Experimental Study on Local Bearing Capacity of Stainless Steel Tubes

Literature Overview

This 2016 study by Chen Xixiang, Wang Xintao, Yuan Yuan, and Chen Yu from Yangtze University investigates the local bearing (flange bearing) performance of hollow stainless steel tubes and concrete-filled stainless steel tubes under concentrated loads. Thirty-four test specimens (26 hollow and 8 concrete-filled) were subjected to static loading to examine web crippling behavior under various boundary conditions, load positions, support plate widths, and web slenderness ratios. Published in the Journal of Guangxi University (Natural Science Edition), this research addresses an important design issue for stainless steel structural members: the local bearing capacity at connection points where concentrated loads are applied.

Core Technical Findings

Support Plate Width Effects

The width of the support plate has a significant positive effect on local bearing ultimate capacity. Wider support plates distribute the concentrated load over a larger area of the tube web, reducing local stress concentration and delaying web crippling. This finding is consistent with classical bearing capacity theory and provides clear design guidance for connection detailing.

Web Slenderness Ratio Effects

The study reveals an inverse relationship between web slenderness and local bearing capacity: tubes with lower h/t ratios (50) exhibit higher ultimate capacity but lower ductility, while tubes with higher h/t ratios (75) show lower capacity but greater ductility. This trade-off is important for design optimization, as ductility may be more critical in seismic applications while capacity is paramount in gravity-load-dominated structures.

Load Position Effects

The position of the applied load significantly influences local bearing capacity:

Internal loading (IG and ITF) provides higher capacity than end loading (EG and ETF) because internal loading benefits from boundary restraint from the tube ends, which provides additional confinement to the web panel.

Concrete-Filled Stainless Steel Tubes

The most significant finding is that concrete filling provides a very substantial improvement in local bearing capacity. The concrete core:

• Provides direct bearing resistance under the support plate
• Restrains web outward buckling through lateral confinement
• Increases effective web thickness by composite action
• Enhances overall ductility through concrete's compressive behavior

Technical Analysis

Web Crippling Mechanism

Local bearing failure in hollow tubes occurs through web crippling, which involves:

1. Initial local yielding beneath the support plate
2. Propagation of plastic zones along the web
3. Development of buckling modes (inward or outward)
4. Formation of plastic hinges and ultimate failure

The load position determines the boundary conditions for the web panel:

• Internal loading: Web panel is restrained at both ends by the tube flanges, creating a continuous beam analogy
• End loading: Web panel is restrained at one end only, creating a cantilever analogy with reduced capacity

Stainless Steel Material Considerations

Stainless steel exhibits different material behavior compared to carbon steel, which affects local bearing capacity:

• Higher yield strength: Typically 2–3 times that of carbon steel, providing higher bearing resistance
• Strain hardening: Significant strain hardening capacity improves post-yield performance
• Lower elastic modulus: Approximately 190–200 GPa (vs. 206 GPa for carbon steel), slightly reducing buckling resistance
• Different stress-strain curve: No distinct yield plateau, requiring modified design approaches

Design Standards Comparison

Engineering Practice Integration

Connection Design Recommendations

Based on the study findings, the following practical recommendations emerge for stainless steel tube connections:

1. Support plate sizing: Use wider support plates to maximize local bearing capacity, with minimum widths typically specified as 1.5–2.0 times the tube wall thickness plus the bolt hole diameter.
2. Load position optimization: Where structurally possible, arrange connections to apply loads internally rather than at tube ends to benefit from boundary restraint.
3. Web slenderness control: For capacity-critical connections, select tubes with lower h/t ratios (≤50) despite the reduced ductility.
4. Concrete filling consideration: For high-capacity connections in stainless steel tubes, concrete filling provides the most effective capacity enhancement.

Manufacturing Quality Requirements

From a steel pipe manufacturing quality perspective:

• Wall thickness uniformity: Local bearing capacity is directly proportional to wall thickness; variations in t around the tube circumference can create weak points.
• Surface quality: Surface defects (dents, scratches) can initiate local yielding and reduce bearing capacity.
• Geometric accuracy: Ovality affects the effective web height and thus the slenderness ratio; tight tolerances are important.
• Material certification: Stainless steel grade verification (e.g., 304, 316, duplex) is essential for accurate design calculations.

Welding Considerations for Stainless Steel Tubes

When stainless steel tubes are joined for structural applications:

• GTAW (Tungsten Inert Gas Welding) is preferred for maintaining material properties
• Heat input control is critical to prevent sensitization and intergranular corrosion
• Back purging with inert gas is necessary to prevent oxidation of the root pass
• Post-weld heat treatment may be required for certain grades to restore corrosion resistance

Key Questions and Reflections

The finding that concrete-filled stainless steel tubes show very significant improvement in local bearing capacity raises questions about the optimal concrete grade and fill quality. Unlike carbon steel CFST members where concrete strength is a primary design parameter, in stainless steel applications, the very high strength of the stainless steel tube may dominate the composite behavior, potentially making concrete grade less critical. However, the concrete's contribution to bearing resistance and web restraint remains important.

The ductility-capacity trade-off between h/t ratios of 50 and 75 is particularly relevant for seismic design. In seismic regions, ductility is often more important than peak capacity, as it allows energy dissipation through inelastic deformation. This suggests that for seismic applications, higher h/t ratios (up to 75) may be preferred despite lower ultimate capacity, provided that the reduced capacity is compensated through other design measures.

The internal loading advantage (IG > ITF > EG ≈ ETF) has practical implications for structural configuration. In multi-story buildings with stainless steel tube columns, beam-column connections should be designed to apply loads internally rather than at column ends. This may influence the selection of connection types and the arrangement of structural frames.

Study Insights and Implications

This comprehensive study provides essential experimental data for the design of local bearing connections in stainless steel tube structures. The identification of key influencing parameters (support plate width, web slenderness ratio, load position, and concrete filling) offers clear design guidance for practitioners. The significant benefit of concrete filling for local bearing capacity suggests that concrete-filled stainless steel tubes should be considered for applications requiring high-capacity connections, such as heavy industrial structures or bridge applications. For stainless steel pipe manufacturers, this research highlights the importance of dimensional accuracy, surface quality, and material consistency in structural applications where local bearing performance is critical. The study also demonstrates the value of stainless steel tubes in structural applications, where their high strength, corrosion resistance, and aesthetic appeal can be leveraged through proper connection design that accounts for local bearing behavior.