Buckling Bearing Capacity of Steel Tubes in Square Steel Tube Concrete Columns

Literature Overview

This study by Guo Lanhui, Rong Qin, and Zhang Sumai from the School of Civil Engineering at Harbin Institute of Technology addresses a critical yet often overlooked issue in the design of steel tube concrete (STC) columns: the local buckling behavior of the steel tube itself when confined by concrete. Published in the Journal of Harbin Institute of Technology (Vol. 43, No. 10, 2011, pp. 6–11) and supported by the National Natural Science Foundation of China (Grant No. 59808004), the research investigates how concrete infill modifies the buckling characteristics of square steel tubes under axial compression. The authors conducted experimental tests on 24 specimens—both empty square steel tubes and concrete-filled counterparts—with width-to-thickness ratios spanning from 50 to 125—and supplemented the experimental data with finite element analysis using Abaqus.

Core Technical Findings

The fundamental observation is that while increasing the width-to-thickness ratio of a steel tube reduces steel consumption and improves economic efficiency, exceeding a certain threshold inevitably triggers local buckling. The study demonstrates that concrete infill fundamentally alters the buckling mode of the steel tube and significantly enhances its buckling capacity. Notably, the magnitude of capacity enhancement grows with increasing width-to-thickness ratio, meaning that the benefit of concrete confinement becomes more pronounced in thinner-walled sections.

Experimental Program Summary

Buckling Mode Transformation

When the steel tube is empty, local buckling manifests as conventional plate buckling of individual flanges with half-wave patterns governed by the classical elastic buckling stress. Once concrete is introduced, the concrete core acts as a continuous lateral support against the inner surface of the steel tube, effectively reducing the effective buckling length of each plate element. This transforms the buckling mode from a single half-wave to multiple half-waves with shorter wavelengths. The concrete also provides radial confinement that delays the onset of plastic buckling and redistributes stress concentrations at corners.

Process and Standards Analysis

From a manufacturing and design perspective, this study has direct implications for the selection of steel tube wall thickness in CFST columns. Current design codes such as GB 50017 (Standard for Design of Steel Structures) and GB 50010 (Code for Design of Concrete Structures) impose slenderness ratio limits on steel tubes in CFST columns primarily based on stability considerations. However, the local buckling capacity of the steel tube itself—particularly for high width-to-thickness ratios—is not always adequately captured by these simplified provisions.

The study's finding that buckling capacity improvement increases with width-to-thickness ratio suggests that the allowable slenderness limit for CFST columns could potentially be relaxed compared to hollow steel columns, provided the concrete infill quality is guaranteed. This is particularly relevant for large-diameter CFST columns used in high-rise buildings and bridge piers, where reducing wall thickness without compromising stability is a major design objective.

Engineering Practice Considerations

• Concrete filling must be complete and dense to ensure effective confinement; voids or honeycombing within the tube can drastically reduce the buckling enhancement effect.
• The corner regions of square tubes experience complex stress states; manufacturing tolerances at corner welds (for welded square tubes) or roll-formed corners (for cold-formed tubes) must be controlled to prevent premature local buckling initiation.
• For ERW or HFW welded square tubes used in CFST applications, the weld seam integrity is critical because the weld zone represents a potential weakness for buckling initiation under compressive loading.

Key Questions and Reflections

One question that arises from this study is how the results would differ for circular steel tubes, where the confinement effect is more uniform. Another consideration is the influence of concrete strength grade on buckling capacity enhancement—higher-strength concrete may provide stiffer confinement but could also be more brittle, potentially affecting the ductility of the buckling response. Furthermore, the study focuses on short columns under pure axial compression; the interaction between local buckling and overall column buckling in slender CFST columns remains an area requiring further investigation.

From a manufacturing standpoint, the study reinforces the importance of dimensional accuracy in steel tube production. Variations in wall thickness, particularly at weld seams, can create localized weak zones that initiate buckling at stresses below the theoretical uniform-section prediction. Quality control measures such as ultrasonic thickness testing and hydrostatic pressure testing become more important when the width-to-thickness ratio approaches the upper limits identified in this research.

Study Insights and Implications

This research provides valuable quantitative data for the design of thin-walled CFST columns. The conclusion that concrete infill can significantly enhance buckling capacity—especially for high width-to-thickness ratios—offers a pathway for material optimization in structural applications. For steel tube manufacturers, the study underscores the need for precise wall thickness control and uniform material properties throughout the tube cross-section. For structural engineers, the findings support more rational utilization of steel in CFST designs, potentially reducing overall material costs while maintaining structural safety. The correlation between experimental results and finite element predictions validates the numerical modeling approach for future parametric studies covering a wider range of geometries and loading conditions.