Stainless Steel Tubes Could Reshape How Concrete Columns Are Designed

Stainless steel is quietly becoming one of the most interesting materials in structural engineering.

The research, led by Wenli Yuan, Jia Wang and C. S. Cai, focused on concrete-filled stainless steel tubular columns, abbreviated CFSST. The authors set out to establish an appropriate design method for evaluating the bearing capacity of these columns, with the goal of improving the rationality and safety of structural design and supporting the broader adoption of stainless steel in civil engineering.

To do this, the team built finite element models in ABAQUS, the industry-standard simulation platform, and calibrated them against laboratory tests previously reported by Azad and colleagues in 2021. Those tests covered sixteen specimens across four geometric sizes, two stainless steel types and four loading conditions, and the new study selected three representative specimens as reference members. The models deliberately incorporated the messy realities of real construction: initial geometric imperfections measured with a laser displacement transducer, material nonlinearities captured through the concrete damaged plasticity model, and welding residual stresses in the tube walls. The stainless steels examined were the austenitic grade A304 and the lean duplex grade LDX2101, both chosen for their high strength and relatively low cost.

Unlike carbon steel, stainless steel does not show a distinct yield plateau, so the yield point had to be defined at a plastic strain of 0.2 percent. The researchers converted nominal stress and strain values from tensile tests into the true stress and true plastic strain format that ABAQUS requires, because the difference between the two becomes significant under large deformation. For the concrete, they used a stress–strain relationship based on the Chinese specification GB 50010–2010 for unconfined concrete and a confinement-aware model proposed by Tao and co-workers in 2013 for the concrete inside the tube, where the steel shell restrains internal crack propagation and significantly enhances ductility.

When the simulated columns were compared with the physical tests, the agreement was close. The numerical predictions reproduced the observed failure modes and the load–displacement curves with reasonable accuracy, and the axial ultimate loads matched the measured values within an acceptable range. The authors were candid about the limits of their approach: because the finite element model introduces imperfections using the first eigenvalue buckling mode scaled to the measured magnitude, rather than mapping the exact measured shape, the predicted buckling location could differ from what appeared in the tests. Slightly higher predicted capacities and differences in pre-peak stiffness were attributed to idealised assumptions about fabrication tolerances, end contact conditions and material parameters. Still, the validated models gave the team a reliable foundation for a much larger parametric investigation.

That parametric study covered seventy columns in compression, spanning square and circular sections, four concrete grades from C30 to C80, and both stainless steel grades. The parameters explored were the slenderness ratio, which gauges the likelihood of global buckling; the width-to-thickness ratio of the tube wall, which governs local buckling; and the confinement factor, which quantifies how strongly the tube restrains the concrete. Higher concrete strength and stronger confinement significantly increased load capacity, whereas increases in slenderness ratio and width-to-thickness ratio reduced structural stability and lowered the ultimate capacity the column could reach before instability took over.

Under identical dimensional and material conditions, square sections exhibited higher capacities than circular sections. The authors explain this through the effective sectional area, noting that capacity is positively related to it, and pointing out that the circular group in their study had a smaller tube wall thickness and was therefore more prone to local buckling, which offset the benefit of more uniform confinement. Because the proportional differences between the two shapes varied considerably across configurations, the researchers concluded that square and circular CFSST members should be calculated separately rather than linked through a simple proportional coefficient.

The most consequential part of the work, however, is what it says about existing design codes. The team compared their numerical results against four international standards: the European EN 1994-1-1, the American AISC 360–16, the Australian AS/NZS 2327:2017 and the Chinese GB 50936-2014. All four were developed primarily for concrete-filled carbon steel tubular columns, and the comparison revealed that they are not fully applicable to stainless steel. For circular sections, all four codes were conservative, though the size of the margin varied considerably with the width-to-thickness ratio.

In response, the authors proposed a modified design formula that accounts for the cross-sectional size effect, the width-to-thickness ratio, the confinement effect and the slenderness ratio in a single framework. The formula reduces the stainless steel strength as a function of the width-to-thickness ratio, applies a confinement influence coefficient calibrated separately for square and circular sections, and introduces a slenderness reduction factor. When the simulated ultimate capacities were compared with predictions from this new formula, the trendline sat slightly above and close to the reference line, suggesting generally safe predictions with a moderate safety margin without causing unnecessary material waste. The team also validated the equations against experimental data on CFSST columns collected from the published literature, finding satisfactory accuracy for the available tests, and noted that some previously proposed formulae tend to yield unconservative predictions within the parameter ranges covered here.

The study does carry caveats worth remembering. Only a limited number of high slenderness ratios were included in the parametric analysis, because once slenderness exceeds a certain level the axial capacity becomes increasingly governed by global stability and its sensitivity to further increases decreases markedly. The authors caution that extrapolating the proposed equations beyond the covered ranges should be undertaken with care and preferably supported by additional experimental or numerical verification, and they recommend further verification if the equations are applied to concretes stronger than those considered. Because the work is entirely numerical and theoretical, grounded in existing test data rather than new physical experiments, its conclusions describe model-based predictions validated against a specific experimental dataset.

Subject of Research: Numerical analysis and design formulae for concrete-filled stainless steel tubular columns under axial compression

Article Title: Numerical analysis and design of concrete-filled stainless steel tubular columns in compression

Article References: Yuan, W., Wang, J., & Cai, C. S. (2026). Numerical analysis and design of concrete-filled stainless steel tubular columns in compression. Advances in Bridge Engineering, 7(1), Article 30. https://doi.org/10.1186/s43251-026-00219-2

Image Credits: AI Generated

DOI: 10.1186/s43251-026-00219-2

Keywords: concrete-filled stainless steel tubes, CFSST columns, finite element analysis, ABAQUS, axial compression, slenderness ratio, width-to-thickness ratio, confinement effect, design codes, stainless steel, structural engineering, bearing capacity