Finite Element Analysis of Axially Compressed Centrifugal Concrete-Filled Steel Tubes

Literature Overview

This paper by Jin Weilang and Yuan Weibin from the Institute of Structural Engineering at Zhejiang University, published in the Journal of Zhejiang University (Engineering Science) in 2005 (Volume 39, Issue 10, pages 1571-1575), presents a three-dimensional finite element analysis of centrifugal concrete-filled steel tubes (CCFT) under axial compression. The study builds upon earlier experimental work on steel-concrete composite structures, specifically incorporating the effect of interfacial slippage between the steel tube and the centrifugally cast concrete core. The paper is classified under TU398 and addresses the ultimate bearing capacity of CCFT axial compression members.

Core Technical Content

The centrifugal casting process produces concrete with a unique microstructure characterised by densification toward the inner surface of the steel tube and porosity gradients that differ from conventionally cast concrete. This non-uniform density distribution has significant implications for the mechanical behaviour of CCFT members under axial loading. The authors developed a three-dimensional finite element model that accounts for three-dimensional slippage at the steel-concrete interface using non-linear spring elements, while simultaneously capturing the non-linear constitutive behaviour of both the centrifugal concrete and the steel tube.

Key Technical Parameters and Modelling Approach

The model was validated against experimental results, and the authors report good agreement between the theoretical predictions and the measured behaviour. The model successfully captures the bonding mechanism between the steel tube and the centrifugal concrete throughout the loading process, from initial elastic response through yield, hardening, and ultimate failure.

Interpretation of Technical Points

The inclusion of slippage effects in the finite element model is a significant methodological contribution. In conventional CFST modelling, the steel-concrete interface is often assumed to be perfectly bonded, which can lead to overestimation of the confinement effect and the ultimate load capacity. The centrifugal casting process, however, produces a concrete surface that is denser and potentially more adherent to the steel tube than conventionally cast concrete, but the density gradient means that the interface properties are directionally dependent. The use of non-linear spring elements to simulate three-directional slippage allows the model to capture the progressive loss of interface integrity under increasing axial compression, which is particularly important for predicting the post-peak behaviour and the ductility of the member.

The paper also highlights that the centrifugal concrete exhibits different stress-strain characteristics compared to normal concrete, primarily due to the density gradient and the preferential alignment of aggregate particles during the centrifugal casting process. This results in higher compressive strength near the steel-concrete interface and lower strength toward the core, which creates a complex stress distribution within the cross-section that cannot be accurately captured by uniform concrete models.

Process and Standards Analysis

The centrifugal casting process for steel tubes involves rotating the mould at high speeds (typically 1000-3000 rpm) while pouring concrete, which generates centrifugal forces that compact the concrete against the inner surface of the steel tube. The process parameters, including rotational speed, pouring rate, and concrete mix design, directly influence the quality of the final composite member. Relevant standards for centrifugal concrete products include GB/T 2864 and ISO 12573, which specify requirements for the manufacturing process, dimensional tolerances, and mechanical properties of centrifugally cast concrete components.

The paper's approach to modelling the steel-concrete interface aligns with the general framework established in GB 51248-2017 (Code for Design of Concrete-Filled Steel Tubular Structures), which provides guidelines for the interaction between steel tubes and concrete cores, including the calculation of the confinement effect and the determination of the ultimate load capacity.

Integration with Engineering Practice

The findings of this study have direct relevance to the design and assessment of CCFT members used in bridge piers, building columns, and offshore structures. The finite element model developed by the authors can serve as a basis for more detailed parametric studies to investigate the effects of varying the centrifugal casting parameters, steel tube geometry, and concrete mix design on the structural performance. For engineering practice, the following recommendations can be derived:

1. The interfacial bond strength between the steel tube and centrifugal concrete should be measured experimentally for each specific production batch, rather than relying on assumed values.
2. The non-uniform density distribution in centrifugal concrete should be accounted for in design calculations, particularly for members subjected to bending or eccentric loading where the stress distribution is non-uniform across the cross-section.
3. The slippage behaviour at the interface should be considered in the assessment of seismic performance, as the progressive loss of bond can significantly affect the energy dissipation capacity of the member.

Key Questions and Reflections

A critical question arising from this study is whether the non-linear spring element model used to simulate interface slippage is sufficiently general to capture the complex three-dimensional behaviour of the steel-concrete interface under combined loading conditions. The current model focuses on axial compression, but in practice, CCFT members are often subjected to combined axial, bending, and shear loads. Additionally, the paper does not address the effect of fatigue loading or cyclic loading on the interface behaviour, which is important for applications in seismic zones.

Study Insights and Implications

This paper makes a valuable contribution to the understanding of CCFT member behaviour by explicitly incorporating interface slippage into the finite element analysis. The good agreement between the model predictions and experimental results validates the modelling approach and provides confidence in its application to more complex loading scenarios. For practising engineers, the key implication is that the centrifugal casting process produces a composite member with unique mechanical characteristics that cannot be adequately described by conventional CFST design models. The development of more sophisticated numerical models that account for the density gradient, interface slippage, and complex loading conditions will be essential for the full exploitation of centrifugal CFST technology in structural engineering applications.