Axial Compression Behaviour of High-Strength Cold-Bent Rectangular Steel Tube Concrete Short Columns

Literature Overview

This paper by Rao Yulong et al. (2019), published in the Journal of Huaqiao University (Natural Science), presents experimental results on the axial compressive capacity of high-strength cold-bent rectangular steel tube concrete (CFHCST) short columns. The study was funded by the National Natural Science Foundation of China (Grant No. 51778065) and the Hubei Provincial Department of Education Science and Technology Research Project (D20151304). The research addresses an increasingly relevant topic in modern structural engineering: how high-strength steels interact with confined concrete in cold-formed rectangular sections under pure axial compression.

Core Technical Parameters and Experimental Design

The experimental programme varied three key parameters: steel tube wall thickness, steel grade strength, and cross-sectional dimensions. The columns were tested under pure axial compression to evaluate load-displacement and load-strain responses. The authors classified the loading process into three stages: elastic, elasto-plastic, and failure, which is consistent with conventional CST (concrete-filled steel tube) column behaviour.

Interpretation of Key Findings

The most significant insight from this work is the trade-off relationship between capacity and ductility that emerges when high-strength steel is combined with cold-formed rectangular sections. As the width-thickness ratio decreases, the external confinement provided by the steel tube to the core concrete increases substantially, enhancing the ultimate load capacity. However, the reduced wall thickness relative to the section width leads to local buckling at lower strains, thereby reducing overall ductility. This observation is particularly important for seismic design applications where ductile behaviour is paramount.

The increase in steel strength yields a more pronounced improvement in ultimate capacity compared to geometric modifications. This aligns with the fundamental relationship between yield strength and axial capacity in composite members. However, higher strength steels typically exhibit reduced elongation at fracture, which propagates into the composite member as reduced energy absorption capacity during the post-peak phase.

Standards and Design Implications

The findings have direct relevance to the design provisions in GB 50936-2014 (Code for Design of Concrete-Filled Steel Tubular Structures) and GB/T 50755-2012 (Technical Code for Cold-Formed Steel Structures). The study suggests that when high-strength steels (e.g., Q460, Q550, or higher) are used in cold-bent rectangular CST columns, the current design equations may overestimate ductility while potentially underestimating the confinement effect. Engineers should consider applying reduction factors for ductility when selecting high-strength cold-formed sections for seismic zones.

The width-thickness ratio limitation specified in GB/T 50755 for cold-formed sections must be carefully evaluated in the context of concrete-filled applications. The concrete infill provides additional restraint that may allow somewhat relaxed limits compared to hollow cold-formed sections, but this benefit diminishes at higher steel grades where the steel itself becomes more brittle.

Engineering Practice Considerations

In practical engineering, the selection of high-strength cold-bent rectangular CST columns requires careful balance. For non-seismic or low-seismic applications (e.g., industrial buildings, transmission towers), the capacity advantage of high-strength steel with reduced wall thickness is highly attractive from an economic standpoint. For seismic applications, the designer must ensure that the member can undergo the required inter-storey drift without local buckling failure. The experimental evidence from this paper supports the recommendation that a minimum width-thickness ratio of approximately 30-35 should be maintained for Q460 grade steel in seismic detailing, even though the concrete infill provides supplementary confinement.

Study Insights and Reflections

The paper effectively demonstrates that the traditional CST column design philosophy, which benefits from the composite action between steel and concrete, must be adapted when high-strength materials are introduced. The fundamental mechanism of confinement remains valid, but the deformation capacity of the system becomes governed by the more brittle component. Future research should investigate hybrid approaches, such as using high-strength steel for the flanges and moderate-strength steel for the webs, to simultaneously optimise capacity and ductility. The experimental methodology, while sound, would benefit from additional tests incorporating cyclic loading to better characterise the seismic performance of these members.