Axial Compression Behavior of Rectangular Thin-Walled CFST Short Columns

Literature Overview

This study by Zhang Zhongjie et al. (2020) from Yantai University investigates the axial compression performance of 32 rectangular thin-walled steel tube concrete (CFST) short column specimens. The research is supported by the Shandong Provincial Natural Science Foundation (ZR2019PEE033) and published in the Journal of Yantai University. The experimental matrix varies three key parameters: the section aspect ratio (length-to-width ratio), concrete compressive strength, and steel tube wall thickness. The authors analyze failure processes, failure modes, load-bearing capacity, deformation behavior, and the collaborative working mechanism between the steel tube and the confined core concrete.

Core Findings and Technical Interpretation

The most significant finding is that during loading, the long side of the thin-walled rectangular steel tube undergoes buckling first, and the ultimate failure mode and failure load are directly governed by this buckling event. This is a critical insight because it means that the structural behavior of rectangular CFST columns is fundamentally different from circular or square CFST columns, where the confinement is more uniform and the buckling behavior is less directional.

The aspect ratio of the rectangular section emerges as the dominant geometric parameter. A higher aspect ratio means the long side is more susceptible to local buckling, which in turn reduces the confinement effect that the steel tube provides to the core concrete. The study demonstrates that the wall thickness and the section aspect ratio have a pronounced influence on the confinement effectiveness. Thin-walled tubes with high aspect ratios provide relatively weak lateral confinement compared to circular or square sections of equivalent cross-sectional area.

A particularly noteworthy finding is that the load-bearing capacity and deformation performance depend not only on the aspect ratio and wall thickness but also on the ratio of steel tube compressive strength to core concrete compressive strength. This introduces an additional dimension to the design equation that is often overlooked in practice. When the steel tube is relatively weaker compared to the concrete, the confinement mechanism is compromised earlier, and the column reaches its peak load at lower strains.

Confinement Zone Model and Load Capacity Formula

The authors developed a confinement zone model for the core concrete, which partitions the cross-section into differently confined regions based on the geometry of the rectangular tube. In a rectangular section, the corners receive the most effective confinement due to the combined lateral restraint from both adjacent walls, while the mid-length of the long side receives the least confinement because of the greater slenderness of that wall segment. This model allows for a more realistic estimation of the confined concrete strength distribution across the section.

Based on this model, the authors derived a load capacity calculation formula for rectangular thin-walled CFST axial compression members. The formula integrates the contributions of the steel tube and the confined core concrete, accounting for the non-uniform confinement distribution. This is a meaningful improvement over existing formulas that treat the confinement as uniform across the entire cross-section, which is more appropriate for circular sections but less accurate for rectangular ones.

Engineering Practice Implications

In practical engineering applications, this study has several important implications. First, when selecting rectangular CFST columns for structural applications, the aspect ratio should be carefully controlled to avoid excessive long-side buckling. A practical guideline would be to limit the aspect ratio to below 1.5 for thin-walled sections. Second, the wall thickness must be selected not merely based on strength requirements but also to ensure adequate confinement of the core concrete. Third, designers should consider the relative strength of the steel tube and the concrete when selecting material grades, as an overly strong concrete relative to a thin-walled steel tube can lead to premature buckling of the tube and loss of confinement.

Key Questions and Reflections

The study raises several questions that deserve further investigation. First, the test specimens are short columns, meaning the slenderness ratio is low and the failure is governed by material and local buckling behavior rather than overall column buckling. How do these findings translate to slender CFST columns where global instability may dominate? Second, the study focuses on axial compression, but in real structures, CFST columns are almost always subjected to combined axial and bending loads. The interaction between bending and the non-uniform confinement in rectangular sections is likely to be more complex than in circular sections. Third, the confinement zone model, while innovative, needs validation against a larger dataset that includes different aspect ratios and wall thicknesses beyond the tested range.

Study Insights and Outlook

This research makes a valuable contribution to the understanding of rectangular thin-walled CFST short columns. The identification of the long-side buckling as the governing failure mechanism is practically important for engineers who must choose between circular, square, and rectangular CFST sections. The confinement zone model represents a step toward more refined analytical models that account for geometric non-uniformity. Future work should extend these findings to slender columns, eccentrically loaded members, and cyclic loading conditions to provide a more comprehensive design framework for rectangular CFST structures.