Elastic and Elasto-Plastic Buckling Analysis of Concrete-Filled Steel Tubes

Literature Overview

This 2006 paper by Guo Lanhui, Zhang Sumei, and KIM Wha-Jung, published in the Journal of Harbin Institute of Technology, investigates the buckling behavior of concrete-filled square steel tubes using the finite strip method. The research examines both elastic and elasto-plastic buckling modes under two distinct stress states: uniform compressive stress and stress gradient conditions. The study provides quantitative insights into how concrete infill influences the local buckling capacity of steel plates and quantifies the detrimental effects of residual stresses on critical buckling stress.

Methodology and Analytical Framework

The finite strip method is employed as a computationally efficient yet accurate approach for analyzing plate buckling in thin-walled structural members. The analysis considers:

1. Uniform compressive stress — representing the axial loading condition where all cross-sectional fibers experience equal compressive stress.
2. Stress gradient — representing bending or combined loading conditions where stress varies across the plate width.

Both elastic buckling (linear stability analysis) and elasto-plastic buckling (accounting for material yielding) are evaluated, providing a comprehensive understanding of the buckling behavior across the full range of stress levels.

Key Findings on Concrete Infill Effect

The most significant finding of this study is the threshold behavior of concrete infill on buckling capacity:

This threshold behavior has important implications for structural design. For stocky steel tubes (width-to-thickness ratio below 30), the concrete infill does not provide meaningful buckling enhancement because the steel plate is already sufficiently thick to resist local buckling independently. However, for slender plates (ratio above 30), the concrete confinement effect becomes increasingly significant, with the maximum improvement reaching 2.58 times the capacity of the equivalent empty steel tube.

Residual Stress Effects

The study quantifies the detrimental effect of residual stresses on buckling capacity:

• Residual stresses reduce buckling capacity in both the elastic and elasto-plastic stages.
• Higher residual stress magnitudes result in lower critical buckling stresses.
• The effect is more pronounced in the elasto-plastic buckling regime.

This finding is directly relevant to practical fabrication, where residual stresses are introduced through steel rolling, welding, and forming operations. Engineers must account for these stresses when predicting the actual buckling capacity of fabricated concrete-filled steel tube members.

Engineering Practice Implications

The findings of this study have direct applications in the design and assessment of concrete-filled steel tube columns and panels:

1. Section proportioning — For width-to-thickness ratios below 30, designers can rely on the steel tube alone for buckling resistance without requiring concrete confinement effects. For ratios above 30, the concrete infill contribution should be explicitly included in capacity calculations.
2. Residual stress management — Post-fabrication stress relief procedures should be considered for slender concrete-filled steel tube members where residual stresses significantly compromise buckling capacity.
3. Bending resistance — The study confirms that concrete infill effectively improves the flexural buckling capacity of steel plates, which is relevant for beam-column members and panel elements.

Study Insights and Independent Reflections

The threshold behavior identified in this study—where concrete infill provides no buckling benefit below a width-to-thickness ratio of 30—challenges the common assumption that concrete filling always improves structural performance. This finding suggests that for stocky steel tubes, the added weight and cost of concrete filling may not be justified from a buckling perspective, and designers should evaluate the structural economics accordingly. The quantification of residual stress effects provides a practical basis for specifying stress relief procedures in fabrication specifications, particularly for slender members where the buckling capacity margin may be compromised. The finite strip method employed in this analysis offers a valuable computational tool that balances accuracy with efficiency, making it suitable for parametric design studies and preliminary capacity assessments in engineering practice. The maximum improvement factor of 2.58 times provides a useful upper bound for capacity predictions in slender concrete-filled steel tube sections, enabling engineers to establish realistic design expectations and avoid over-reliance on confinement effects.