Axial Compression Performance of Stiffened Round-Rectangular Concrete-Filled Steel Tube Short Columns

Literature Overview

This paper by Ren Zhigang, Zhang Ling, and Li Qi from the School of Civil Engineering and Architecture, Wuhan University of Technology, published in the Journal of Wuhan University of Technology (2022, Vol. 44, No. 11, pp. 45-52), presents an experimental and numerical investigation into the axial compression behaviour of round-rectangular concrete-filled steel tube (RRCFST) short columns incorporating various stiffening measures. The research was supported by the National Natural Science Foundation of China (Grant No. 51778512). The authors designed and fabricated four RRCFST short column specimens and conducted axial compression tests, supplemented by finite element (FE) simulations using ABAQUS that were validated against experimental data. The parametric study examined the influence of different stiffening strategies—straight ribs, T-shaped ribs, and diaphragms—on load-bearing capacity, as well as the effects of flat-segment width-to-thickness ratio, section aspect ratio, and diaphragm width-to-thickness ratio.

Core Technical Findings

The study establishes that among the three stiffening measures investigated, the diaphragm (transverse partition plate) provides the greatest enhancement to axial compression capacity. This is attributed to the diaphragm's ability to distribute the confining pressure more uniformly across the concrete core and to effectively restrain local buckling of the steel tube flat segments. The parametric analysis reveals several important quantitative relationships:

The finding that the flat-segment width-to-thickness ratio has no influence on the diaphragm-enhanced capacity is particularly noteworthy. This suggests that once a diaphragm is introduced, the local buckling resistance of the flat segments becomes less critical, as the diaphragm effectively redistributes stresses and prevents premature local failure. The progressive increase in capacity enhancement with increasing aspect ratio (11% to 16%) indicates that taller sections benefit more from diaphragm stiffening, likely because the diaphragm provides additional restraint against lateral deformation over longer unsupported lengths.

Finite Element Modelling and Validation

The ABAQUS-based FE model was calibrated against the four experimental specimens, ensuring that the numerical predictions captured the key mechanical responses including load-displacement curves, failure modes, and ultimate capacity. The validated model was then used for parametric analysis, which is a standard and efficient approach in structural engineering research. The FE model likely incorporated nonlinear material models for both the steel tube (with elastic-plastic behaviour) and the concrete core (with confined concrete constitutive models such as Mander or Kent-Park), as well as contact interactions between the steel tube and concrete interface. The use of parametric studies in FE allows for a comprehensive exploration of design variables that would be impractical to test experimentally due to cost and time constraints.

Engineering Practice Implications

From an engineering design perspective, this research provides valuable guidance for the selection of stiffening measures in RRCFST columns used in building frames, bridge piers, and offshore structures. The recommendation to prefer diaphragms over rib stiffeners for maximum capacity enhancement is practically significant, as diaphragms are relatively straightforward to fabricate and weld. However, engineers must consider the additional welding work, potential distortion during welding, and the need for adequate weld quality control—particularly for the diaphragm-to-tube joint, which is a critical stress concentration zone.

In terms of welding practice, the diaphragm-to-tube connection typically requires full-penetration butt welds or high-quality fillet welds depending on the design requirements. The weld quality directly affects the effectiveness of the diaphragm stiffening, as any lack of fusion or porosity in the weld would compromise the load transfer mechanism. Quality assurance measures such as ultrasonic testing (UT) and radiographic testing (RT) should be applied to these critical welds, in accordance with standards such as GB/T 11345 or ASME V.

Study Insights and Reflections

This research contributes meaningfully to the understanding of RRCFST behaviour, but several questions remain open for future investigation. First, the study focuses on short columns (high slenderness ratio), and the behaviour of long columns with diaphragm stiffening under combined axial and bending loads would be more representative of real structural applications. Second, the effect of seismic loading on diaphragm-stiffened RRCFST columns has not been addressed, yet ductility and energy dissipation are critical performance criteria for seismic regions. Third, the parametric study could be extended to include the number and spacing of diaphragms, as well as the interaction between multiple diaphragms in longer columns. The finding that smaller diaphragm width-to-thickness ratios yield greater capacity improvement suggests that thinner, more numerous diaphragms may be optimal, but this must be balanced against fabrication feasibility and weld quality considerations.