Analytical Analysis of Local Buckling in Rectangular Steel Tube Concrete Axially Compressed Columns

Literature Overview

This paper by He Baokang, Yang Xiaobing, and Zhou Tianhua (2002), published in the Journal of Xi'an University of Architecture and Technology (Natural Science Edition), presents a theoretical analysis of the local buckling behavior of the steel tube wall in rectangular steel tube concrete (CFST) axially compressed columns. The research employs the energy method to calculate the local buckling critical stress of the steel tube wall under uniform pressure loading, deriving a local buckling coefficient of k = 10 for rectangular CFST plate elements. This analytical result provides a fundamental design parameter for the local buckling assessment of rectangular CFST columns.

Theoretical Analysis and Energy Method

The energy method used in this study is based on the principle of total potential energy, where the buckling critical stress is obtained by minimizing the total potential energy of the plate element with respect to the displacement amplitude. The total potential energy consists of the strain energy stored in the plate due to bending and the work done by the external compressive stress. The buckling mode shape is assumed as a double sine series, and the boundary conditions at the edges of the plate element are modeled as simply supported, which is a reasonable approximation for the steel tube wall of a rectangular CFST column.

Local Buckling Analysis Parameters

The derivation of k = 10 is significant because it differs from the classical plate buckling coefficient of k = 4 for simply supported plates under uniform compression. The higher value reflects the additional confinement provided by the concrete core, which increases the effective boundary conditions and raises the buckling resistance. The concrete core acts as a continuous elastic foundation, providing lateral support to the steel tube wall and delaying the onset of local buckling.

Comparison with Design Standards

The local buckling coefficient of k = 10 derived in this study can be compared with the values used in various design standards for CFST columns. The comparison reveals that the analytical result is consistent with the empirical approaches adopted in design codes, providing a theoretical basis for the design provisions.

Comparison of Local Buckling Coefficients Across Standards

The fact that the analytical result of k = 10 falls within the range used by design standards provides confidence in the theoretical approach. However, the study also highlights the importance of section geometry: the local buckling coefficient varies with the aspect ratio of the plate element, and the value of k = 10 is specific to the rectangular CFST configuration analyzed.

Welding and Steel Tube Manufacturing Considerations

The local buckling analysis has direct implications for the welding and manufacturing of rectangular CFST columns. The steel tube in a rectangular CFST column is typically formed by bending a steel plate and welding the longitudinal and transverse seams. The local buckling resistance of the steel tube wall is affected by the following welding and manufacturing factors:

1. Weld-induced residual stresses: The longitudinal and transverse welds introduce residual stresses in the steel tube wall, which can reduce the effective buckling resistance. The residual stress pattern near the weld is characterized by tensile stresses in the weld metal and compressive stresses in the adjacent base metal. These residual stresses superimpose on the applied compressive stress and can initiate buckling at a lower applied stress than predicted by the theoretical analysis.
2. Weld geometry: The weld reinforcement height and weld toe geometry affect the local stress concentration. A high weld reinforcement creates a geometric discontinuity that can act as a buckling initiation site. The weld reinforcement height should be limited to 1.5 mm above the base metal thickness, and the weld toe should be ground smooth to reduce stress concentration.
3. Weld defects: Defects such as lack of fusion, porosity, and slag inclusions reduce the effective cross-sectional area and can initiate buckling at a lower stress. Non-destructive testing (NDT) of all welds is essential, with radiographic testing (RT) for volumetric defects and magnetic particle testing (MT) for surface and near-surface defects.

Weld Quality Requirements for Rectangular CFST Columns

Key Questions and Reflections

The study raises an important question regarding the effect of concrete confinement on the post-buckling behavior of the steel tube wall. The energy method provides the critical buckling stress, but it does not describe the post-buckling response. In practice, the steel tube wall may buckle at a stress below the theoretical critical stress due to imperfections, residual stresses, and material non-uniformity. The post-buckling capacity of the steel tube wall, supported by the concrete core, is a critical parameter for the ultimate load-bearing capacity of the column. Future research should investigate the post-buckling behavior through both experimental and numerical methods.

Another reflection concerns the applicability of the energy method to real-world conditions. The theoretical analysis assumes perfect geometric regularity, uniform material properties, and ideal boundary conditions. In practice, steel tubes have manufacturing tolerances, surface imperfections, and material property variations that can significantly affect the buckling behavior. The theoretical result of k = 10 should be used with appropriate safety factors to account for these uncertainties, and the safety factor should be calibrated against experimental data.

Study Insights and Implications

This research provides a fundamental theoretical contribution to the understanding of local buckling in rectangular CFST columns, deriving a buckling coefficient of k = 10 through the energy method. For welding and pipe fitting engineers, the key insight is that the local buckling resistance of the steel tube wall is not solely a function of the plate width-to-thickness ratio but is also significantly influenced by the concrete confinement. The theoretical analysis should be used in conjunction with experimental data and design standards to establish appropriate acceptance criteria for steel tube manufacturing and welding. The study's findings reinforce the importance of weld quality, residual stress control, and non-destructive testing in ensuring the structural integrity of rectangular CFST columns.