higher than carbon steel.