Experimental Study on Local Bearing Capacity of Stainless Steel Tubes

Literature Overview

This comprehensive experimental study by Chen Xixiang, Wang Xintao, Yuan Yuan, and Chen Yu from Yangtze University (published in 2016, Volume 41, Issue 5, pp. 1330-1341 in the journal "Journal of Guangxi University, Natural Science Edition") investigates the local bearing capacity of hollow stainless steel tubes and concrete-filled stainless steel tubes. Supported by the National Natural Science Foundation of China (Grants 51278209 and 51478047) and Yangtze University's Engineering Science Development Fund (Grant 15j0402), the study tested 26 hollow stainless steel tube specimens and 8 concrete-filled stainless steel tube specimens under various loading configurations.

Core Technical Findings

Loading Configuration Effects

The study examined four distinct loading configurations and their influence on local bearing capacity:

This hierarchy reflects the different restraint conditions and stress distribution patterns associated with each configuration. Internal loading provides greater lateral restraint to the loaded web region compared to external loading.

Web Slenderness Effects

The inverse relationship between capacity and ductility with web slenderness is consistent with the well-known behavior of steel plate elements, where slender plates exhibit earlier buckling at lower stress levels but with greater deformation capacity.

Support Plate Width Effect

Increasing the support plate width significantly improves the local bearing capacity of hollow stainless steel tubes. This is attributed to the broader stress distribution achieved with wider bearing plates, which reduces peak stress concentrations in the web.

Concrete-Filled Effect

The introduction of concrete infill provides a very significant improvement in local bearing capacity, confirming the well-established benefit of composite action in steel tube members.

Material Properties and Stainless Steel Specific Considerations

Stainless steel exhibits distinct material behavior compared to carbon steel that directly influences local bearing performance:

• Higher strain hardening exponent: Stainless steel grades (such as 304L, 316L, or 2205 duplex) exhibit significant strain hardening, allowing them to sustain higher stresses after initial yielding.
• Reduced elastic range: The yield-to-ultimate strength ratio is typically lower for stainless steel, providing greater post-yield deformation capacity.
• Temperature sensitivity: Stainless steel has lower modulus of elasticity and different thermal expansion characteristics, which may affect long-term performance under thermal cycling.
• Corrosion resistance: The primary advantage of stainless steel in structural applications, particularly in marine, chemical, or atmospheric environments where carbon steel would require protective coatings.

Failure Mode Analysis

The observed failure modes in the tests provide critical insight into the local bearing mechanism:

1. Web crippling: The primary failure mode for slender webs, characterized by progressive buckling of the web panel under concentrated loading.
2. Flange yielding: Observed in configurations with high flange stress concentrations, particularly in external loading cases.
3. Corner deformation: At the flange-web junction, plastic hinge formation may occur under high load levels.
4. Concrete crushing (for CFST specimens): The confined concrete core may crush before the steel tube reaches its ultimate capacity, particularly in specimens with high concrete strength and thin tube walls.

Design Code Comparisons

The experimental results can be benchmarked against existing design provisions:

The study's findings contribute to the validation and potential revision of stainless steel design provisions, particularly regarding the web slenderness limits and the concrete-filling enhancement factors.

Welding and Fabrication Implications

For stainless steel tube fabrication, several welding-specific considerations arise:

1. Welding process selection: GTAW (Tungsten Inert Gas Welding) is preferred for stainless steel tubes due to its clean, precise arc and excellent penetration control. GMAW with solid stainless wire is also common for thicker sections.
2. Heat input control: Excessive heat input can cause sensitization in austenitic stainless steels (chromium carbide precipitation at grain boundaries), reducing corrosion resistance. Interpass temperature should be maintained below 150°C for susceptible grades.
3. Filler metal selection: For 304L stainless tubes, ER308L filler wire is standard; for 316L, ER316L is used. For duplex grades (2205), specialized duplex fillers (such as ER2209) are required to maintain the austenite-ferrite balance.
4. Post-weld treatment: For high-strength applications, solution heat treatment (1050-1100°C followed by rapid cooling) may be required to restore full mechanical properties in the HAZ.
5. Corrosion control at welds: Weld decay and intergranular corrosion are primary concerns. Proper welding sequence, purge gas (argon) for root passes, and post-weld pickling/passivation are essential quality measures.

Study Insights and Reflections

The systematic investigation of loading configuration effects provides valuable data for practical design, as real-world connections and load applications rarely conform to idealized symmetric loading. The finding that internal one-flange loading provides the highest capacity is particularly relevant for connection design, where asymmetric loading is common.

The significant enhancement from concrete infill validates the continued development of stainless steel concrete-filled tube (SSCFST) systems for applications requiring both corrosion resistance and high structural capacity. The combination of stainless steel's corrosion resistance with concrete's compressive strength and thermal mass creates a versatile structural system suitable for aggressive environments such as marine platforms, chemical processing facilities, and coastal infrastructure.

The web slenderness-ductility trade-off highlights the importance of performance-based design for stainless steel structures. Rather than simply restricting slenderness ratios, designers can leverage the inherent ductility of slender stainless steel sections to achieve adequate performance through appropriate deformation capacity provisions. This philosophy aligns with modern performance-based seismic design approaches that prioritize deformation capacity over elastic strength.

The research contributes meaningfully to the growing body of knowledge on stainless steel structural members, supporting the increasing adoption of stainless steel in structural engineering where long-term durability and low maintenance are paramount design objectives.