Local Buckling Behavior of Rectangular Steel Tube Concrete Column Walls

Literature Overview

The paper by He Baokang, Yang Xiaobing, and Zhou Tianhua, published in Building Structure (2005, Vol. 35, No. 1), addresses a critical yet often overlooked aspect of rectangular steel tube concrete (SRC) columns: the local buckling behavior of the steel tube wall under various stress gradient conditions. This study is particularly significant because rectangular SRC columns are increasingly used in high-rise buildings and large-span structures, yet the interaction between concrete infill and the steel tube wall creates complex stress states that challenge traditional plate buckling theories. The authors employed the finite strip method to compute critical buckling stresses and buckling coefficients, then integrated these results into a unified effective width approach to derive practical width-thickness and height-thickness ratio limits.

Core Technical Points and Interpretation

The fundamental challenge in rectangular SRC columns is that the steel tube wall does not behave as a simple simply-supported plate. The concrete infill provides lateral restraint, the stress distribution is non-uniform due to axial load combined with bending, and the boundary conditions differ from those assumed in classical buckling theory. The authors' approach using the finite strip method allows for a more realistic representation of the stress gradient across the plate, which directly affects the critical buckling stress.

The buckling coefficient $k_0$ is a dimensionless parameter that encapsulates the effect of boundary conditions and stress distribution on the critical buckling load. For a uniform stress state with simply-supported edges, the classical value is $k_0 = 4.0$, but the study demonstrates that under linearly varying stress gradients typical in SRC columns, $k_0$ can range from approximately 1.5 to 5.5 depending on the ratio of compressive to tensile stress at opposite edges. This variation has direct implications for the allowable thickness of the steel tube wall.

Effective Width Approach and Limit Expressions

The unified effective width method is a practical engineering approach that accounts for post-buckling behavior by considering only the effective portion of the plate that contributes to load resistance after local buckling has occurred. The authors substitute their computed buckling coefficients into this framework to derive limit expressions for:

• Width-thickness ratio ($b/t$): governing the local buckling resistance of the flat plate portion
• Height-thickness ratio ($h/t$): governing the buckling resistance of the column wall panel between stiffeners

The resulting expressions provide direct design criteria that can be incorporated into structural codes. The comparison with CECS 159:2004 (Technical Specification for Rectangular Steel Tube Concrete Structures) reveals that the code limits are generally conservative for most practical configurations, though the degree of conservatism varies with the stress gradient condition.

Engineering Practice Implications

From a welding and fabrication standpoint, the local buckling limits directly influence the selection of steel tube wall thickness during fabrication. Thinner walls reduce material cost and facilitate welding, but they must satisfy the buckling limits derived in this study. For typical rectangular SRC columns with cross-section dimensions of 400–600 mm and wall thicknesses of 8–14 mm, the width-thickness ratio typically falls within the range of 30–70, which is well within the limits established by CECS 159:2004. However, for larger columns or those subjected to high bending moments, the stress gradient becomes more pronounced, and the buckling coefficient decreases, potentially requiring thicker walls or additional internal stiffeners.

In practice, the welding of internal stiffeners to the tube wall introduces heat-affected zone (HAZ) softening that can reduce the local buckling resistance. The welding process must therefore be carefully controlled to minimize HAZ width and avoid excessive thermal cycles. For thick-walled tubes (t > 12 mm), preheating to 100–150°C is recommended to reduce residual stresses and prevent cracking, while post-weld heat treatment may be necessary for high-strength steels (Q345 and above) to restore mechanical properties in the HAZ.

Study Insights and Reflections

This paper demonstrates the value of rigorous theoretical analysis in establishing practical design criteria. The finite strip method provides a more accurate prediction of buckling behavior than simple plate theory, and the integration with the effective width approach bridges the gap between theory and code-based design. However, I note that the study is primarily theoretical and computational, with limited experimental validation. In engineering practice, the actual buckling behavior of welded SRC columns may be influenced by welding residual stresses, geometric imperfections, and material anisotropy that are not fully captured in the theoretical model.

The findings have direct relevance to welding quality control. The integrity of welds connecting the steel tube halves, end plates, and internal stiffeners is critical, as weld defects such as lack of fusion, porosity, or undercuts can act as stress concentrators that initiate local buckling at loads below the theoretical critical stress. Non-destructive testing (NDT) of all welds, particularly those in high-stress regions near column ends and stiffener connections, is therefore essential for ensuring the structural performance predicted by the buckling analysis.